Transcript View/Open - Oregon State University

NUTRITIONAL STATUS AND LIPID PROFILES IN ACTIVE
WOMEN WITH AND WITHOUT MENSTRUAL DYSFUNCTION
RB Biegler, J Chang, DL Finders, EE Sperber, LC Kam, CP Guebels, MM Manore, FACSM.
Department of Nutrition and Exercise Sciences, Oregon State University, Corvallis, OR.
ABSTRACT
Exercise-induced menstrual dysfunction (ExMD) is prevalent (6-79%) in active women and may result from low energy availability. Unfavorable lipid profiles have
been reported in women with ExMD; low energy intake may also increase risk for poor nutritional status. PURPOSE: To compare nutritional status (diet, blood) and
lipid profiles in active women with ExMD 1) before and after a 6-mo intervention that provided a daily fortified CHO-PRO supplement (360 kcal/d; 300 mg/d calcium
[Ca]) and 2) to Eumenorrheic controls (EU). METHODS: Menstrual status was confirmed by measuring reproductive hormones. In the ExMD group (n=8;
age=23±3y, VO2max=49±6 mL/kg/min, body fat= 22±5%), energy intake and expenditure were assessed at pre- (0-mo) and post-intervention (6-mo) using 7-d
weighed food and physical activity records. EU athletes (n=10; age= 24±5y, VO2max=51±5 mL/kg/min, body fat=23±4%) were only measured at 0-mo. RESULTS:
For ExMD women, the addition of a CHO-PRO supplement to the diet increased Ca intake (pre=1320±571 mg/d, post=1725±555 mg/d, p=0.02). Dietary
cholesterol (chol) (pre=193±130g/d; post=339±240 g/d, p=0.04) also increased, but this was not due to the CHO-PRO drink (chol=15mg/325 ml). At 6-mo, Ca
intake was significantly higher in ExMD vs. EU (1211±385 mg/d, p=0.04), but not at 0-mo. Vitamin/mineral supplement use was more prevalent in EU (50%)
compared to ExMD (13-38%, range during intervention). At 6-mo, mean Total chol (188 mg/dL) and LDL-chol (103mg/dL) were higher in ExMD compared to EU
(Total chol=154 mg/dL; LDL-chol=84 mg/dL; p<0.05), yet mean cardiac risk was similar (p>0.05; ExMD=2.6; EU=3.0) due to higher HDL-chol levels in ExMD (71
mg/dl) vs. EU (53 mg/dL). In the ExMD group, 25-38% had low iron status during the intervention vs. 20% of EU (0-mo). No other differences in blood micronutrient
status were observed. CONCLUSION: Although, Total chol and LDL-chol were higher in ExMD vs. EU, overall both groups had low cardiac risk. Ca intake was
higher in ExMD group due to the added Ca in the CHO-PRO supplement. No other group differences in diet or blood profiles were observed. In conclusion, ExMD
may negatively increase some blood lipid parameters but this did not translate into increased cardiac risk.
INTRODUCTION
The prevalence of exercise-induced menstrual dysfunction (ExMD) is high (6-79%) in active women (Beals & Hill, 2006; Beals & Manore, 2002). Highest
rates have been reported in aesthetic (30-43%) and endurance (27-32%) sports, particularly those where a lean physique may offer a competitive advantage
(Beals & Hill, 2006; Beals & Manore, 2002; Nichols, Sanborn, & Essery, 2007; Torstveit & Sundgot-Borgen, 2005). Some studies (Friday, Drinkwater,
Bruemmer, Chestnut, & Chait, 1993; Rickenlund, Eriksson, Schenck-Gustafsson, &Hirchberg, 2005) have reported an association between ExMD and
unfavorable lipid profiles, while others (Hinton, Rector, Peppers, Imhoff, & Hillman, 2006; Hoch, et al., 2003) report no differences in lipid profiles between
ExMD and Eumenorheic athletes (EU). Elevated blood lipids may increase cardiac risk: ratio of Total chol (CHOL) to high-density lipoprotein chol (HDLchol). In addition to increased cardiac risk, women with ExMD may be at risk for poor nutritional status due to low energy intake. Dietary intake of
micronutrients, notably B-vitamins (folate, B-6, B-12), iron, zinc, and bone building nutrients (calcium, magnesium, and vitamin D), may be insufficient in this
population (Manore, 1999, 2000, 2002; Woolf & Manore, 2006). Exercise may increase the need for some of these micronutrients (Manore, 2000, 2002).
