Secondary Logo

Journal Logo

High Protein Diets, Calcium Economy, and Bone Health

Kerstetter, Jane E. PhD, RD; O'Brien, Kimberly O. PhD; Insogna, Karl L. MD

Clinical Nutrition Issues
Free

High protein diets are known to increase urinary calcium excretion in humans. A commonly held hypothesis is that the excess urinary calcium originates from bone because the acid generated by dietary protein requires buffering by bone. According to this hypothesis, the long-term consequence of a high protein diet would be to increase bone resorption, decrease bone density, and increase fractures. However, there are significant scientific data (including cross-sectional, longitudinal, and dietary intervention studies) in humans that suggest that high dietary protein is not detrimental to bone. In fact, many studies show that increasing protein is beneficial to skeletal health and there are several physiological mechanisms that support this observation.

From the School of Allied Health, University of Connecticut, Storrs, Conn (Kerstetter)

The Johns Hopkins Bloomberg School of Public Health, Center for Human Nutrition, Baltimore, Md (O'Brien)

The Yale University School of Internal Medicine, New Haven, Conn. (Insogna)

This work was supported by grants from the US Department of Agriculture (00–35200–9579, 97–35200–4420, 94–37200–0668), the National Institutes of Health (DK52128–03, NIH MO1-8RR00125, NIH 5P30AR46032–04), the Catherine Weldon Donaghue Women's Health Investigator Program at Yale University (to KLI), The Patrick & Catherine Weldon Donaghue Medical Research Foundation (to KLI), and the University of Connecticut (to JEK).

Corresponding author: Jane E. Kerstetter, PhD, RD, School of Allied Health, 358 Mansfield Rd, University of Connecticut, Storrs, CT 06269 (e-mail: Jane.Kerstetter@uconn.edu).

OSTEOPOROSIS and low bone mass (osteopenia) affect almost 44 million men and women older than 50 years in the United States, and this number is expected to grow to 52 million by 2010. 1 The health problem is reaching near epidemic proportions. In response, The Institute of Medicine revised and released the recommended dietary allowances, RDAs (known collectively as the dietary reference intakes, DRIs), for the bone-related nutrients (calcium, phosphorus, magnesium, fluoride, and vitamin D) before the other nutrients. The DRIs reflect both state-of-the-art understanding and the substantial gaps in the scientific data base for nutritional factors that influence skeletal health. The DRIs cite the extensive number of human and animal studies addressing the role of dietary calcium and vitamin D in acquiring and maintaining skeletal mass through the life span. In contrast, our understanding of how other dietary components, such as protein, affect the skeleton is still limited. In this review, we will focus on how dietary protein influences calcium metabolism and how those processes may affect bone health.

Our capacity to measure changes in skeletal homeostasis has improved considerably in the last decade. We now have better ways to noninvasively assess rates of skeletal formation and resorption, to quantify bone mass and measure calcium homeostasis. Using these tools, a complex picture of protein's effect on calcium and bone metabolism is emerging. It is frequently suggested that a high protein diet is detrimental to bone health, particularly if the protein is from animal sources. However, recent epidemiological and clinical data do not necessarily support these hypotheses. This review will develop the hypothesis that dietary protein is an important regulator of calcium metabolism and that high protein diets may be less detrimental than previously thought.

Back to Top | Article Outline

HOW MUCH PROTEIN AND CALCIUM DO WE CONSUME?

The mean protein intake for adult men and women in the United States and the percentage of individuals consuming each level of protein are summarized 2 in Table 1. For purposes of this review, we have identified low, medium, high, and very high protein diets in comparison to the RDA as defined in Table 1. The most current RDA for protein remains at 0.8 g protein/kg in the adult. 3 Between 15% and 25% of adult men (20–59 years) and 5% and 10% of adult women (20–59 years) consume more than double the RDA for protein. As seen from Table 1, dietary protein generally declines, when expressed in absolute grams/day, over the age of 60 years.

Table 1

Table 1

The majority of adult men and women in the United States consume less than 100% of the 1989 RDA for calcium (800 mg for men and women older than 25 years). 2 Therefore, when considering the impact of dietary protein on calcium metabolism, it should be kept in mind that most Americans consume a diet rich in protein, but poor in calcium.

Back to Top | Article Outline

DIETS MODERATE IN PROTEIN DO NO HARM

There is uniform agreement that at moderate levels of protein intake (roughly 100%–150% of the RDA or 0.8–1.2 g protein/kg), measures of calcium and skeletal metabolism remain normal. 4–8 Approximately 30% to 50% of adults in the United States have protein intakes that are considered moderate. Using a 4-day experimental model where nutrients were controlled, we have also documented normal calcium homeostasis when healthy adults consumed a diet containing 1.0 g protein/kg and 20 mmol calcium. 4 Measures of the PTH-1-α-hydroxylase axis and urinary calcium excretion were within the normal ranges under these moderate conditions.

