Next Article in Journal
Nutritional Recommendations for Cardiovascular Disease Prevention
Next Article in Special Issue
Vitamin B12 Deficiency in Relation to Functional Disabilities
Previous Article in Journal
Fatty Acid and Phytosterol Content of Commercial Saw Palmetto Supplements
Previous Article in Special Issue
Vitamin B12 Metabolism during Pregnancy and in Embryonic Mouse Models

Nutrients 2013, 5(9), 3634-3645; doi:10.3390/nu5093634

Biological Variability and Impact of Oral Contraceptives on Vitamins B6, B12 and Folate Status in Women of Reproductive Age
Jennifer O. McArthur 1, HoMan Tang 1, Peter Petocz 2 and Samir Samman 1,*
Discipline of Nutrition and Metabolism, School of Molecular Bioscience, The University of Sydney, Sydney, NSW 2006, Australia; E-Mails: (J.O.M.); (H.T.)
Department of Statistics, Macquarie University, Ryde, NSW 2112, Australia; E-Mail:
Author to whom correspondence should be addressed; E-Mail:; Tel.: +61-2-9351-2476; Fax: +61-2-9351-4726.
Received: 25 June 2013; in revised form: 9 August 2013 / Accepted: 12 August 2013 /
Published: 16 September 2013


: Vitamins B6, B12 and folate play crucial metabolic roles especially during the reproductive years for women. There is limited reporting of within-subject variability of these vitamins. This study aimed to determine the within and between subject variability in serum vitamins B6, B12, folate and erythrocyte folate concentrations in young women; identify factors that contribute to variability; and determine dietary intakes and sources of these vitamins. Data were obtained from the control group of a trial aimed at investigating the effect of iron on the nutritional status of young women (age 25.2 ± 4.2 year; BMI 21.9 ± 2.2 kg/m2). The coefficients of variability within-subject (CVI) and between-subject (CVG) for serum vitamins B6, B12 and folate, and erythrocyte folate were calculated. Food frequency questionnaires provided dietary data. CVI and CVG were in the range 16.1%–25.7% and 31.7%–62.2%, respectively. Oral contraceptive pill (OCP) use was associated (P = 0.042) with lower serum vitamin B12 concentrations. Initial values were 172 ± 16 pmol/L and 318 ± 51 pmol/L for OCP and non-OCP users, respectively; with differences maintained at four time points over 12 weeks. BMI, age, physical activity, alcohol intake and haematological variables did not affect serum or erythrocyte vitamin concentrations. Vitamin B12 intakes were derived from traditional and unexpected sources including commercial energy drinks. Young women using OCP had significantly lower serum vitamin B12 concentrations. This should be considered in clinical decision making and requires further investigation.
vitamin B6; vitamin B12; folate; variability; oral contraceptives; women

1. Introduction

Vitamins B6, B12 and folate play crucial inter-related roles in DNA synthesis throughout the lifecycle especially during childhood, adolescence and the reproductive years for women [1,2,3]. Studies have shown an inverse correlation between the intake of folic acid during pregnancy and infants born with neural tube defects [4,5]; a higher incidence of neurological disorders when vitamin B12 deficiency persists [6]; and anaemia, depression and confusion present with vitamin B6 deficiency [3]. The concentrations of the vitamins in the circulation reflect an individual’s storage of these vitamins and their dietary intakes. For vitamin B12, intakes are determined by the predominant animal sources available to a population and the preferences of individuals [7,8]. While investigating the effects of meat or iron supplementation on biomarkers of nutritional status, we identified a failure of this population to meet dietary recommendations for vitamins B6, B12 and folate [9]. Accordingly concern over meeting intake benchmarks is justified particularly for vitamin B12 as vegetarianism is increasing in popularity among teenagers [10] with the prevalence of vegetarian tendencies amongst female adolescents being as high as 37%. As vitamin B6 is not limited to animal foods, there is some lessening of the impact of this trend. The vitamin B6 non-animal sources include processed cereals, fruits and vegetables [11]. Folate fortification of baked goods is supplementing the intakes of high folate foods such as green leafy vegetables. Whether the predominant dietary sources of vitamins B6, B12 and folate for young women are reflective of the foods analysed as having a high content of these vitamins is uncertain.

The analysis of control group data from a recently completed trial [9] will enable us to explore the within- and between-subject variability of vitamins B6, B12 and folate concentrations in women of reproductive age. The availability of data on diet and aspects of lifestyle will help to identify potential determinants of variability in vitamin concentrations.