Lastly, low iron status is also prevalent in active women (~35% of population) (Brownlie, Utermohlen, Hinton, & Haas, 2004; Dubnov & Constantini, 2004;
Hinton, Giordano, Brownlie, & Haas, 2000; Sinclair & Hinton, 2005; Zhu &Haas, 1998). Inadequate dietary iron intake, diets with low iron bioavailability, and
excessive iron loss may contribute to the high incidence of iron deficiency in this population (Manore, 2002).
PURPOSE
The primary purpose of this study was to:
1) Determine nutritional status, lipid profiles, and iron status of active women with ExMD before and after a 6-mo diet intervention that
provided a daily fortified CHO-PRO supplement (360 kcal/d; 20 g/d PRO, 54g/d CHO, 300 mg/d calcium [Ca]).
2) Compare nutritional status, lipid profiles, and iron status between women with ExMD and active Eumenorrheic controls (EU).
Table 1. Comparison of CHO-PRO supplement to low fat chocolate milk.
RESULTS: ExMD vs. EU
RESULTS: ExMD 0-mo vs. 6-mo
•The diet intervention, which provided an additional 360 kcal/d, increased mean body weight and BMI (p=0.03); increases in body weight were primarily as fat
mass (+2.1±2.1% body fat; p=0.03) (see Table 2).
•Mean dietary protein intake [% of energy intake (EI)] increased from 0-mo to 6-mo (p<0.05). See Table 3a for a summary of dietary energy and macronutrient
intake compared to recommendations.
•At 0-mo, mean dietary calcium intake (1320 mg/d) was below the recommended amount for active women with ExMD (1500 mg/d) (Nattiv, et al., 2007);
however, significant improvements were observed with the addition of the CHO-PRO supplement by 6-mo (1725 mg/d; p=0.02). Intakes of folate, B-6, B-12, iron,
magnesium, phosphorus, and zinc were similar before and after the intervention (p>0.05). Mean dietary intake of vitamin D was below the recommendation at
both time points (0-mo and 6-mo), but sun exposure was not measured during the study. Mean magnesium intake was < Recommended Daily Allowance (RDA)
at 0-mo only (Table 3b).
•No changes were observed in blood lipids or cardiac risk from 0-mo to 6-mo (p<0.05). As shown in Figure 1, triglycerides (TG), Total cholesterol (CHOL), highdensity lipoprotein cholesterol (HDL-chol), low-density lipoprotein cholesterol (LDL-chol), and very low density lipoprotein cholesterol (VLDL-chol) were all within
the normal reference ranges.
•Serum folate and B-12 did not change due to the intervention (p>0.05). One participant that was taking a B-12 supplement (100mcg/d) at 0-mo had serum B-12
values above the reference range and was advised to discontinue taking the supplement.
•Iron status remained similar pre- and post-intervention (p>0.05). At 0-mo, one participant was in iron depletion (low ferritin) and another had iron deficiency
anemia (low serum iron, high TIBC, low % saturation, low ferritin). By 6-mo, the individual with iron depletion remained depleted and the second participant
improved her iron status to iron deficiency without anemia. One participant became iron deficient without anemia (Table 4).
•All 8 participants with ExMD resumed menses over the 6-mo diet intervention (mean time to first menses = 2.63 mo, range 1-7 mo).
Table 2. Participant characteristics.
Table 3b. Dietary micronutrient intake.
Mean (SD)
ExMD (n=8)
0-mo
6-mo
22.6 (3.3)
N/A
13.5 (2.0)
N/A
Description
Age (y)
Age at menarche (y)
Weight (kg)
BMI (kg/m2)
FFM (kg)
Body Fat (%)
Training Volume (h/wk)
VO2max (mL/kg/min)
EB (kcal/d)
EA (kcal/kg FFM/d)
•Women with ExMD and active EU controls were similar in age, age at menarche, body composition, training volume, and VO2max (p>0.05). Although EB
(kcal/d) was higher for EU than ExMD at 0-mo, differences did not reach significance (p=0.06). There were no significant between-group differences for EA
(p>0.05). See Table 2 for participant characteristics.