Back to Top | Article Outline

HIGH DIETARY PROTEIN INCREASES URINARY CALCIUM

Eighty years ago, Sherman 9 first observed that feeding an all-meat diet to humans increased urinary calcium. There was tremendous interest during the 1970s and 1980s in the relationship between dietary protein and calcium metabolism and approximately 30 human studies addressing this topic were published during those 2 decades. It is now clearly established that increasing dietary protein increases urinary calcium. We have reviewed data from 26 clinical intervention trials in adult humans where the diet was controlled, dietary protein was manipulated (150 g protein and less), and urinary calcium was measured (Fig 1). 4,5,10–33 Despite varied experimental designs, most studies reported a positive relationship between protein intake and urinary calcium and the overall association is strong (P < .001, r = 0.7). The linear regression equation from Figure 1 is as follows:

Figure 1

Figure 1

where urine calcium is expressed as mmol/d and dietary protein is g/d, slope = 3.208E-02, and y-intercept is 1.501. These data clearly establish that dietary protein is an important regulator of urinary calcium excretion, at least as important as dietary calcium. 16,34

Because protein and phosphorus are typically found together in foods, a high protein diet (typically from meat) generally also means a high phosphorus diet and phosphorus is hypocalciuric. Because of the phosphorus effect, those studies that use meat (as opposed to purified proteins) as a way of increasing dietary protein may not observe the increase in urinary calcium during a high meat diet 5,17,35,36 (although there are divergent studies on this issue). 4,22,26,28,30,37 Nevertheless, it is fair to say that high phosphorus intakes (typically concurrent with the high protein intake), tends to blunt, but not clearly ameliorate the rise in urinary calcium in response to high protein diets. 38

Back to Top | Article Outline

HIGH PROTEIN DIETS: HARMFUL TO THE SKELETON?

While it is clear that increasing dietary protein increases urinary calcium, the question remains, where does the additional urinary calcium originate? Three potential sources are the diet, intestine, and bone, or any combination thereof. In most of the experiments shown in Figure 1, dietary calcium is well controlled and, therefore, diet can be easily ruled out as a source. The second option that changes in intestinal calcium absorption underlie high-protein-diet–induced hypercalciuria has generally been considered unlikely. Most human balance studies report no difference in calcium absorption when dietary protein is altered 5,6,12–16,19,22 (although there are a few exceptions). 11,20,39 Consequently, it is thought that the additional urinary calcium excreted in response to a high protein diet results from increased bone resorption. The long term consequence of “leaching” calcium from bone would be a reduction in bone mass and increased risk for fragility fractures. Figure 2 summarizes a widely held traditional view on how dietary protein affects calcium homeostasis and skeletal metabolism. The strength of the data supporting this formulation are reviewed below.

Figure 2

Figure 2

Dietary protein, because it is rich in the sulfur-containing amino acids, clearly increases endogenous acid production. 40 The richer the protein source is in sulfur amino acids, the more fixed acid it generates. The endogenous acids are produced by the following reaction 41:

It is argued that the acid load generated by a high protein diet reduces tubular calcium reabsorption leading to hypercalciuria. 41 In addition, it is thought that, particularly in older individuals, buffering of some of the fixed acid load in bone leads to a loss of skeletal mass. Theoretically, increasing dietary protein from 75 to 125 g would increase urine calcium (and decrease calcium balance) by 60 mg daily, which equates to 1% to 2% bone loss annually in an adult. Should the increase in dietary protein persist for a decade, then the calculated 10% to 20% loss in bone would be significant. Conversely, ingesting foods that generate alkali should result in a reduction of net acid excretion and decreased urine calcium excretion. 40 Net acid excretion can be measured in a 24-hour urine and is well known to parallel dietary protein. The potential renal acid load (PRAL) of a diet can be estimated from the composition of the diet using established formulas. 42–44 Depending on the composition of the diets, high protein diets generally produce a greater PRAL than low or moderate protein diets.

Nonetheless, the scientific data supporting the conclusion that the skeleton is called upon to buffer the fixed acid generated by a high protein diet is largely indirect. Acutely exposing cultured calvariae in vitro to an acidic environment causes the release of bone calcium because of simple physicochemical dissolution. If exposed chronically, a low pH in the media increases the activity of osteoclasts (the cells that break down bone) and decreases osteoblastic activity (the cells that stimulate bone formation). 41,45 Likewise, exposing calvariae to alkalotic conditions decreases calcium efflux from bone, probably by decreasing osteoclastic resorption and increasing osteoblastic activity. 46

The human studies regarding acid-base manipulation and the skeleton are consistent with the cell studies. For example, supplementation with potassium bicarbonate in humans improved calcium balance and reduced indirect makers of bone resorption. 24,47 This observation was confirmed, most recently, by Maurer and colleagues, 48 who showed that supplementing healthy men and women with potassium and sodium bicarbonate improved calcium retention and reduced markers of bone resorption.

While it seems clear that pharmacologic manipulation of acid-base status both in vitro and in vivo can affect calcium homeostasis, it remains less certain whether the considerably less extreme alterations induced by a typical diet can produce the same effect. A number of factors tend to ameliorate the potential deleterious effect of fixed acid generated by a high protein diet. We eat our high protein diets over a course of an entire day, 12 to 15 hours, and so the acid generating foods are “dribbled in” rather than “dumped in.” Human diets are complex, containing both acid and alkaline generating capacities. The capacity of the renal and respiratory systems to buffer an acid load is substantial. Given all of these factors, several critical questions remain. In those adults that chronically consume a high protein diet, is there enough endogenous acid produced by that diet to damage the skeleton? Are there data to show that bone mineral density (BMD) declines or bone resorption increases under high dietary protein conditions, as would be predicted? To date, it has been difficult to definitively show, in well-controlled intervention studies, that dietary protein affects the skeleton. In one short and one intermediate-length intervention study in humans using isotopic methods to quantitate calcium kinetics, dietary protein did not affect calcium retention or bone turnover. 36,49 Additionally, there are several large, cross-sectional 50–58 and longitudinal studies 8,59–61 suggesting that BMD is higher in those who consume a high protein diet. These studies are reviewed in the next section.

