During lactation, many women initiate or resume an exercise program in order to return to their prepregnancy fitness level. Lovelady et al. (20) and Dewey et al. (8) have reported that lactating women can exercise and still produce adequate milk for infant growth. However, there is a paucity of information on the effects of exercise and weight loss on maternal nutritional status during lactation. Lactating women who initiate a weight loss and exercise program may be at risk for developing nutritional deficiencies due to the increased metabolic demands of exercise, lactation, and energy restriction.
One nutrient in particular that may be of concern to this population is vitamin B-6. Numerous studies (2,16,17,26,27) have shown that average dietary intakes of vitamin B-6 in lactating women were below the Recommended Dietary Allowance of 2.0 mg (15). Because some researchers have reported a correlation between concentration of vitamin B-6 in the breast milk and maternal intake of vitamin B-6 (6,16,27,29), a low intake of vitamin B-6 may lead to reduced vitamin B-6 concentrations in breast milk. Marginal vitamin B-6 status in lactating women and infants (as determined by concentrations of vitamin B-6 in the breast milk of less than 600 nmol·L) has been implicated in infant seizures (3,29) and impaired behavioral development (22). Heiskanen et al. (13) reported that infants with marginal vitamin B-6 status (as determined by reduced activity of enzymes involving vitamin B-6 in infant erythrocytes) had slower linear growth after 6 months of age.
The active coenzyme form of vitamin B-6 is pyridoxal 5′-phosphate (PLP). Enzymes that depend on PLP to function include glycogen phosphorylase and aminotransferases, enzymes involved in glycogenolysis and amino acid metabolism, respectively. Increases in plasma PLP during exercise have been documented (7,14,21). Because exercise induces glycogenolysis and transamination of amino acids, it has been speculated that the increase in plasma PLP results from mobilization of stored PLP for use by the muscle (14) or liver (21). This mobilization of stored PLP leads to increased plasma PLP, which may convert to 4-pyridoxic acid and be excreted in the urine. Persons who exercise frequently (resulting in increased vitamin B-6 loss in the urine) and have consistently low vitamin B-6 intakes may be at risk for vitamin B-6 deficiency (21).
There has been little research examining the effect of exercise on vitamin B-6 status during the reproductive period. Yates et al. (33) examined the effect of an exercise program on vitamin B-6 status in pregnant women. The exercise was walking at 70% of age-predicted maximum heart rate for 30 min·d-1, 3 d·wk-1, for 8 wk. They found no indications of a deficiency; however, women in their study supplemented their diet with 10 mg of vitamin B-6 each day, approximately five times the RDA for vitamin B-6 during pregnancy.
Breastfeeding women have an increased need for vitamin B-6 because: 1) it is essential for the growing infant and the mother, and 2) the content of vitamin B-6 in the breast milk is related to maternal intake of the vitamin. However, reports indicate that average maternal intake of vitamin B-6 is inadequate during lactation (2,16,17,26,27). Women who restrict their food intake in order to lose weight may also decrease their intake of vitamin B-6. In addition, exercise may increase vitamin B-6 utilization and excretion (7,14,21) and result in further increased need for the lactating woman who exercises frequently. Therefore, the purpose of this study was to determine the effect of energy restriction and exercise on the vitamin B-6 status of overweight lactating women receiving a daily multivitamin supplement containing 2.0 mg of vitamin B-6.
Subjects and Research Design
Twenty-two women were recruited from a study on the effects of weight loss in overweight lactating women on infant growth (19). Criteria required for inclusion into the study included the following: healthy (free of chronic disease), sedentary (had not exercised more than once per week during the previous 3 months), nonsmoking, exclusively breastfeeding, delivered a full-term infant weighing ≥ 2500 g, and did not deliver by Cesarean section. Because the focus of this intervention was on the prevention of obesity, women with a body mass index (BMI) ≥ 25 and ≤ 30 kg·m-2 at 3 wk postpartum were recruited. At 4 wk postpartum, women were randomly assigned to the experimental group (WG), which received the weight loss program, or to the control group (CG), which was instructed not to restrict their energy intake and not to perform vigorous aerobic exercise more than once per week. All women were given a multivitamin supplement which contained 2.0 mg vitamin B-6 in the form of pyridoxine-HCl and at least 50% of the Recommended Dietary Allowances for other vitamins for lactating women, 1 wk before baseline measurements and throughout the remainder of the study. The supplementation of B-6 to all women in the study was an ethical decision, based upon the association found between low vitamin B-6 status of infants and slow growth (13). Mothers were asked weekly if they were consuming the supplements as prescribed. Measurements were completed in the Human Performance Laboratory. Maternal plasma and milk vitamin B-6 status, anthropometric measurements, and dietary intake records were assessed at baseline, midpoint, and endpoint (4–6 wk, 9–11 wk, and 14–16 wk postpartum, respectively); cardiovascular fitness was measured at baseline and endpoint only. Maternal weight and infant weight and length were also measured weekly by a research assistant in the participants’ homes. Informed, written consent was obtained from each subject, and the experimental procedures were in accordance with the policy statements of the American College of Sports Medicine and the Human Subjects Committee of the Institutional Review Board at the University of North Carolina at Greensboro.