•Dietary protein intake of EU (1.3 g/kg/d) was adequate, but significantly lower than ExMD at 6-mo (1.8g/kg/d; p=0.04). The percent of energy from fat was
higher in EU (34%) than ExMD at both time points (0-mo: 29%; 6-mo: 30%); although, only statistically significant at 0-mo (p=0.045). Mean intakes of
carbohydrate were similar between ExMD and EU (p>0.05); however, EU (4.6g/kg/d) were below the recommendation of 5 g/kg/d for endurance female
athletes (see Table 3a).
•Dietary intake of calcium was lower in EU than ExMD participants at 6-mo (p=0.04) (see Table 3b). Intakes of folate, B-6, and B-12 were similar between
groups and met recommendations; however, mean vitamin D intake was inadequate (<600 IU/d) for both groups. Mineral intakes were adequate and
comparable between groups (p>0.05).
•CHOL and HDL-chol were lower in EU than ExMD at both time points (p<0.05). LDL-chol was similar between EU and ExMD at 0-mo (p=0.15), but was
significantly lower than ExMD at 6-mo (p=0.04). There were no differences in cardiac risk between the groups (p>0.05). See Figure 1 for more details.
•Blood levels of folate, vitamin B-12, and vitamin D were similar in EU vs. ExMD (0-mo and 6-mo, p>0.05).
•There were no significant between-group differences in blood iron status parameters (p>0.05) (see Figure 2). Two EU participant were classified as iron
depleted (low ferritin), compared to 2 ExMD participants with poor iron status at 0-mo (n=1 iron depletion, n=1 iron deficiency anemia) and 3 ExMD
participants at 6-mo (n=1 iron depletion, n=2 iron deficiency without anemia).
Description
EU (n=10)
0-mo*
24.1 (4.6)
12.7 (1.3)
62.4 (7.8)
64.0 (8.0)a
66.8 (9.3)
22.3 (2.5)
48.5 (4.6)
22.9 (2.5)a
48.4 (4.8)
23.2 (2.8)
51.0 (5.0)
22.0 (4.7)
7.4 (3.2)
49.0 (5.8)
-510 (361)
37 (10)
24.1 (3.9)a
7.1 (3.4)
49.3 (6.0)
-44 (707)
45(15)
23.2 (4.4)
7.4 (3.6)
50.6 (5.2)
-171 (459)
38 (10)
Mean (SD)
Recommended
ExMD (n=8)
EU (n=9)#
Intake
0-mo
6-mo
0-mo*
Vitamin B-6 (mg/d)
Folate (mcg/d)
Vitamin B-12 (mcg/d)
Iron (mg/d)
Vitamin D (IU/d)
Calcium (mg/d)
Magnesium (mg/d)
Phosphorus (mg/d)
#
#
See footnotes on right.
Table 3a. Dietary energy and macronutrient intake.
Zinc (mg/d)
3.6 (2.8)
532 (468)
14 (25)
29 (15)
379 (321)
1320 (571)
288 (115)
912 (383)
13 (8)
2.6 (1.0)
426 (236)
7 (4)
22 (5)
383 (316)
1725 (555)a
330 (69)
1034 (386)
16 (5)
3.4 (2.3)
449 (207)
8 (5)
24 (9)
385 (314)
1211 (385)c
365 (194)
1098 (471)
14 (6)
1.3
400
2.4
18
600
1000(+)
310
700
8
‡
Dietary Reference Intakes for Calcium, Phosphorous, Magnesium, Vitamin D, and Fluoride (1997);
Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic
Acid, Biotin, and Choline (1998); and Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic,
Boron,Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and
Zinc (2001); and Dietary Reference Intakes for Calcium and Vitamin D (2011). These reports may
be accessed via www.nap.edu.