Back to Top | Article Outline

HIGH PROTEIN DIETS: GOOD FOR THE SKELETON?

Cross-sectional studies

The results of cross-sectional studies evaluating the association of dietary protein and bone density are relatively consistent. There are at least 9 such studies where BMD is the primary outcome showing that a high protein diet is associated with a high BMD (not a low BMD, as might be predicted). 50–58 There are few studies showing no association, 62,63 but only one study showing a negative association. 64

For example, using the NHANES III data base we found, in 1882 non-Hispanic white women aged 50 years and older, that after adjusting for age and body weight, the higher the protein intake, the higher is the hip BMD (Fig 3). 55 When the analysis was restricted to women with intakes of calcium greater than 800 mg/d, the relationship between dietary protein and hip bone density was still observed suggesting that the effect of protein was not due to concurrently low intakes of dietary calcium. Consistent with these data, Munger et al found, in a prospective observational study, that 55-year-old to 69-year-old women consuming the highest amounts of protein, particularly animal protein, had the lowest risk of hip fracture. 65

Figure 3

Figure 3

Back to Top | Article Outline

Longitudinal studies

Several long-term longitudinal studies have evaluated the relationship between typical protein intake and BMD in adults. Almost all have found that subjects (generally older men and women) consuming the highest protein intakes also showed the least amount of bone loss. 8,59–61 For example, Hannan and colleagues 60 studied 615 participants in the Framingham Osteoporosis Study over a 4-year period of time. In this elderly group, higher protein intake was significantly associated with lower rates of bone loss at the hip and spine. Persons in the highest quartile for protein intake showed the lowest rates of bone loss (Fig 4). The bone sparing effect persisted after all known potentially confounding variables were controlled. These findings are consistent with earlier work of Freudenheim, who reported that a high protein intake was associated with better preservation of bone density at the wrist in 35-year-old to 65-year-old women. 59 Dawson-Hughes et al, in a longitudinal intervention study, examined the effect of usual protein intake on BMD in 342 older adults. Subjects were either randomized to taking 12.5 mmol of calcium (as calcium citrate malate) with vitamin D daily or a placebo. The subject's typical protein intake was divided into tertiles. In the calcium supplemented group, increasing dietary protein exerted a positive effect on total body BMD. 8 There is consensus in the longitudinal studies that higher protein intakes are associated with favorable skeletal outcomes.

Figure 4

Figure 4

Back to Top | Article Outline

Isotopic studies

Many clinical intervention studies have used a calcium balance approach to measure protein's effects on calcium homeostasis. The limitation of such an approach is the relative insensitivity of balance studies to small changes in absorption. In particular, a critical component of balance studies is the measurement of fecal calcium. It is very difficult to accurately quantitate fecal calcium excretion given the differences in intestinal transit time and variation caused by dietary changes.

Isotopic studies are more sensitive than balance studies, but have not been extensively used to assess protein's effect on calcium in part because of the expense, specialized equipment, and expertise required. However, recently there have been 2 isotopic studies specifically designed to evaluate the impact of dietary protein on calcium retention and indices of bone metabolism. Roughead et al, 36 using isotopic calcium tracers, compared the effects of a high and low meat diet on body calcium retention. Fifteen healthy postmenopausal women consumed both high and low meat diets for 8 weeks in a randomized, cross-over study design. At the end of this time period, there was no statistically significant difference in 47Ca retention when subjects consumed a diet containing 12% versus 20% of the energy as protein. There were also no differences in measures of bone turnover. In fact, there appeared to be a slight trend (albeit not significant) toward improved 47Ca calcium retention on the high meat diet. Clearly, over the 8-week duration of this study, a high meat diet was not detrimental to bone.

A second calcium isotopic study was conducted by our research group and initial results from that study have been reported. 49 In this acute study, 13 adult women consumed both a medium protein (1 g protein/kg) and a high protein (2.1 g protein/kg) diet in random order for a week. All experimental diets were designed to contain 20 ± 0 mmol calcium and 100 ± 0 mmol sodium. The phosphorus intake on the low protein diet averaged 35 ± 1 mmol, whereas on the high protein diet it was 38 ± 0 mmol. Using dual stable calcium isotopes at the end of each intervention, we measured bone formation, resorption, balance, and calcium absorption. As expected, when subjects consumed the high protein diet, they developed hypercalciuria. There was no change in kinetic measures of bone formation, bone resorption, or bone balance between the medium and the high protein diets. This study provides direct evidence that the increase in urinary calcium excretion in response to a high protein diet is not due to an increase in bone resorption. Rather, an increase in intestinal calcium absorption explains almost all of the increment in urinary calcium following a high protein diet (as detailed in the next section titled “Intestinal Calcium Absorption”). In both isotopic studies, 36,49 calcium intake was controlled (600–800 mg) and close to the average consumed by US women, albeit lower than current recommendations. 66

Taken together, the 2 isotopic studies 36,49 do not support the traditional hypothesis that a high protein diet in adults adversely affects calcium retention or skeletal homeostasis. These 2 studies, the first to use isotopic methodology in well-controlled dietary intervention, directly address the effects of dietary protein on bone. Isotopic methods of assessing calcium homeostasis are considered by many to be the gold standard because of their sensitivity. The acute increase in intestinal calcium absorption from the high protein diets 49 may not persist, because there was no change in intestinal calcium absorption at 8 weeks in the Roughead study, 36 thereby raising the question of intestinal adaptation over time.