Anthropometric and Body Composition Measurements
Women were weighed in bathing suits to the nearest 100 g with a stationary beam balance scale. Maternal height was measured without shoes, using a stationary stadiometer. Body composition was determined by underwater weighing. Before submersion in the tank, women’s residual lung volume was measured using an oxygen dilution technique (31). Body density and percent fat were calculated using the formulas of Brozek et al. (4). Infants were weighed nude on a portable digital scale (SECA, Inc., Columbia, MD) to the nearest 10 g, and their length was measured to the nearest 0.1 cm using a portable pediatric measuring board.
All foods and beverages consumed for three consecutive days were weighed on portable digital gram scales (Ohaus, Inc., Florham Park, NJ). Women recorded their dietary intake by speaking into a tape recorder. Nutritionist IV software program (version 2.0, N2 Computing, Salem, OR), manufacturer’s information, and other food composition tables were used to analyze dietary intake.
A modified Balke protocol (1) was used to estimate cardiovascular fitness. After a 5-min warm-up on the treadmill, the speed was increased to 5.4–5.9 km·h-1 and remained constant for the duration of the test, whereas the grade of the treadmill was increased by 2.5% every 2 min. Heart rates and perceived exertion levels were recorded every minute. The test continued until the heart rate reached 85% of predicted maximum heart rate reserve [(maximal heart rate − resting heart rate) (0.85) + resting heart rate]. Predicted oxygen consumption (V̇O2 mL·kg-1·min-1) was calculated for each heart rate after every 2 min at each grade level using the following equation for walking: (3.5 mL O2·kg-1 ·min-1) + (speed in m·min-1 × 0.1) + (grade × m·min-1 × 1.8) (1). Predicted V̇O2 mL·kg-1 at maximum heart rate was calculated using a linear regression equation with heart rate as the independent variable and V̇O2 mL·kg-1 as the dependent variable.
Weight Loss Program
Energy intake and exercise intensity were determined individually for each woman in the WG. Because no equations have been developed specifically for lactating women to estimate energy requirements, the Harris-Benedict equation was used (12). This equation is used often to estimate energy requirements of adults. It considers gender, age, weight, and height to estimate resting energy expenditure. This estimate was multiplied by a moderate activity factor of 1.35. Moderate activity level was chosen because the majority of women were caring for small children, cooking meals, and cleaning their homes. They were “sedentary” in regard to having no regular aerobic exercise program more than once per week. An additional 630 kcal were added to the energy expenditure estimate to provide for the extra energy needed for lactation. This number was based on the National Research Council’s estimation that the average energy content of breast milk is 70 kcal·100 mL-1 and the daily milk volume is 750 mL with an 80% production efficiency (24). Energy intake prescription was determined by subtracting 500 kcal from the average of the reported daily baseline intake and estimated energy requirements. Women were prescribed a diet containing approximately 25% of energy from fat, 20% from protein, and 55% from carbohydrate and no less than 1800 kcal·d-1. Women in the WG were given six commercially made, low-fat, and low-calorie frozen dinners each week to assist with dietary compliance.
Subjects exercised four times each week at an intensity of 65–80% of their heart rate reserve. Women wore heart rate monitors (Polar, Inc., Port Washington, NY) during exercise to ensure they were exercising at the prescribed intensity. Initial exercise sessions were 15 min in duration and included a 5-min warm-up and 5-min cool-down. Participants increased the duration of exercise by at least 2 min every day until they were exercising for 45 min within their target heart range, usually by the end of the fifth week of the intervention. Exercise sessions included brisk walking, jogging, or aerobic dancing. A research assistant monitored heart rate and compliance at each exercise session.
