Low-carbohydrate, higher-fat diets such as the Atkins diet resurfaced during the past decade as more and more individuals searched for a means to decrease body weight at a time when overweight and obesity were viewed as increasing problems in the United States. The attractiveness of this particular diet strategy stems from the relatively rapid decrease in body weight seen during the first few days and weeks when carbohydrate intake is severely diminished, while fat and protein intake are increased. A commonly touted “metabolic advantage” of low-carbohydrate diets (LCDs) over traditional diets suggests that more weight may potentially be lost when matched calorie for calorie with other diets higher in carbohydrate content, due in part to the higher thermogenic cost of protein that makes up a larger percentage of many LCDs .
Sedentary, overweight individuals are a common group of individuals that utilized the LCD to achieve desirable health outcomes commonly associated with reductions in body weight and body fat. Consequently, this group is also the most frequently reported in the scientific literature regarding outcomes determining the impact LCDs have on health. Much of the available research seems to suggest improvements in most metabolic health parameters after consumption of a short-term LCD (< 6 months) in both normolipidemic and hyperlipidemic individuals. Decreases in fasting serum triglyceride levels [2–5] have been reported in conjunction with consuming an LCD, as well as increases in high-density lipoprotein cholesterol (HDL-C) levels [5,6], as HDL-C has been shown to increase when dietary carbohydrate is replaced by saturated, monounsaturated, or polyunsaturated fat in the diet . Previous studies also suggest improvements in insulin sensitivity as serum glucose and insulin levels decrease after consuming an LCD [7,8].
Although LCDs are more frequently consumed by the average weight-conscious consumer, some athletes may opt for that dietary pattern to potentially enhance endurance performance and/or to rapidly affect body composition. Data are contradictory regarding the performance effects, with most suggesting a lack of improvement [9•], but the weight loss results are fairly supportive . This review discusses the potential health risks of low-carbohydrate and high-fat diets and the effect of this dietary practice on aspects of athletic performance.
Potential Health Risks
Despite the reported benefits, some have warned of potential negative health consequences associated with consuming a diet low in carbohydrate and higher in fat and protein. Protein-rich foods, particularly animal-based protein foods, often contain higher levels of saturated fat and cholesterol that may adversely affect low-density lipoprotein cholesterol (LDL-C) levels and increase the risk for cardiovascular disease, although research demonstrates that lipid–lipoprotein levels improve when carbohydrate intake is decreased [6,11,12]. LCDs tend to be low in dietary fiber and may increase the risk of constipation, irritable bowel, diverticulitis, and other intestinal issues. Other concerns associated with LCDs include potential metabolic strain on liver and kidney function (ie, kidney stones) and potential electrolyte or pH imbalances, although recent evidence does not support the latter issues . LCDs may lack required vitamins, minerals, and essential fatty acids, but that risk can be decreased by supplementing these nutrients. The majority of high-fat and LCD research has focused on short-term (< 1 year) metabolic adaptations; as such, the long-term benefits and potential consequences are still poorly understood. Moreover, severe carbohydrate restriction is only recommended for a short period of time, and carbohydrate-containing foods are usually added back into these diets (eg, Atkins and South Beach).
Aside from health outcomes, LCDs have recently found their way into the athletic arena and have warranted attention from a few investigators as a potential mechanism to improve endurance performance. It has been a subject of recent debate as to what type of diet is best for an athletic population, and one of the recent approaches in the scientific community has been to examine the effects of LCDs, more commonly referred to as high-fat diets, on performance parameters in athletes. This approach has primarily focused on endurance athletes such as competitive cyclists and distance runners. It is a common perception among athletes that a diet low in carbohydrate and high in fat will negatively affect exercise performance. Proponents of LCDs suggest that this dietary practice provides large amounts of lipid as substrate for ATP synthesis, thus potentially decreasing the reliance on limited muscle glycogen stores and ultimately delaying muscle glycogen breakdown during exercise.