Back to Top | Article Outline

Protein supplementation studies

In several clinical trials where hip fracture patients were supplemented with protein, there was a reduction in the degree of bone loss 67,68 and a significant improvement in the rate of recovery. 69–71 For example, Schurch et al 69 studied 82 patients (mean age = 80.7 ± 7.4 years) who had recently sustained an osteoporotic hip fracture. These patients had self-selected very low protein intakes (approximately 40 g). The administration of additional 20 g of protein per day was associated with significant attenuation of proximal femur bone loss in the fractured hip. 1 year, bone loss rates were 50% lower in the protein-supplemented individuals than in those that received an isocaloric control supplement. Interestingly, those patients that received the protein supplements spent less time in the rehabilitation ward than those who received the control supplement.

Back to Top | Article Outline

POTENTIAL MECHANISMS TO EXPLAIN WHY DIETARY PROTEIN IS BONE PROTECTIVE

There are at least 3 mechanisms by which dietary protein could exert a positive effect on bone: via an improvement in intestinal calcium absorption, an increase in insulin-like growth-factor 1 (IGF-1), or protein's supportive role in bone protein matrix, or any combination of these 3 factors. Data bearing on each mechanism are reviewed below.

Back to Top | Article Outline

Intestinal calcium absorption

As noted above, we have found that a high protein diet induces an acute increase in intestinal calcium absorption while kinetic measure of bone turnover remains unaltered. We initially studied intestinal calcium absorption in 7 young women as they consumed a high (2.1 g/kg) and low protein (0.7 g/kg) diet for 4 days. 28 Since then, using the same protocol, we have studied 13 additional healthy women (10 young and 3 postmenopausal). The results in all 20 of these study subjects are summarized 72 in Figure 5. Most notably, calcium absorption during the low protein diet averaged 18.4% ± 1.3%, significantly lower than during the high protein diet: 26.3 ± 1.5% (P < .0001, paired t-test). The change in intestinal calcium absorption from low to high protein (8%) explains ~80% of the change in urinary calcium excretion between the low and high protein diets (3.4 to 5.4 mmol) since dietary calcium was held constant in this study at 20 mmol. Therefore, the conclusion based on balance studies that dietary protein does not affect intestinal calcium absorption, is likely to be incorrect (at least acutely). It is possible that balance studies could not detect intestinal calcium absorption differences because of the very high interindividual variability in basal rates of intestinal calcium absorption and the relative insensitivity of the methodology. Our paired design controlled for the former limitation and the use of stable calcium isotopic methodology represent an improvement in sensitivity vis a vis the standard balance approach.

Figure 5

Figure 5

If increasing dietary protein improves intestinal calcium absorption without stimulating bone breakdown, the availability of more calcium, particularly in a protein-sufficient state may favor new bone formation. At the very least, the improvement in intestinal calcium absorption induced with increasing dietary protein may help to counteract the fall in the efficiency of calcium absorption that occurs with aging. This would suggest that menopausal women, in whom impaired intestinal calcium absorption is common, would benefit from an adequate intake of dietary protein (as well as sufficient calcium). Others have agreed that adequate protein intake is essential for bone health in older adults. 73

Back to Top | Article Outline

Insulin-like growth factor 1

IGF-1 has emerged as a key mediator of bone growth. 74 The principal regulator of skeletal metabolism, on a day to day basis is parathyroid hormone (PTH). Parathyroid hormone has complex effects on mineral homeostasis. However, one of PTH's prominent effects, at least when the skeleton is exposed to the hormone in transient daily bursts, is to stimulate bone growth. 75–78 This anabolic effect of PTH requires IGF-1. 79,80 In fact, PTH induces the expression of IGF-1 in bone, and IGF-1 is thought to be a key mediator of PTH-dependent bone growth. 81 IGF-1 also functions in the kidney to stimulate the renal transport of inorganic phosphate and the production of 1,25-dihydroxy vitamin D. 82

Dietary protein is an important regulator of circulating IGF-1 levels. 83 A low protein diet decreases the production and action of IGF-1, whereas protein supplements or a high protein diet increase serum IGF-1 levels. 84 Serum IGF-1 levels are positively correlated with BMD in adult men and women. 85–89 It is possible, therefore, that a high protein diet contributes to bone by increasing circulating IGF-1 levels. 83,90

Back to Top | Article Outline

Protein as a structural component

Bone consists of an organic matrix (or osteoid) in which salts of calcium and phosphate are deposited and, in combination with hydroxyl ions, form special crystals called hydroxyapatite. The matrix is primarily collagen protein fibers. Therefore, protein is an important structural component of bone, accounting for approximately half of bone volume. Because the adult skeleton is constantly remodeling itself, adequate dietary protein is required to provide substrate for adequate new bone formation.