Vitamin B-6 Analysis
Assessment of vitamin B-6 status was determined by measuring concentrations of plasma and milk vitamin B-6, and plasma pyridoxal 5′-phosphate (PLP), as well as the evaluation of erythrocyte alanine transaminase (EALT) activity with and without additional PLP. Women collected milk samples by manually expressing approximately 25 mL of milk into collection tubes during the first feeding after 5:00 a.m. They wrapped the tubes in foil to reduce exposure to light. The milk was transported to the Human Performance Laboratory and frozen at −70°C until analysis.
Blood samples were drawn from an antecubital vein in the morning, after an overnight fast. Plasma was frozen at −70°C until analysis. Stimulation of erythrocyte alanine transaminase (EALT) was analyzed by the method of Gella et al. (10). Briefly, transamination of alanine to pyruvate was monitored with and without excess PLP. Subjects were considered to have an adequate vitamin B-6 status if the measured pyruvate formed by the erythrocytes exposed to PLP was no more than 25% greater than the pyruvate formed by the erythrocytes not exposed to PLP (18).
Plasma PLP was analyzed using an enzymatic assay employing tyrosine decarboxylase apoenzyme and 14C tyrosine (5). Plasma total vitamin B-6 was analyzed using a microbiological assay (23). Concentrations of plasma PLP > 30 nmol·L-1 and total vitamin B-6 > 40 nmol·L-1 were considered adequate (18). Milk vitamin B-6 was also analyzed using a microbiological assay (Saccharomyces uvarum) (25). Concentrations > 600 nmol·L-1 were considered adequate (22).
Data were analyzed using SPSS PC (SPSS, Inc., Chicago, IL). Initial characteristics of age, parity, height, weight, BMI, prepregnancy weight, pregnancy weight gain, and infant birth weight of groups were compared with multivariate analysis and race by chi-square analysis. Changes over time and between groups were evaluated with repeated measures analysis of variance. A Bonferroni correction was used to correct for alpha-inflation. Because there were 15 variables analyzed, we tested at a significance level of P ≤ 0.003 for each analysis. Multiple regression was used to determine if plasma vitamin B-6 and maternal intake of total vitamin B-6 predicted milk vitamin B-6 concentrations at each time point. Significance was defined as P ≤ 0.02. Results are reported as means and standard errors of the means.
There were no significant differences in baseline characteristics of women between the WG and CG (see Table 1). Data on the women’s weight, body composition, energy intake, cardiovascular fitness, and vitamin B-6 status during the weight-loss intervention are shown in Table 2. Only two women reported taking oral contraceptives during the study. Women in the WG lost 4.4 ± 0.4 kg, which was significantly more than the CG’s loss of 0.9 ± 0.5 kg (P < 0.001). This loss was predominantly fat mass (−3.8 ± 0.4 and −0.8 ± 0.3 kg for the WG and CG, respectively, P ≤ 0.001). Both groups also lost fat-free mass; however, the loss was not significantly different between the groups or over time. There were no significant differences between the two groups in infant weight or length gain during the study.
One woman in the CG did not keep complete dietary records for all three measurement periods. Therefore, she was excluded from dietary analysis. Women in the WG decreased their energy intake by 672 ± 146 kcal·d-1 compared with women in the CG who did not change their energy intake (P = 0.007). Changes in dietary intake of protein, vitamin B-6, and the ratio of vitamin B-6 to protein were not significant, nor were there significant differences between groups. Three women reported irregular intake of the vitamin supplements during the study. That is why the total vitamin B-6 intakes vary slightly from the predicted value (i.e., dietary intake plus 2.0 mg from the supplement). All but one of the women in the WG were able to exercise 4 d a week. The exercise program improved cardiovascular fitness: predicted maximal oxygen consumption increased by 12% among women in the WG, as compared with 3% among the control women (P = 0.09).
There were no significant differences between groups or over time in plasma PLP or total vitamin B-6 concentrations, or in erythrocyte alanine transamination (EALT) activity; however, there was a trend for vitamin B-6 concentration in milk to increase over time in both groups (P = 0.05). The plasma PLP and the plasma total vitamin B-6 values at the midpoint were higher than the baseline and end measurements. There were two women (one in each group) whose values were higher than two standard deviations from the mean. When these women were removed from data analysis, the mean and standard error of the mean values of PLP decreased from 75.3 ± 16.0 to 70.0 ± 10.7 in the WG and from 81.2 ± 14.8 to 60.1 ± 5.4 in the CG. The plasma total vitamin B-6 decreased from 91.4 ± 16.9 to 86.8 ± 12.7 in the WG and from 99.2 ± 16.9 to 75.9 ± 7.5 in the CG. The results of repeated measures analysis of variance were the same as previously stated. That is, the values were not significantly different over time or between groups. Because there were no differences between groups, data from all subjects were grouped together to determine if plasma vitamin B-6 and maternal intake of total vitamin B-6 would predict milk vitamin B-6 concentrations at each time point. They predicted milk vitamin B-6 concentrations (r2 = 0.37, P = 0.02) only at the end measurements.