It is generally assumed that a carbohydrate intake of 7 to 10 g/kg/d  is necessary for endurance athletes to restore muscle and liver glycogen stores to ensure sufficient glucose availability for skeletal muscle contraction during endurance-type aerobic exercise . Consumption of a high-carbohydrate diet has been shown in earlier studies to increase carbohydrate oxidation and muscle glycogenolysis during exercise . Consuming large amounts of carbohydrate may provide adequate substrate to fuel daily training and competition needs, and potentially increases the relatively low amount of carbohydrate stored in the body as glycogen.
Carbohydrate and fat are the primary substrates for skeletal muscle metabolism during rest and exercise. Their contribution to total oxidative metabolism is dependent on a variety of factors, including exercise intensity, exercise duration, diet, and other factors such as training status . LCDs result in metabolic and hormonal adaptations that may improve fat oxidation and promote glycogen sparing in exercising skeletal muscle. Similar to endurance training adaptations, there is a shift toward a greater reliance on fat oxidation for fuel at rest and during exercise on an LCD, which may be due to a combination of increased oxidative enzymes, increased mitochondrial density, greater storage and utilization of intramuscular triglyceride, and enhanced muscular uptake of plasma free fatty acids [18–21]. In addition, LCDs have been shown to increase resting human skeletal muscle pyruvate dehydrogenase (PDH) kinase activity and decrease the amount of active PDH , which in turn decreases carbohydrate oxidation. These combined mechanisms would lead to a reduction in muscle glycogenolysis and carbohydrate oxidation and contribute to greater utilization of free fatty acids during exercise.
Havemann et al.  reported respiratory exchange ratio (RER) values after adaptation to a low-carbohydrate, higher-fat diet. Eight cyclists ingested either an LCD for 7 days followed by a high-carbohydrate diet on day 8 or a high-carbohydrate diet alone for 8 days. During exercise, the RER values at rest and during exercise were decreased on the LCD and the corresponding blood data demonstrated increased plasma free fatty acid levels when compared with ingestion of a higher-carbohydrate diet for 8 days . These results represent specific examples demonstrating increased fat utilization at rest and during exercise with adaptation to an LCD, even when carbohydrate intake is restored.
In examining the effects of an LCD on exercise performance, a few animal studies have demonstrated that increased availability of fatty acids delays the development of exhaustion in rats subjected to prolonged exercise [24,25]. This has led investigators to examine differing amounts of carbohydrate intake on human performance outcomes. One study, in which competitive cyclists were confined to a metabolic ward and provided a eucaloric LCD for 4 weeks designed for weight maintenance (83% fat, 15% protein, and < 3% carbohydrate) and supplemented with additional key minerals (1 g/d K+, 3 g/d Na+, 600 mg Ca+, 300 mg Ma+, and a multivitamin) showed no loss of VO2max or endurance exercise capacity (time to exhaustion at 60%–70% VO2max) when compared with baseline assessments despite diminished pre-exercise muscle glycogen content . Lambert et al.  demonstrated that 2 weeks of adaptation to a high-fat (67%), low-carbohydrate (7%) diet in five endurance-trained male cyclists nearly doubled exercise time to exhaustion at approximately 60% VO2max when compared with a low-fat (12%), high-carbohydrate (74%) diet, whereas muscle power during supramaximal exercise (30-second Wingate test) and high-intensity cycling exercise to exhaustion at approximately 90% VO2max were not impaired after the LCD despite low pre-exercise glycogen levels.
Pitsiladis and Maughan  examined the effects of more moderate changes in dietary carbohydrate in trained male cyclists and experienced triathletes. Subjects in this study consumed an isoenergetic diet that consisted of either 70% carbohydrate or 40% carbohydrate and exercised to exhaustion at either 90% VO2max (n = 7 cyclists) or 80% VO2max (n = 5 triathletes). The results of this study showed no differences in cycling exercise times to exhaustion and no differences in rate of perceived exertion, heart rate, or oxygen uptake between conditions. Despite the performance enhancement found in a few studies, it has also been demonstrated by some that LCDs have no effect on exercise performance in either trained or untrained individuals [9•,29–31].