Back to Top | Article Outline

THE REMAINING QUESTIONS

Paradoxically, when fracture is the principal outcome, high-protein intakes are associated with higher rates of fracture in most epidemiologic studies 91–95 (except for Munger et al, 65 as noted above). If a high protein diet is associated with high BMD as most of the epidemiological data show and as most of the clinical trials support, it is very difficult to explain why a high protein intake would be associated with an increased risk for fracture. Perhaps protein intake is tracking with an unmeasured risk factor for osteoporotic fracture. For example, because protein foods tend to be relatively expensive, perhaps it is a surrogate for socioeconomic status. It is known that fracture risk increases with socioeconomic status. Explaining this apparent paradox is critical to a full understanding of how dietary protein affects the skeleton.

Do the short-term and intermediate-length studies of Kerstetter et al 49 and Roughead et al 36 accurately predict the long-term effects on mineral metabolism of a high protein diet? The answer is not known. The fact that the majority of the epidemiological data show a higher incidence of fracture in those consuming the most protein, suggests that the question is worth pursuing in a longitudinal study. Such a study would be difficult and expensive to execute properly, but it is the only way that this important question can be answered.

Back to Top | Article Outline

SUMMARY AND CONCLUSIONS

Available evidence indicates that diets moderate in protein (in the approximate range of 1.0–1.5 g protein/kg) are associated with normal calcium metabolism and presumably are sufficient for bone health. Based on our work and that of others, this appears to be true in men, young women, and postmenopausal women. 5,6,96 Approximately 40% to 50% of adults in the United States consume dietary protein that could be considered moderate. Between 15% and 25% of adult men and between 5% and 10% of adult women (20–59 years) are consuming exceptionally high protein diets, more than 1.6 g protein/kg. Are these diets detrimental to skeletal health? Based on available evidence, our answer to that question is “probably not.” Clearly, humans become hypercalciuric during high protein diets but, at least acutely, this is solely due to an increase in intestinal calcium absorption. 49 At an intermediate time point (2 months), it appears that there is still no evidence for a detrimental effect on bone. 36 There is a wide variety of epidemiological and clinical data showing that dietary protein is associated with improved bone and plausible cellular mechanisms for such a relationship exist.

Bone is complex tissue that changes slowly. As such, it is difficult to design and conduct well-controlled nutrition studies in humans to quantitate the effect of one nutrient on bone. Whereas calcium and vitamin D remain our most important bone-related nutrients, there are other nutrients or food components that may also play a role in skeletal health. 97 Given the rising incidence of osteoporosis in our culture and the clear impact that dietary protein has on calcium metabolism, it is imperative that we gain a better understanding of the complex interplay between dietary protein and the skeleton.