This study demonstrated that a moderate weight loss of approximately 0.5 kg·wk-1 in overweight, lactating women did not affect vitamin B-6 status or infant growth. This is an important finding because compromised vitamin B-6 status of the mother may result in low concentrations of vitamin B-6 in the breast milk. This could adversely affect the growth and/or behavioral development of the infant. Women in the WG, even while restricting their intake to an average of 1662 ± 103 kcal·d-1, were able to maintain a dietary intake of 1.8 ± 0.3 mg·d-1 of vitamin B-6, similar to that of control women. This intake was somewhat higher than those reported in other studies of lactating women (2, 17, 26). However, at the end of the study, 45% (5/11) of women in the WG and 40% (4/10) of women in the CG consumed less than the estimated average requirement of 1.7 mg of vitamin B-6·d-1 during lactation (15). In spite of these low intakes, average concentrations of plasma PLP, total vitamin B-6, and milk concentrations were adequate throughout the study. This was likely due to the additional daily intake of 2.0 mg of vitamin B-6 provided in the supplement. At the end of the study the average total vitamin B-6 intake (dietary and supplement) to protein ratio was 0.052 in the WG and 0.045 in the CG, well above the suggested value of 0.016 for adequate vitamin B-6 status (18).
Average baseline plasma PLP concentrations (55.5 and 73.7 nmol·L, WG and CG, respectively) were higher than those reported by Andon et al. (2). Lactating women in their study (who were consuming an average of 1.46 mg of vitamin B-6 from dietary intake and not receiving supplementary vitamin B-6) had mean plasma PLP concentrations of 34 nmol·L-1. However, our plasma PLP concentrations were similar to those reported by Chang and Kirksey (6) among lactating women at 1–2 months postpartum with vitamin B-6 intakes of approximately 3.5 mg from dietary and supplement sources. At the end of the study, the mean plasma PLP concentrations (63.3 and 70.0 nmol·L-1, WG and CG, respectively) were slightly lower than those reported by Chang and Kirksey (6) (94 nmol·L) at 4 months postpartum with similar vitamin B-6 intakes.
Total vitamin B-6 concentrations in breast milk increased over time in both groups. This was also observed by Chang and Kirksey (6) during the first 6 months of lactation. Lower milk concentrations at 1 month postpartum may indicate that women are replenishing their own stores of vitamin B-6 that were depleted during pregnancy. The correlation between plasma PLP and vitamin B-6 concentrations in milk increased during this study and the study by Chang and Kirksey (6). This suggests that once maternal stores are repleted, there is a stronger correlation between maternal plasma and milk concentrations of vitamin B-6.
Previous studies of supplementation with vitamin B-6 during lactation have reported a strong relationship between vitamin B-6 concentration of milk and maternal intake of vitamin B-6 (6,16,27,29). These studies provided subjects with a wide range of vitamin B-6 intakes, usually from 2 to 20 mg of B-6·d-1. In contrast, subjects in this study were consuming a narrower range of vitamin B-6: approximately 2–5 mg·d-1 and there was no relationship between dietary or dietary and supplemental intake of vitamin B-6 and concentrations of vitamin B-6 in breast milk. Andon et al. (2) also reported no relationship between maternal vitamin B-6 intake and vitamin B-6 concentration in breast milk of women not receiving vitamin B-6 supplementation. Based on results of supplementation studies, Chang and Kirksey (6) have recommended a daily supplement of 2.5–4.0 mg of vitamin B-6 to maintain adequate concentrations of vitamin B-6 in breast milk as well as maternal plasma PLP concentrations.
There was no significant difference in infant growth between the two groups. Infant weight gain was similar to that reported in other studies on the growth of breastfed infants from birth to four months postpartum (30). Heiskanen et al. (13) reported slower linear growth after 6 months of age among infants with marginal vitamin B-6 status. We did not measure vitamin B-6 concentrations of the infants’ plasma; however, the adequate gain in length of our infants may indirectly reflect adequate vitamin B-6 status.