LCDs generally lead to greater total energy deficits than traditional or habitual diets, and thus may affect diet adherence and exercise performance. However, Horvath et al.  have demonstrated that trained male and female runners may consume fewer overall calories on a low-fat diet (16% total energy) and have reduced endurance performance when compared with those on medium- (31%) and high-fat (44%) diets when allowed to eat ad libitum. Also, Muoio et al.  suggest that restriction of dietary fat may be detrimental to endurance performance as these investigators found that treadmill running time to exhaustion in six trained men was greatest after a diet higher in fat (38%) versus two diets higher in carbohydrate (61% and 73%).
Low-carbohydrate, high-fat diets may lead to greater utilization of fat as fuel during exercise, thereby leading to a glycogen-sparing effect and potentially improving endurance exercise capacity. The low amount of glycogen stored in the human body, which is approximately 300 to 400 g in skeletal muscle and 70 to 100 g in the liver , poses a limitation in the ability to maintain a high power output during prolonged endurance exercise. It has been argued that a consequence of an LCD may be a decline in pre-exercise muscle glycogen content, especially in untrained individuals, which may defeat the purpose of creating the glycogen-sparing effect in the first place. However, Lambert et al.  demonstrated enhanced endurance capacity in endurance-trained individuals on an LCD even in the face of diminished pre-exercise glycogen levels. The metabolic adaptations associated with a short-term LCD combined with an already enhanced oxidative system, including an up-regulation of mitochondrial oxidative enzymes and increased mitochondrial density, suggests endurance athletes may still be able to perform similar amounts of physical work on an LCD even in the face of potentially more difficult perceived effort.
The available research on the effects of an LCD on endurance performance is still limited and the results equivocal, and there are few data regarding the effects on high-intensity exercise performance. The research that has been done to date has focused mainly on time-to-exhaustion exercise trials, with a few studies utilizing time trial performance as a primary outcome. Few studies have performed a crossover study design where all subjects received each dietary treatment. Furthermore, most studies have used a pre–post testing design and have not controlled for deviations in daily training.
What remains relatively unknown is the effect high-fat diets have on daily training; the vast majority of research has utilized a pre–post study design. The athletes are tested before beginning the diet modification and then tested again at the end of the diet. If high-fat diets lower glycogen and hinder performance based on a pre–post testing design, how certain is it that the observed differences are not due to reductions in training volume? For example, if someone does less work on a daily basis while following a high-fat diet, then that individual would likely have a reduced performance effort due to a decreased training stimulus. Therefore, differences in performance may be due to chronic changes in substrate utilization and not necessarily acute changes during the final testing session. It is imperative that studies that utilize a pre–post testing design insure that work and training are similar between diet groups; otherwise, it is difficult to know account for differences in training volume.
Because most studies have used time-to-fatigue or time trial methods to test performance, little is understood regarding how these diets affect day to day training. As most training bouts neither continue to the point of fatigue nor are completed at maximal effort, it would be useful to know how these diets alter daily exercise, if at all. Because most training sessions occur at intensities less than what occurs during competition, the negative effects previously observed may not occur when training.
We recently reported [35•] a case study of a professional triathlete consuming a grain-based diet and an LCD, each lasting 2 weeks with a wash-out period between the diets. The athlete was not able to perform the same training volume while on the carbohydrate-restricted diet compared with what he was able to complete on the grain-based diet. There are several limitations with a design of this nature (eg, biased toward preferred diet, history, and training effect); however, this case report does provide evidence that fluctuations in daily training may influence performance outcomes in addition to or separate from the diet intervention. Thus, research designs need to incorporate control mechanisms, such as supervising the training sessions, to insure that any performance differences are due to the diet and not training volume, which is often unsupervised. Although the change in training volume was affected by the diet, it is just difficult to tease out how much each contributes to performance.
Weight loss and body composition are issues that have received little research attention regarding this diet strategy in the athletic arena. Many athletes gain fat weight during the off-season and have to decrease the gained body fat to attain an optimal level for in-season performance. Based on the available literature, it appears that an LCD may offer an athlete a faster means of attaining the desired body composition.
Whereas the athletic and exercise literature has almost exclusively investigated the short-term performance change following an LCD with less regard to body composition, the health-related literature has numerous studies pertaining to the body composition and weight effects of this diet strategy. One limitation regarding this aspect of LCDs is the issue of controlling for weight stability.