2. Experimental Section

2.1. Participants

Healthy active women (18–35 years) taking no medications, other than OCP, or vitamin and mineral supplements, were enrolled in a randomized controlled trial that was investigating the impact of diet or supplements on the iron status of women of reproductive age [9]. All procedures involving human subjects were approved by The University of Sydney Human Ethics Review Committee. Written, informed consent was obtained from all volunteers prior to their participation in the study.

2.2. Blood Collection and Analysis

Blood samples were collected from all volunteers initially (week 0) then at four weekly intervals (weeks 4, 8 and 12). All samples were taken between 0730 and 0930 h from an antecubital vein. Veni-puncturists were from a single collection centre with pre-set protocols for collection and sample handling. The potential pre-analytical variation was minimised, i.e., the women were in the fasted state (10 to 12 h), and reported not to have engaged in vigorous activity in the preceding 12 h, and did not consume alcohol for 24 h prior to the blood collection. Subjects were in the supine position during the blood collection. On each occasion, blood samples were collected into vacutainer tubes (Becton Dickinson, Franklin Lakes, NJ, USA): untreated tubes for the analysis of serum folate and vitamin B12 concentrations, and EDTA-coated tubes for the analysis of erythrocyte folate and serum vitamin B6 concentrations. Samples that were destined for the analysis of vitamin B6 were collected in tubes that were shielded from light. All blood samples were kept on ice for up to 2 h, and centrifuged at 1500 g for 10 min at 5 °C.

The concentrations of vitamin B12, serum- and erythrocyte-folate were determined using an automated system (UniCel DxI Immunoassay System, Beckman Coulter Inc., CA, USA). Plasma vitamin B6 (pyridoxal-5-phosphate) concentrations were determined using an HPLC method (Chromsystems Instruments and Chemicals GmbH, Munich, Germany). The inter-assay CV for vitamins B6, B12 and folate were 6%, 11.2% and 12.8%, respectively. The reference intervals were: serum vitamin B6, 35–110 nmol/L; serum vitamin B12, 150–750 pmol/L; serum folate, 8–25 nmol/L; and erythrocyte folate, >550 nmol/L [12].

2.3. Dietary and Exercise Data

A validated FFQ [13] was used to collect dietary data on 2 occasions. The first occasion (week 0) was at study commencement, reporting on dietary intake for the previous 12 weeks, and the second was at study conclusion (week 12). Intakes of vitamins B6, B12 and folate were examined further and the highest five contributing foods for each vitamin were determined. To achieve this for individual participants, the contributing foods for each of the three vitamins were identified and ranked. The participants reported their activity levels using the validated International Physical Activity Questionnaire Short Form [14]. The frequency and duration of activity enabled the calculation of the metabolic equivalent of task (MET).

2.4. Statistical Analyses

The variability in vitamin concentrations is influenced by the intrinsic biological variation (within-subject variance) expressed as the coefficient of variation (CVI); and the variance of the means among subjects (between-subject variance) expressed as the coefficient of variation (CVG). Biological variations were reported as CVI, CVG, index of individuality (CVI/CVG) and reference change value (RCV). The RCV is a measure of difference used to monitor serial data and is known as the critical difference [15]. Coefficients of variation (CV) were calculated using Minitab statistical software [16]. All other statistical calculations were carried out using SPSS [17]. Repeated-measures analyses were carried out for the longitudinal vitamin data, including possible explanatory variables as factors (e.g., OCP use) or covariates (e.g., BMI). Regression analysis was used to identify possible relationships between serum vitamin B6, vitamin B12 and folate and erythrocyte folate concentrations with the equivalent dietary vitamin intake, alcohol intake, BMI, age, MET, OCP use and haemoglobin concentrations. A probability value of P < 0.05 was set for statistical significance.

3. Results

Complete data sets were available for 22 participants, age 25.2 ± 4.2 (mean ± SD) years and BMI 21.9 ± 2.2 kg/m2. Alcohol consumption ranged from 0 to 75.7 g/day with median intakes of 5.5 and 3.0 g/day at weeks 0 and 12, respectively. Physical activity ranged from 4.0 to 240 MET h/week with median activity of 33.6 and 33.3 MET h/week at weeks 0 and 12, respectively. Mean corpuscular volume (MCV) and haemoglobin (Hb) levels were collected at each time period. There were no significant differences between OCP users or non-OCP users for MCV (89.1 ± 2.7 fL and 90.4 ± 3.5 fL) or Hb (127 ± 6 and 132 ± 9 mmHg).