Four women had two or more vitamin B-6 indices below adequate (18) during the study. One woman in the WG had marginal plasma PLP and total vitamin B-6 concentrations at all time points, as well as an increased EALT activity at the end of the study. This subject was consuming a low amount of vitamin B-6 due to inconsistent supplementation and low dietary intake (1.5 mg). Another woman in the WG had low plasma PLP and total vitamin B-6 concentrations only at the end of the study. She also reported low dietary intake of vitamin B-6 (1.4 mg). However, milk concentrations of vitamin B-6 were well above normal in both of these subjects. Only two women in the study (both in the CG) had milk concentrations below 600 nmol·L-1. One of these women had increased EALT activity at the beginning and middle of the study, which were the two time points her milk vitamin B-6 concentrations and dietary intake of vitamin B-6 were low. By the end of the study, her milk concentrations and all of her vitamin B-6 indices were normal. She also reported taking oral contraceptive agents (low in estrogen), beginning at 6 wk postpartum. Only one other woman reported taking oral contraceptive agents and her indicators of vitamin B-6 status were normal throughout the study. The woman in the CG with low plasma PLP, total vitamin B-6, and milk vitamin B-6 concentrations at all time points throughout the study had a low dietary intake of vitamin B-6 and reported taking thyroid medication. There have been reports of pyridoxine deficiency in hyperthyroidism in humans (32). The compromised status of these four subjects suggest a need to increase vitamin B-6 intake through diet or supplementation during lactation among women with a low dietary intake of vitamin B-6 or those taking thyroid medication.
Although athletes generally appear to have adequate dietary vitamin B-6 intakes, a few studies have reported marginal vitamin B-6 status in athletes (9,11,28). Telford et al. (28) reported that 51 of 86 subjects had values below normal for vitamin B-6 status, as measured by stimulation of erythrocyte glutamate oxaloacetate transaminase activity (if stimulation was ≥ 20% with added PLP, the subjects were considered deficient in vitamin B-6). Vitamin B-6 status was improved in 23 of the 27 athletes receiving a daily supplement of 100 mg of vitamin B-6 for 7–8 months, compared with only 6 of the 24 athletes not receiving a supplement. In a cross-sectional study, Guilland et al. (11) reported that 35% of 55 athletes had increased erythrocyte aspartate aminotransferase stimulation and 17% had low plasma PLP concentrations (<34.4 nmol·L-1). In contrast to these studies, all of the women in the WG in our study had adequate baseline values of plasma PLP and total vitamin B-6, and erythrocyte alanine transaminase activity. In addition, women in our study did not exercise as vigorously or frequently as the athletes in the above studies.
Vitamin B-6 has vital roles in glycogenolysis, gluconeogenesis, and transamination of amino acids, all of which increase during exercise. Studies examining the effect of acute exercise have shown that exercise induces an increase in plasma PLP, plasma 4PA, and urinary excretion of 4 PA (7,14,21). Because of this, it has been speculated that athletes would require more vitamin B-6 in the diet to accommodate the increases in these metabolic pathways. However, the research on the effect of exercise on vitamin B-6 status is very limited. Fogelholm (9) reported that vitamin B-6 status was affected negatively in only one of 21 women during a 24-wk exercise program. These results are similar to our study; i.e., the vitamin B-6 status decreased in 2 of 11 women. Yates et al. (33) found no effect of walking from 22 to 30 wk gestation on vitamin B-6 status of pregnant women; however, all subjects in that study received a supplement of 10 mg of vitamin B-6·d-1.
In summary, overweight lactating women on an energy restriction and exercise program may lose weight at a rate of 0.5 kg·wk-1 without affecting their vitamin B-6 status, the concentration of vitamin B-6 in their breast milk, or the growth of their infants. However, these results are dependent on adequate dietary intake of vitamin B-6 coupled with a supplementary intake of 2 mg of B-6·d-1. Further research is needed to evaluate the effect of energy restriction and exercise on the vitamin B-6 status of unsupplemented lactating women.
This research was funded by grants from the National Institutes of Health (HD34222), the North Carolina Agricultural Research Service, and the Institute of Nutrition of The University of North Carolina. We are indebted to the women who participated in this study, to the team of student research assistants, and to Ellen Sunbery, for her technical assistance.
Address for correspondence: Cheryl Lovelady, Ph.D., Department of Nutrition and Foodservice System, The University of North Carolina at Greensboro, P.O. Box 26170, Greensboro, NC 27402-6170; E-mail: Cheryl_Lovelady@uncg.edu.
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Keywords:© 2001 Lippincott Williams & Wilkins, Inc.
OBESITY,; WEIGHT LOSS,; PYRIDOXAL 5′-PHOSPHATE,; BODY COMPOSITION,; INFANT GROWTH