When attempting to reduce body weight quickly, it appears that higher-fat LCDs may offer the better means of accomplishing that goal (Fig. 1). Athletes followed Phase 1 of the Atkins diet or a grain-based diet (focus on whole-grains with limited refined carbohydrate and desserts) for 2 weeks. The LCD contained 56% fat and 13% carbohydrate and the grain-based diet contained 28% fat and 56% carbohydrate (Haub and Cook; Unpublished data). There was significant (P < 0.05) weight reduction following the LCD phase; however, others observed that only increasing fat content alone did not reduce body weight in a group of cyclists . Furthermore, during our study, both body weight and lean mass decreased to a greater extent with the higher-fat diet. Thus, it may not be advantageous to merely reduce body weight by following an LCD if preservation of lean mass is desired.
It is evident that higher-carbohydrate intake tends to elicit fewer perturbations in athletic performance compared with low carbohydrate intake. Many report that restricting carbohydrate intake elicits improvements in oxidative mechanisms, especially fat oxidation. However, any performance benefits of this approach have not been demonstrated consistently [9•]. Weight loss is one area in which LCDs might be of benefit compared with higher-carbohydrate diets. Research consistently demonstrates that LCDs elicit faster weight loss than low-fat diets; however, it remains to be fully understood if the more rapid weight loss improves athletic performance.
This was partially supported by funds received from the Kansas Wheat Commission and a Beginning Grant-in-Aid (0560026Z) from the American Heart Association.
References and Recommended Reading
Papers of particular interest, published recently, have been highlighted as: • Of importance •• Of major importance
1. Feinman RD, Fine EJ: “A calorie is a calorie” violates the second law of thermodynamics.Nutr J
2. Larosa JC, Fry AG, Muesing R, Rosing DR: Effects of high-protein, low-carbohydrate dieting on plasma lipoproteins and body weight.J Am Diet Assoc
3. Sondike SB, Copperman N, Jacobson MS: Effects of a low-carbohydrate diet on weight loss and cardiovascular risk factor in overweight adolescents.J Pediatr
4. Brehm BJ, Seeley RJ, Daniels SIR, D'Alessio DA: A randomized trial comparing a very low carbohydrate diet and a calorie-restricted low fat diet on body weight and cardiovascular risk factors in healthy women.J Clin Endocrinol Metab
5. Foster GD, Wyatt HR, Hill JO, et al.
: A randomized trial of a low-carbohydrate diet for obesity.N Engl J Med
6. Volek JS, Sharman MJ: Cardiovascular and hormonal aspects of very-low-carbohydrate ketogenic diets.Obes Res
2004, 12(Suppl 2)
7. Mensink RP, Katan MB: Effect of dietary fatty acids on serum lipids and lipoproteins. A meta-analysis of 27 trials.Arterioscler Thromb
8. Samaha FF, Iqbal N, Seshadri P, et al.
: A low-carbohydrate as compared with a low-fat diet in severe obesity.N Engl J Med
9.• Erlenbusch M, Haub M, Munoz K, et al.
: Effect of high-fat or high-carbohydrate diets on endurance exercise: a meta-analysis.Int J Sport Nutr Exerc Metab
This article provides a statistical review of human trials through 2004. Compares the relevant studies and illustrates the effects of macronutrient intake on exercise performance.
10. Nordmann AJ, Nordmann A, Briel M, et al.
: Effects of low-carbohydrate vs low-fat diets on weight loss and cardiovascular risk factors: a meta-analysis of randomized controlled trials.Arch Intern Med
11. Haub MD, Wells AM, Campbell WW: Beef and soy-based food supplements differentially affect serum lipoprotein-lipid profiles because of changes in carbohydrate intake and novel nutrient intake ratios in older men who resistive-train.Metabolism
12. Yancy WS Jr, Olsen MK, Guyton JR, et al.
: A low-carbohydrate, ketogenic diet versus a low-fat diet to treat obesity and hyperlipidemia: a randomized, controlled trial.Ann Intern Med
13. Yancy WS Jr, Olsen MK, Dudley T, Westman EC: Acid-base analysis of individuals following two weight loss diets.Eur J Clin Nutr
2007, Epub ahead of print.