Energy/Nutrients
Energy (kcal)
Carbohydrate (g)
Protein (g)
Fat (g)
Calcium (% DV)
Vitamin D (% DV)
Iron (% DV)
Folic Acid (% DV)
Vitamin B6 (% DV)
Vitamin B12 (% DV)
360
54
20
8
30
25
10
20
20
216
36
11
3
39
69
5
4
11
49
% DV =% of Daily Values
METHODS
Endurance-trained women (n=18) were recruited from the Willamette Valley and divided into two groups based on menstrual status. Participation criteria included
being 18-35y, exercising regularly (minimum 7 h/wk), no use of oral contraceptives or hormone therapy for the last 6 months, and a score <14 on the Eating
Disorder Inventory (EDI-2) assessment (Garner & Olmstead, 1984). Active women with ExMD participated in a 6-mo diet intervention, which provided a daily
CHO-PRO supplement (360 kcal/d; 20 g/d PRO, 54 g/d CHO, 300 mg/d Ca). See Table 1 for a comparison of the CHO-PRO supplement to low fat chocolate
milk. Self-reported menstrual dysfunction was confirmed by measuring reproductive hormones in the blood (estradiol, progesterone, luteinizing hormone, follicle
stimulating hormone, prolactin) and determination of ovulation status (ClearBlue ® Easy Fertility Monitor). ExMD participants (n=8) were assessed at baseline (0mo) and post-intervention (6-mo) for energy intake and expenditure, body composition, VO2max, and blood parameters (iron status, lipid profile, folate, vitamin B12, vitamin D); active EU controls (n=10) completed the same measurements at 0-mo only. Cardiac risk was reported as the ratio of Total cholesterol (CHOL)/
high-density lipoprotein cholesterol (HDL-chol). Body composition was determined by dual-energy x-ray absorptiometry (DXA) (Hologic QDR-4500 Elite A
Waltham, MA) and a continuous graded exercise test was used to assess VO2max using indirect calorimetry (TrueOne 2400; ParvoMedics Metabolic Cart,
Sandy, UT). Dietary energy (EI) and nutrient intake were quantified using 7-d weighed food records; we screened for under-reporters using the Goldberg cut-off
(1.35 x Basal Metabolic Rate [BMR]) (Goldberg, et al. 1991). One EU participant was dropped from the dietary intake data due to under-reporting. Physical
activity logs were kept on the same 7-d and allowed for estimations of total and exercise energy expenditure (TEE and EEE, respectively). All records were
analyzed using a computerized diet and activity analysis program (Food Processor SQL version 9.91, 2006; ESHA Research).
Energy balance (EB; EB= EI-TEE) and energy availability (EA; EA=EI-EEE) were calculated from the analyzed records, adjusting for resting metabolic rate
(RMR) (measured 2x using indirect calorimetry) and running energy expenditure (REE) (typical training speeds; indirect calorimetry). For the ExMD group, onesided paired t-tests were performed to determine changes over time (0-mo vs 6-mo) for parameters we hypothesized would improve (EI, EB, EA, body weight,
and micronutrients provided by the supplement); all other ExMD comparisons over time were performed using 2-sided paired t-tests. Between-group
comparisons (ExMD 0-mo vs. EU, ExMD 6-mo vs. EU) were made using two-sided unpaired t-tests. Data were summarized as means and standard deviations.
Statistical significance was set at p<0.05.
Description
ExMD (n=8)
EU (n=9)# Recommended Intake
0-mo
6-mo
0-mo*
a
Energy intake (kcal/d) 2312 (325) 2694 (541) 2430 (524)
variable
Carbohydrate (g/kg/d) 5.0 (1.2)
5.4 (1.0)
4.6 (1.0)
≥5 g/kg/d
53 (7)
51 (6)
Protein (g/kg/d)
1.4 (0.2)
1.8 (0.5)
Protein (%)
Fat (g/kg/d)
Fat (%)
Saturated fat (%)
15 (4)
1.2 (0.3)
29 (3)
8.3 (2.5)
193 (130)
17 (2)a
1.5 (0.6)
30 (5)
8.8 (2.0)
339 (240)a
Carbohydrate (%)
Cholesterol (mg/d)
50 (5)
55-65%
1.3 (0.3)c endurance:1.2-1.4 g/kg/d
strength:1.6-1.7 g/kg/d
15 (3)
10-35%
1.4 (0.4)
N/A
34 (5)b
20-35%
b
10.8 (2.2)
<10%
259 (132)
<300 mg/d
‡ADA-ACSM
Position Stand on Nutrition and Athletic Performance (2009);
Manore (2002); Dietary Guidelines for Americans (2010).
See further footnotes on right..
Figure 1. Lipid profile
*ExMD (0-mo) vs. EU; p<0.05.†ExMD (6-mo) vs. EU; p<0.05. ExMD, exercise-induced menstrual
dysfunction (n=8); EU, eumenorrheic active controls (n=10). TG, triglycerides; CHOL, Total cholesterol;
cardiac risk= CHOL/HDL-chol.