3.1. Blood Analyses

A number of participants had vitamin B6 concentrations higher than the reference interval and the mean vitamin B6 concentration at week 0 was significantly higher than week 12 (P = 0.005) (Figure 1a). No other differences between mean values for any of the vitamins were noted.

Nutrients 05 03634 g001 1024
Figure 1. Mean serum vitamins B6 (a), vitamin B12 (b), folate (c), and erythrocyte folate (d) concentrations (n = 22)..Reference intervals (shaded) [12], mean (♦) and outliers (□, ○)

Click here to enlarge figure

Figure 1. Mean serum vitamins B6 (a), vitamin B12 (b), folate (c), and erythrocyte folate (d) concentrations (n = 22)..Reference intervals (shaded) [12], mean (♦) and outliers (□, ○)
Nutrients 05 03634 g001 1024

The interquartile range for vitamin B6 demonstrated a spread at week 0 which decreased at subsequent sampling points (Figure 1a). The median vitamin B12 concentrations were in the lower 8%–22% of the reference interval, increasing in week 4. Frequency distributions for vitamin B12 were positively skewed and had two of the collections multi-modal (weeks 8 and 12) (Figure 1b). Serum- and erythrocyte-folate concentrations demonstrated similar characteristics. Serum folate demonstrated the greatest symmetry despite being multi-modal for two time-points (Figure 1c). Of the four analytes, erythrocyte folate had the largest interquartile range at week 0 (Figure 1d).

3.1.1. Variability

The between-subject biological variations for the analysed vitamins were greater than the within-subject variations (Table 1) and the indices of individuality (CVI/CVG) were in the range of 0.26 (vitamin B12) to 0.81 (vitamin B6). The RCV at 49.8%–72.8% are influenced by the large within-subject variations of these vitamins. Maximum variances for each of the vitamins were 174% (vitamin B6; participant viii), 114% (vitamin B12; participant xvi), 153% (serum folate; participant x); and 147% (erythrocyte folate; participant i).

Table Table 1. CVI, CVG and RCV for the serum vitamins B6, B12 and folate; and erythrocyte folate (n = 22).

Click here to display table

Table 1. CVI, CVG and RCV for the serum vitamins B6, B12 and folate; and erythrocyte folate (n = 22).
Vitamin B6 (nmol/L)31.725.70.8172.8
Vitamin B12 (pmol/L)
Folate (nmol/L)36.820.40.5560.7
Folate (nmol/L)

3.1.2. Relationships

Regression analysis showed that vitamin B12 concentrations over four time-points was determined by BMI (P = 0.031) and when tested by repeated-measures ANOVA, serum vitamin B12 values showed a significant difference between OCP users and non-users (P = 0.042) (Figure 2). Serum vitamin B6, showed a linear decline over time (P = 0.017) irrespective of OCP use. Serum vitamin B6, folate and erythrocyte folate concentrations did not show any relationships with their respective dietary intake, BMI, age, alcohol intake, MET or haemoglobin concentrations (data not shown).

Nutrients 05 03634 g002 1024
Figure 2. Serum vitamin B6 (a), vitamin B12 (b) and folate (c), and erythrocyte folate (d) concentrations (mean ± SE) at four time-points for OCP a users Nutrients 05 03634 i001 (n = 9) and non-OCP users ■ (n = 13). Missing data for one OCP user at four weeks.

Click here to enlarge figure

Figure 2. Serum vitamin B6 (a), vitamin B12 (b) and folate (c), and erythrocyte folate (d) concentrations (mean ± SE) at four time-points for OCP a users Nutrients 05 03634 i001 (n = 9) and non-OCP users ■ (n = 13). Missing data for one OCP user at four weeks.
Nutrients 05 03634 g002 1024

a Combined OCP containing ethyl estradiol (30−35 mcg) and drospirenone (3 mg); * P = 0.042.