14. Burke LM, Cox GR, Culmmings NK, Desbrow B: Guidelines for daily carbohydrate intake: do athletes achieve them?Sports Med
15. Costill DL, Hargreaves M: Carbohydrate nutrition and fatigue.Sports Med
16. Galbo H, Holst JJ, Christensen NJ: The effect of different diets and of insulin on the hormonal response to prolonged exercise.Acta Physiol Scand
17. Jeukendrup AE: Modulation of carbohydrate and fat utilization by diet, exercise and environment.Biochem Soc Trans
18. Helge JW, Ayre K, Chaunchaiyakul S, et al.
: Endurance in high-fat-fed rats: effects of carbohydrate content and fatty acid profile.J Appl Physiol
19. Hurley BF, Nemeth PM, Martin WH 3rd, et al.
: Muscle triglyceride utilization during exercise: effect of training.J Appl Physiol
20. Jeukendrup AE, Saris WH, Wagenmakers AJ: Fat metabolism during exercise: a review--part II: regulation of metabolism and the effects of training.Int J Sports Med
21. Kiens B, Essen-Gustavsson B, Christensen NJ, Saltin B: Skeletal muscle substrate utilization during submaximal exercise in man: effect of endurance training.J Physiol
22. Peters SJ, Leblanc PJ: Metabolic aspects of low carbohydrate diets and exercise.Nutr Metab (Lond)
23. Havemann L, West SJ, Goedecke JH, et al.
: Fat adaptation followed by carbohydrate loading compromises high-intensity sprint performance.J Appl Physiol
24. Hickson RC, Rennie MJ, Conlee RK, et al.
: Effects of increased plasma fatty acids on glycogen utilization and endurance.J Appl Physiol
25. Rennie MJ, Winder WW, Holloszy JO: A sparing effect of increased plasma fatty acids on muscle and liver glycogen content in the exercising rat.Biochem J
26. Phinney SD: Ketogenic diets and physical performance.Nutr Metab (Lond)
27. Lambert EV, Speechly DP, Dennis SC, Noakes TD: Enhanced endurance in trained cyclists during moderate intensity exercise following 2 weeks adaptation to a high fat diet.Eur J Appl Physiol Occup Physiol
28. Pitsiladis YP, Maughan RJ: The effects of alterations in dietary carbohydrate intake on the performance of high-intensity exercise in trained individuals.Eur J Appl Physiol Occup Physiol
29. Burke LM, Hawley JA: Effects of short-term fat adaptation on metabolism and performance of prolonged exercise.Med Sci Sports Exerc
30. Carey AL, Staudacher HM, Cummings NK, et al.
: Effects of fat adaptation and carbohydrate restoration on prolonged endurance exercise.J Appl Physiol
31. Pogliaghi S, Veicsteinas A: Influence of low and high dietary fat on physical performance in untrained males.Med Sci Sports Exerc
32. Horvath PJ, Eagen CK, Fisher NM, et al.
: The effects of varying dietary fat on performance and metabolism in trained male and female runners.J Am Coll Nutr
33. Muoio DM, Leddy JJ, Horvath PJ, et al.
: Effect of dietary fat on metabolic adjustments to maximal VO2 and endurance in runners.Med Sci Sports Exerc
34. Sherman WM, Wimer GS: Insufficient dietary carbohydrate during training: does it impair athletic performance?Int J Sport Nutr
35.• Rosenkranz RR, Cook CM, Haub MD: Endurance training on low carbohydrate and grain based diets: a case study.Int J Sport Nutr Exerc Metab
2007, 17:in press.
This case study details one athlete's experience with a higher-fat and lower-carbohydrate diet compared with a lower-fat and higher-carbohydrate diet. The novelty of this paper is that it focuses on how the diets affect daily training instead of just one performance trial.
36. Brown RC, Cox CM, Goulding A: High-carbohydrate versus high-fat diets: effect on body composition in trained cyclists.Med Sci Sports Exerc