See further footnotes below.
Mean (SD)
Low Fat Chocolate
CHO-PRO Supplement (325 mL)
Milk (325 mL)
+
Footnotes for Tables 2-3.
ExMD, exercise-induced menstrual dysfunction; EU, eumenorrheic endurance-trained
control. BMI, body mass index; FFM, fat free mass (DXA); VO2max, maximal aerobic
capacity (indirect calorimetry). %, percent of total energy intake from 7-d diet records.
EB, energy balance (EB= energy intake [EI]- total energy expenditure [TEE])
EI, total dietary energy intake from analysis of 7-d weighed diet records
TEE, total energy expenditure from 7-d activity logs adjusted for measured RMR and
running energy expenditure.
RMR, resting metabolic rate (measured 2x at each time point; indirect calorimetry).
EA, energy availability (EA= energy intake [EI] – exercise energy expenditure [EEE]).
Training volume, all minutes of exercise (planned + unplanned) >4.0 METs from 7-d activity logs.
EEE, all kcals expended during exercise (planned + unplanned) >4.0 METs from 7-d activity logs.
*EU were measured at baseline (0-mo) only and compared to ExMD at 0-mo and 6-mo.
#one EU participant was left out due to under-reporting (Goldberg cut-off: 1.35 x basal metabolic rate).
(+)The Female Athlete Triad Position Stand (Nattiv, et al., 2007) recommends that active
women with amenorrhea consume1500mg/d of Calcium.
aSignificant difference between ExMD at 0-mo vs. 6-mo (p<0.05).
bSignificant difference between EU vs. ExMD at 0-mo (p<0.05).
cSignificant difference between EU vs. ExMD at 6-mo (p<0.05).
Table 4. Comparison of iron status.
Stage
Fe, iron; TIBC, total iron-binding capacity; saturation %, transferrin carrier; ferritin,
iron storage. ExMD, exercise-induced menstrual dysfunction (n=8); EU, active
eumenorrheic control (n=10).
AND
CONCLUSION
Although CHOL and LDL-chol were significantly higher in ExMD participants compared to EU controls (Figure 1), both groups had a low cardiac
risk. Overall, cardiac risk ranged from 2.5 -3.0, well below the normal range of 3.7-5.6. In addition, mean blood lipids were within recommended
ranges. Calcium was the only dietary micronutrient that significantly improved with the 6-mo intervention (ExMD); the increase in calcium was
largely due to the Ca (300 mg/d) in the CHO-PRO supplement. Other bone-related micronutrients (Vitamin D, magnesium, phosphorus) were not
significantly different over the intervention and between groups. Poor iron status was observed in a small percentage of our participants, but
overall status was lower than reported for active women. Eighty-two percent of our participants consumed at or above the RDA for iron (18 mg/d).
All macronutrients intakes were within recommended ranges. In conclusion, ExMD may negatively affect some blood lipid parameters compared
to EU; however, this does not translate into an increased cardiac risk.
ACKNOWLEDGEMENTS
The authors would like to acknowledge the Gatorade Sports Science Institute for funding this study.
We are also grateful for support from the OSU College of Health and Human Sciences and USDA Training Grant.
ExMD
(6-mo)
EU
(0-mo)
Stage I: Iron depletion
1
1
2
Stage II: Iron deficiency without
anemia
0
2
0
Stage III: Iron deficiency with
anemia
1
0
0
ExMD, exercise-induced menstrual dysfunction (n=8); EU, Eumenorrheic active control
(n=10); Iron depletion= low ferritin; Iron deficiency without anemia= low serum iron, low
% saturation; Iron deficiency anemia= low serum iron, high TIBC, low % saturation, low
ferritin.
Figure 2. Iron deficiency panel.
DISCUSSION
ExMD
(0-mo)
REFERENCES
Beals, K. A., & Hill, A. K. (2006). The prevalence of disordered eating, menstrual dysfunction, and low bone mineral density among US collegiate athletes. International Journal of Sport
Nutrition & Exercise Metabolism, 16(1), 1-23.
Beals, K. A., & Manore, M. M. (2002). Disorders of the female athlete triad among collegiate athletes. International Journal of Sport Nutrition and Exercise Metabolism, 12(3), 281-293.