Back to Top | Article Outline

REFERENCES

1. National Osteoporosis Foundation. Disease Statistics. Available at: www.nof.org. Accessed August 2003.
2. Food Surveys Research Group, Beltsville Human Nutrition Research Center, Agricultural Research Service. Supplementary Data Tables, USDA's 1994–96 Continuing Survey of Food Intakes by Individuals. Riverdale, Md: United States Department of Agriculture; 1999.
3. Standing Committee on the Scientific Evaluation of Dietary Reference Intakes Food and Nutrition Board Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrates, Fiber, Fat, Protein and Amino Acids (Macronutrients). Washington, DC: National Academy Press; 2002.
4. Kerstetter JE, Caseria DD, Mitnick ME, et al. Increased circulating concentrations of parathyroid hormone in healthy, young women consuming a protein-restricted diet. Am J Clin Nutr. 1997;66:1188–1196.
5. Spencer H, Kramer L, DeBartolo M, Norris C, Osis D. Further studies of the effect of a high protein diet as meat on calcium metabolism. Am J Clin Nutr. 1983;37:924–929.
6. Heaney RP, Recker RR. Effects of nitrogen, phosphorus, and caffeine on calcium balance in women. J Lab Clin Med. 1982;99:46–55.
7. Heaney RP. Dietary protein and phosphorus do not affect calcium absorption. Am J Clin Nutr. 2000;72:758–761.
8. Dawson-Hughes B, Harris SS. Calcium intake influences the association of protein intake with rates of bone loss in elderly men and women. Am J Clin Nutr. 2002;75:773–779.
9. Sherman H. Calcium requirement of maintenance in man. J Biol Chem. 1920;44:21–27.
10. Johnson NE, Alcantara EN, Linkswiler H. Effect of level of protein intake on urinary and fecal calcium and calcium retention of young adult males. J Nutr. 1970;100:1425–1430.
11. Walker RM, Linkswiler HM. Calcium retention in the adult human male as affected by protein intake. J Nutr. 1972;102:1297–1302.
12. Anand CR, Linkswiler HM. Effect of protein intake on calcium balance of young men given 500 mg calcium daily. J Nutr. 1974;104:695–700.
13. Allen LH, Oddoye EA, Margen S. Protein-induced hypercalciuria: a longer term study. Am J Clin Nutr. 1979;32:741–749.
14. Schuette SA, Zemel MB, Linkswiler HM. Studies on the mechanism of protein-induced hypercalciuria in older men and women. J Nutr. 1980;110:305–315.
15. Hegsted M, Linkswiler HM. Long-term effects of level of protein intake on calcium metabolism in young adult women. J Nutr. 1981;111:244–251.
16. Hegsted M, Schuette SA, Zemel MB, Linkswiler HM. Urinary calcium and calcium balance in young men as affected by level of protein and phosphorus intake. J Nutr. 1981;111:553–562.
17. Spencer H, Kramer L, Osis D, Norris C. Effect of a high protein (meat) intake on calcium metabolism in man. Am J Clin Nutr. 1978;31:2167–2180.
18. Chu JY, Margen S, Costa FM. Studies in calcium metabolism, II: Effects of low calcium and variable protein intake on human calcium metabolism. Am J Clin Nutr. 1975;28:1028–1035.
19. Kim Y, Linkswiler HM. Effect of level of protein intake on calcium metabolism and on parathyroid and renal function in the adult human male. J Nutr. 1979;109:1399–1404.
20. Lutz J, Linkswiler HM. Calcium metabolism in postmenopausal and osteoporotic women consuming two levels of dietary protein. Am J Clin Nutr. 1981;34:2178–2186.
21. Schuette SA, Hegsted M, Zemel MB, Linkswiler HM. Renal acid, urinary cyclic AMP, and hydroxyproline excretion as affected by level of protein, sulfur amino acid, and phosphorus intake. J Nutr. 1981;111:2106–2116.
22. Schuette SA, Linkswiler HM. Effects on Ca and P metabolism in humans by adding meat, meat plus mild, or purified proteins plus Ca and P to a low protein diet. J Nutr. 1982;112:338–349.
23. Draper HH, Piche LA, Gibson RS. Effects of a high protein intake from common foods on calcium metabolism in a cohort of postmenopausal women. Nutr Res. 1991;11:273–281.
24. Lutz J. Calcium balance and acid-base status of women as affected by increased protein intake and by sodium bicarbonate ingestion. Am J Clin Nutr. 1984;39:281–288.
25. Trilok G, Draper HH. Sources of protein-induced endogenous acid production and excretion by human adults. Calcif Tissue Int. 1989;44:335–338.
26. Shapses SA, Robins SP, Schwartz EI, Chowdhury H. Short-term changes in calcium but not protein intake alter the rate of bone resorption in healthy subjects as assessed by urinary pyridinium cross-link excretion. J Nutr. 1995;125:2814–2821.
27. Mahalko JR, Sandstead HH, Johnson LK, Milne DB. Effect of a moderate increase in dietary protein on the retention and excretion of Ca, Cu, Fe, Mg, P, and Zn by adult males. Am J Clin Nutr. 1983;37:8–14.
28. Kerstetter JE, O'Brien KO, Insogna KL. Dietary protein affects intestinal calcium absorption. Am J Clin Nutr. 1998;68:859–865.
29. Zemel MB, Schuette SA, Hegsted M, Linkswiler HM. Role of the sulfur-containing amino acids in protein-induced hypercalciuria in men. J Nutr. 1981;111:545–552.
30. Kerstetter J, Svastisalee C, Caseria D, Mitnick M, Insogna K. A threshold for low-protein-diet-induced elevations in parathyroid hormone. Am J Clin Nutr. 2000;72:168–173.
31. Licata AA. Acute effects of increased meat protein on urinary electrolytes and cyclic adenosine monophosphate and serum parathyroid hormone. Am J Clin Nutr. 1981;34:1779–1784.
32. Margen S, Chu JY, Kaufmann NA, Calloway DH. Studies in calcium metabolism, I: The calciuretic effect of dietary protein. Am J Clin Nutr. 1974;27:584–589.