3.2. Dietary Analyses

In Table 2 individual mean vitamin intakes are presented for both FFQ (weeks 0, 12). Paired t-tests showed no significant differences between week 0 and week 12 for either energy or vitamin intakes. However the absolute intake of vitamin B6 was significantly higher at week 0 compared to week 12 (+0.52 ± 0.61 mg/day, P = 0.001) (Figure 3). The mean intakes for all vitamins were above the recommended dietary intakes (RDI) however intakes less than the RDI were reported for vitamin B6 (13.6%), vitamin B12 (36.3%) and folate (36.3%); and less than the estimated average requirements (EAR) for vitamin B12 (31.8%) and folate (22.7%). There were no significant differences found between OCP and non-OCP users for mean alcohol intakes (data not shown).

For vitamin B12 intakes no significant difference was found between OCP (2.79 ± 1.40 μg) and non-OCP (3.43 ± 1.66 μg) users during the study (source FFQ week 12).

The foods that contributed the intakes of vitamins B6, B12 and folate are ranked in Table 3. These included energy drinks as a major provider of vitamin B12, and reduced fat milk for vitamin B6 and folate.

Table Table 2. Participant age, BMI and intakes energy (MJ/day) and vitamins B6 (mg/MJ), vitamin B12 (μg/MJ) and folate equivalents (μg/MJ) at week 0 and week 12.

Click here to display table

Table 2. Participant age, BMI and intakes energy (MJ/day) and vitamins B6 (mg/MJ), vitamin B12 (μg/MJ) and folate equivalents (μg/MJ) at week 0 and week 12.
week 0week 12
Daily Vitamin Intake/MJDaily Vitamin Intake/MJ
ID n = 22Age yearBMI kg/m2Energy MJ/dayVit B6 mgVit B12 μgFolate Eq μgEnergy MJ/dayVit B6 mgVit B12 μgFolate Eq μg
iii26.319.515.80.100.14 a34.5711.70.10 b0.1438.47
iv c25.925.
v c27.
vi c32.020.610.70.150.5337.596.80.15 b0.3256.56
vii c20.023.310.70.170.22 a26.7512.50.170.2125.63
ix30. a0.4928.78
x c20.
xi c21.821.722.50.160.2561.6324.40.160.2158.86
xii c22.
xv26.822.811.30.10 a0.19 a38.6214.50.100.2636.19
xvi c33.721. b43.877.00.190.1990.79
xviii c34.122.610.70.170.21 a30.569.10.150.1539.75
Min value18.518.
Max value34.125.525.50.610.55223.1926.20.430.61287.99

a = intake < RDI; b = intake < EAR; c = OCP user.

Nutrients 05 03634 g003 1024
Figure 3. Dietary intakes of vitamins B6 mg/day (a), B12 μg/day (b) and folate μg/day (c) at weeks 0 and 12. Data shown as boxplots and includes mean (♦) and outliers (○, ∆).

Click here to enlarge figure

Figure 3. Dietary intakes of vitamins B6 mg/day (a), B12 μg/day (b) and folate μg/day (c) at weeks 0 and 12. Data shown as boxplots and includes mean (♦) and outliers (○, ∆).
Nutrients 05 03634 g003 1024
Table Table 3. Reported contributors of dietary vitamin B6, vitamin B12 and folate.

Click here to display table

Table 3. Reported contributors of dietary vitamin B6, vitamin B12 and folate.
Rank aVitamin B6Vitamin B12Folate
food% bfood% bfood% b
1beef and veal29beef and veal19reduced fat milk16
2asian greens11lamb17tomato (raw)14
3carrot11egg13mixed green salad13
4reduced fat milk9fish13strawberries13
5banana9energy drinks12vegetarian lasagne13

a where Rank 1 is the highest contributor to total intake; b percentage contribution to daily vitamin intake.

4. Discussion

Analyses were undertaken to gain a better understanding of both the within- and between-subject biological variation of vitamins B6, B12 and folate in women of reproductive age. The strengths of the current study over earlier studies are: firstly, the collection of longitudinal data from a homogeneous group of free-living women; and secondly, the use of standardised protocols that aimed to minimise pre-analytical and analytical variability. When compared to previous studies the current findings show consistently higher within-subject variability data for vitamins B6 and B12, and modestly lower variability for serum folate [18,19,20].

Longitudinal data enable the evaluation of the extent of the individual variances. In the present study there were multiple excursions from the reference intervals for all analytes with the exception of erythrocyte folate concentrations, which remained within the reference interval. However for serum vitamins B6, B12 and folate, participants’ values were outside the reference intervals for 1−3 of the four blood collections. Harris [21] argued for a cumulative reporting system for analytical data to accommodate within-subject variations. The present study suggests that more than three samples are required to account for within-subject variability for serum vitamins B6, B12 and folate in women of reproductive age. Additionally, the indices of individuality (CVI/CVG) reported for serum vitamin B12 and serum- and erythrocyte-folate were <0.6 suggesting that the reference intervals are insensitive when applied to individual variations [19].