Brownlie, T. t., Utermohlen, V., Hinton, P. S., & Haas, J. D. (2004). Tissue iron deficiency without anemia impairs adaptation in endurance capacity after aerobic training in previously
untrained women. American Journal of Clinical Nutrition, 79(3), 437-443.
Dubnov, G., & Constantini, N. W. (2004). Prevalence of iron depletion and anemia in top-level basketball players. International Journal of Sport Nutrition and Exercise Metabolism, 14(1), 30-37.
Friday, K. E., Drinkwater, B. L., Bruemmer, B., Chesnut, C., 3rd, & Chait, A. (1993). Elevated plasma low-density lipoprotein and high-density lipoprotein cholesterol levels in amenorrheic
athletes: effects of endogenous hormone status and nutrient intake. Journal of Clinical Endocrinology & Metabolism, 77(6), 1605-1609.
Garner, D. M., & Olmstead, M. P. (1984). Eating Disorder Inventory (EDI) Manual. In I. Psychological Assessment Resources (Ed.), (pp. 1-24).
Goldberg, G. R., Black, A. E., Jebb, S. A., Cole, T. J., Murgatroyd, P. R., Coward, W. A., et al. (1991). Critical evaluation of energy intake data using fundamental principles of energy
physiology: 1. Derivation of cut-off limits to identify under-recording. European Journal of Clinical Nutrition, 45(12), 569-581.
Hinton, P., Rector, R., Peppers, J., Imhoff, R., & Hillman, L. (2006). Serum markers of inflammation and endothelial function are elevated by hormonal contraceptive use but not by exerciseassociated menstrual disorders in physically active young women. Journal of Sport Science and Medicine, 5, 235-242.
Hinton, P. S., Giordano, C., Brownlie, T., & Haas, J. D. (2000). Iron supplementation improves endurance after training in iron-depleted, nonanemic women. Journal of Applied Physiology,
88(3), 1103-1111.
Hoch, A., Dempsey, R., Carrera, G., Wilson, C., Chen, E., & Barnabei, V. (2003). Is there an association between athletic amenorrhea and endothelial cell dysfunction? Medicine &
Science in Sports & Exercise, 3, 377-383.
Manore, M. M. (1999). Nutritional needs of the female athlete. Clinical Sports Medicine, 18(3), 549-563.
Manore, M. M. (2000). Effect of physical activity on thiamine, riboflavin, and vitamin B-6 requirements. The American Journal of Clinical Nutrition, 72(2 Suppl), 598S-606S.
Manore, M. M. (2002). Dietary recommendations and athletic menstrual dysfunction. Sports Medicine, 32(14), 887-901.
Nattiv, A., Loucks, A. B., Manore, M. M., Sanborn, C. F., Sundgot-Borgen, J., & Warren, M. P. (2007). American College of Sports Medicine position stand. The female athlete triad.
Medicine & Science in Sports & Exercise, 39(10), 1867-1882.
Nichols, D. L., Sanborn, C. F., & Essery, E. V. (2007). Bone density and young athletic women. An update. Sports Medicine, 37(11), 1001-1014.
Rickenlund, A., Eriksson, M. J., Schenck-Gustafsson, K., & Hirschberg, A. L. (2005). Amenorrhea in female athletes is associated with endothelial dysfunction and unfavorable lipid
profile. Journal of Clinical Endocrinology & Metabolism, 90(3), 1354-1359.
Sinclair, L. M., & Hinton, P. S. (2005). Prevalence of iron deficiency with and without anemia in recreationally active men and women. Journal of the American Dietetic Association,
105(6), 975-978.
Torstveit, M. K., & Sundgot-Borgen, J. (2005). Participation in leanness sports but not training volume is associated with menstrual dysfunction: a national survey of 1276 elite athletes
and controls. British Journal of Sports Medicine, 39(3), 141-147.
Woolf, K., & Manore, M. M. (2006). B-vitamins and exercise: does exercise alter requirements? International Journal of Sport Nutrition and Exercise Metabolism, 16(5), 453-484.
Zhu, Y. I., & Haas, J. D. (1998). Altered metabolic response of iron-depleted nonanemic women during a 15-km time trial. Journal of Applied Physiology, 84(5), 1768-1775.