33. Pannemans DL, Schaafsma G, Westerterp KR. Calcium excretion, apparent calcium absorption and calcium balance in young and elderly subjects: influence of protein intake. Br J Nutr. 1997;77:721–729.
34. Zemel MB. Calcium utilization: effect of varying level and source of dietary protein. Am J Clin Nutr. 1988;48:880–883.
35. Hunt JR, Gallagher SK, Johnson LK, Lykken GI. High- versus low-meat diets: effects on zinc absorption, iron status, and calcium, copper, iron, magnesium, manganese, nitrogen, phosphorus, and zinc balance in postmenopausal women. Am J Clin Nutr. 1995;62:621–632.
36. Roughead ZK, Johnson LK, Lykken GI, Hunt JR. Controlled high meat diets do not affect calcium retention or indices of bone status in healthy postmenopausal women. J Nutr. 2003;133:1020–1026.
37. Licata AA, Bou E, Bartter FC, West F. Acute effects of dietary protein on calcium metabolism in patients with osteoporosis. J Gerontol. 1981;36:14–19.
38. Kerstetter JE, Allen LH. Protein intake and calcium homeostasis. Adv Nutr Res. 1994;9:167–181.
39. Kaneko K, Masaki U, Aikyo M, et al. Urinary calcium and calcium balance in young women affected by high protein diet of soy protein isolate and adding sulfur-containing amino acids and/or potassium. J Nutr Sci Vitaminol (Tokyo). 1990;36:105–116.
40. Barzel US, Massey LK. Excess dietary protein can adversely affect bone. J Nutr. 1998;128:1051–1053.
41. Bushinsky DA. Acid-base imbalance and the skeleton. Eur J Nutr. 2001;40:238–244.
42. Remer T. Influence of diet on acid-base balance. Semin Dial. 2000;13:221–226.
43. Remer T, Manz F. Potential renal acid load of foods and its influence on urine pH. J Am Diet Assoc. 1995;95:791–797.
44. Remer T, Dimitriou T, Manz F. Dietary potential renal acid load and renal net acid excretion in healthy, free-living children and adolescents. Am J Clin Nutr. 2003;77:1255–1260.
45. Arnett TR, Spowage M. Modulation of the resorptive activity of rat osteoclasts by small changes in extracellular pH near the physiological range. Bone. 1996;18:277–279.
46. Bushinsky DA. Metabolic alkalosis decreases bone calcium efflux by suppressing osteoclasts and stimulating osteoblasts. Am J Physiol. 1996;271:F216–F222.
47. Sebastian A, Harris ST, Ottaway JH, Todd KM, Morris RC Jr. Improved mineral balance and skeletal metabolism in postmenopausal women treated with potassium bicarbonate. N Engl J Med. 1994;330:1776–1781.
48. Maurer M, Riesen W, Muser J, Hulter HN, Krapf R. Neutralization of the acidogenic western diet inhibits bone resorption independent of K-intake and reduces cortisol secretion in humans. Am J Physiol Renal Physiol. 2002;284:F32–F40.
49. Kerstetter JE, Raphael RH, O'Brien KO, Caseria DM, Wall DE, Insogna KL. High protein diets acutely increase intestinal calcium absorption but not kinetic measures of bone resorption. J Bone Miner Res. 2003;18:SA322.
50. Teegarden D, Lyle RM, McCabe GP, et al. Dietary calcium, protein, and phosphorus are related to bone mineral density and content in young women. Am J Clin Nutr. 1998;68:749–754.
51. Lacey JM, Anderson JJ, Fujita T, et al. Correlates of cortical bone mass among premenopausal and postmenopausal Japanese women. J Bone Miner Res. 1991;6:651–659.
52. Geinoz G, Rapin CH, Rizzoli R, et al. Relationship between bone mineral density and dietary intakes in the elderly. Osteoporos Int. 1993;3:242–248.
53. Cooper C, Atkinson EJ, Hensrud DD, et al. Dietary protein intake and bone mass in women. Calcif Tissue Int. 1996;58:320–325.
54. Chiu JF, Lan SJ, Yang CY, et al. Long-term vegetarian diet and bone mineral density in postmenopausal Taiwanese women. Calcif Tissue Int. 1997;60:245–249.
55. Kerstetter JE, Looker AC, Insogna KL. Low protein intake and low bone density. Calcif Tissue Int. 2000;66:313.
56. Ilich JZ, Brownbill RA, Tamborini L. Bone and nutrition in elderly women: protein, energy, and calcium as main determinants of bone mineral density. Eur J Clin Nutr. 2003;57:554–565.
57. Whiting SJ, Boyle JL, Thompson A, Mirwald RL, Faulkner RA. Dietary protein, phosphorus and potassium are beneficial to bone mineral density in adult men consuming adequate dietary calcium. J Am Coll Nutr. 2002;21:402–409.
58. Rapuri PB, Gallagher JC, Haynatzka V. Protein intake: effects on bone mineral density and the rate of bone loss in elderly women. Am J Clin Nutr. 2003;77:1517–1525.
59. Freudenheim JL, Johnson NE, Smith EL. Relationships between usual nutrient intake and bone-mineral content of women 35–65 years of age: longitudinal and cross-sectional analysis. Am J Clin Nutr. 1986;44:863–876.
60. Hannan MT, Tucker KL, Dawson-Hughes B, Cupples LA, Felson DT, Kiel DP. Effect of dietary protein on bone loss in elderly men and women: the Framingham Osteoporosis Study. J Bone Miner Res. 2000;15:2504–2512.
61. Promislow JH, Goodman-Gruen D, Slymen DJ, Barrett-Connor E. Protein consumption and bone mineral density in the elderly: the Rancho Bernardo Study. Am J Epidemiol. 2002;155:636–644.
62. Mazess RB, Barden HS. Bone density in premenopausal women: effects of age, dietary intake, physical activity, smoking, and birth-control pills. Am J Clin Nutr. 1991;53:132–142.
63. Wang MC, Luz Villa M, Marcus R, Kelsey JL. Associations of vitamin C, calcium and protein with bone mass in postmenopausal Mexican American women. Osteoporos Int. 1997;7:533–538.
64. Metz JA, Anderson JJ, Gallagher PN Jr. Intakes of calcium, phosphorus, and protein, and physical-activity level are related to radial bone mass in young adult women. Am J Clin Nutr. 1993;58:537–542.
65. Munger RG, Cerhan JR, Chiu BC. Prospective study of dietary protein intake and risk of hip fracture in postmenopausal women. Am J Clin Nutr. 1999;69:147–152.
66. Standing Committee on the Scientific Evaluation of Dietary Reference Intakes, Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride. Washington, DC: National Academy Press; 1999.
67. Bonjour JP, Schurch MA, Rizzoli R. Proteins and bone health. Pathol Biol (Paris). 1997;45:57–59.
68. Bonjour JP, Schurch MA, Rizzoli R. Nutritional aspects of hip fractures. Bone. 1996;18:139S–144S.
69. Schurch MA, Rizzoli R, Slosman D, Vadas L, Vergnaud P, Bonjour JP. Protein supplements increase serum insulin-like growth factor-I levels and attenuate proximal femur bone loss in patients with recent hip fracture. A randomized, double-blind, placebo-controlled trial. Ann Intern Med. 1998;128:801–809.
70. Tkatch L, Rapin CH, Rizzoli R, et al. Benefits of oral protein supplementation in elderly patients with fracture of the proximal femur. J Am Coll Nutr. 1992;11:519–525.
71. Delmi M, Rapin CH, Bengoa JM, Delmas PD, Vasey H, Bonjour JP. Dietary supplementation in elderly patients with fractured neck of the femur. Lancet. 1990;335:1013–1016.
72. Kerstetter JE, O'Brien KO, Insogna KL. Low protein intake: the impact on calcium and bone homeostasis in humans. J Nutr. 2003;133:855S–861S.
73. Bell J, Whiting SJ. Elderly women need dietary protein to maintain bone mass. Nutr Rev. 2002;60:337–341.
74. Geusens PP, Boonen S. Osteoporosis and the growth hormone-insulin-like growth factor axis. Horm Res. 2002;58(suppl 3):49–55.
75. Meunier PJ. Anabolic agents for treating postmenopausal osteoporosis. Joint Bone Spine. 2001;68:576–581.
76. Morley P, Whitfield JF, Willick GE. Parathyroid hormone: an anabolic treatment for osteoporosis. Curr Pharm Des. 2001;7:671–687.
77. Rosen CJ, Bilezikian JP. Clinical review 123: anabolic therapy for osteoporosis. J Clin Endocrinol Metab. 2001;86:957–964.
78. Rosen CJ, Rackoff PJ. Emerging anabolic treatments for osteoporosis. Rheum Dis Clin North Am. 2001;27:215–233, viii.
79. Bikle DD, Sakata T, Leary C, et al. Insulin-like growth factor I is required for the anabolic actions of parathyroid hormone on mouse bone. J Bone Miner Res. 2002;17:1570–1578.
80. Miyakoshi N, Kasukawa Y, Linkhart TA, Baylink DJ, Mohan S. Evidence that anabolic effects of PTH on bone require IGF-I in growing mice. Endocrinology. 2001;142:4349–4356.
81. Kaji H, Sugimoto T, Kanatani M, Nishiyama K, Nasu M, Chihara K. Insulin-like growth factor-I mediates osteoclast-like cell formation stimulated by parathyroid hormone. J Cell Physiol. 1997;172:55–62.
82. Froesch ER, Schmid C, Schwander J, Zapf J. Actions of insulin-like growth factors. Annu Rev Physiol. 1985;47:443–467.
83. Bonjour JP, Schurch MA, Chevalley T, Ammann P, Rizzoli R. Protein intake, IGF-1 and osteoporosis. Osteoporos Int. 1997;7(suppl 3):S36–S42.
84. Bonjour JP, Ammann P, Chevalley T, Rizzoli R. Protein intake and bone growth. Can J Appl Physiol. 2001;26(suppl):S153–S166.
85. Romagnoli E, Minisola S, Carnevale V, et al. Effect of estrogen deficiency on IGF-I plasma levels: relationship with bone mineral density in perimenopausal women. Calcif Tissue Int. 1993;53:1–6.
86. Nasu M, Sugimoto T, Chihara M, Hiraumi M, Kurimoto F, Chihara K. Effect of natural menopause on serum levels of IGF-I and IGF-binding proteins: relationship with bone mineral density and lipid metabolism in perimenopausal women. Eur J Endocrinol. 1997;136:608–616.
87. Kalu DN, Arjmandi BH, Liu CC, Salih MA, Birnbaum RS. Effects of ovariectomy and estrogen on the serum levels of insulin-like growth factor-I and insulin-like growth factor binding protein-3. Bone Miner. 1994;25:135–148.
88. Langlois JA, Rosen CJ, Visser M, et al. Association between insulin-like growth factor I and bone mineral density in older women and men: the Framingham Heart Study. J Clin Endocrinol Metab. 1998;83:4257–4262.
89. Janssen JA, Burger H, Stolk RP, et al. Gender-specific relationship between serum free and total IGF-I and bone mineral density in elderly men and women. Eur J Endocrinol. 1998;138:627–632.
90. Rizzoli R, Ammann P, Chevalley T, Bonjour JP. Protein intake and bone disorders in the elderly. Joint Bone Spine. 2001;68:383–392.
91. Frassetto LA, Todd KM, Morris RC Jr, Sebastian A. Worldwide incidence of hip fracture in elderly women: relation to consumption of animal and vegetable foods. J Gerontol A Biol Sci Med Sci. 2000;55:M585–M592.
92. Abelow B, Holford T, Insogna K. Cross-cultural association between dietary animal protein and hip fracture: a hypothesis. Calcif Tissue Int. 1992;50:14–18.
93. Feskanich D, Willett WC, Stampfer MJ, Colditz GA. Protein consumption and bone fractures in women. Am J Epidemiol. 1996;143:472–479.
94. Meyer HE, Pedersen JI, Loken EB, Tverdal A. Dietary factors and the incidence of hip fracture in middle-aged Norwegians. A prospective study. Am J Epidemiol. 1997;145:117–123.
95. Hegsted DM. Calcium and osteoporosis. J Nutr. 1986;116:2316–2319.
96. Kerstetter JE, Svastisalee CM, Mitnick ME, Caseria DM, Insogna KL. Dietary protein-induced changes in mineral metabolism are not influenced by age or sex. J Bone Miner Res. 2000;15:S423.
97. Ilich JZ, Kerstetter JE. Nutrition in bone health revisited: a story beyond calcium. J Am Coll Nutr. 2000;19:715–737.
Keywords:

bone; high protein diet; human; hypercalciuria

© 2004 Lippincott Williams & Wilkins, Inc.