The mean vitamin concentrations were not significantly different across the four time points except for vitamin B6 where the initial mean value (week 0) was significantly higher than the sample obtained at week 12. This was supported also by repeated-measures analysis of vitamin B6. It is possible that routine eating behaviours of the participants were disrupted due to the commencement of the university semester and this may explain elevated serum vitamin B6 concentrations at week 0. A change in activity level was considered possibly sufficient to alter vitamin B6 absorption, turnover, metabolism or loss; however, statistical analyses did not identify noteworthy changes in MET [22]. In regression analysis we were unable to identify dietary intake or MET as determinants of variability of vitamin B6.

Oral contraceptives that contain a combination of oestrogen and progestin, that were taken by the participants have metabolic effects unrelated to contraceptive functions, such as effects on carbohydrate and lipid metabolism [23]. Previous studies reported lower concentrations of vitamins B6, B12 or folate in users compared to non-users of OCP [24,25,26]. Green et al. [27] could not support earlier findings for folate and reported no lowering in adolescents; however vitamin B12 levels of OCP users were 33% lower than non-users. In the current study a relationship was identified between OCP use and serum vitamin B12 with significantly lower vitamin B12 concentrations in OCP users at all four time points. Wilson et al. [28] proposes that the lower vitamin B12 is a false indicator of deficiency citing Reidel et al. [29], who assessed modern biomarkers of vitamin B12 status such as methylmalonic acid, holo-transcobalamin and homocysteine concentrations. These data suggested a redistribution of vitamin B12 in OCP users rather than a depletion of vitamin B12. Research by Shojania and Wylie supports lower total transcobalamin I and higher transcobalamin III levels in OCP users [30]. Overall the impact of OCP on vitamin B12 metabolism remains uncertain [28,31]. Data from the present study, using the reference interval for vitamin B12 (150–750 pmol/L) show that 50% of OCP users had concentrations below the reference range but were asymptomatic of deficiency; and secondly, the dietary data show that those who consumed equivalent to the lower quartile vitamin B12 intakes were not OCP users. Serum vitamin B12 data prior to commencement of OCP use were not available preventing further analysis to ascertain whether lower serum vitamin B12 is a consequence of OCP use.

Georgiou et al. [32] reported that college students have higher quality diets compared with their employed non-student peers and graduates. In the current study 50% of the participants were consuming nutrients at levels <RDI for two of the three vitamins, and this may be reflective of the wider female student population. In examining the sources of vitamins B6, B12 and folate in the diet, we identified participants who are consuming foods with modest vitamin contents, in quantities that raise the importance of the food as a key vitamin provider. The established listings of foods that provide quantities of these vitamins are reported in credible sources [3,11] however the dietary data from the present study show that there is limited overlap between these listings and the foods consumed by women in the present study. The substantial intake of reduced-fat gourmet coffee drinks elevates milk to the highest source of folate for the participants and the consumption of caffeinated energy drinks matched fish and egg as important sources of vitamin B12. Public health concerns have been raised regarding the widespread and excessive consumption of caffeinated energy drinks by young adults [33]. However results of the current study, taken together with reports of meat avoidance [34] and dieting practices [35] by young women suggest that reducing the consumption of caffeinated energy drinks may further lower vitamin B12 intakes and increase the percent of women at risk of vitamin B12 deficiency. The sources and dietary intakes of vitamin B12 in women warrant further investigation.

5. Conclusions

This study provides longitudinal data regarding the within-subject vitamin variability for women of reproductive age who were tertiary students. Biochemical data from the current study reinforce earlier findings that clinical decision-making requires serial collections and adherence to standardised protocols. For serum vitamins B6, B12 and folate, at least three collections were required for a meaningful estimation of vitamin status to be reached. Relationships were not identified between the serum and erythrocyte vitamin concentrations and their dietary intakes; BMI, MET and alcohol intakes. However OCP users presented with consistently lower serum vitamin B12 concentrations independent of their dietary intake. The potential for changes in absorption, turnover or storage of vitamin B12 needs clarity to establish the true nutritional vitamin B12 status of OCP users.


The study was supported by a grant-in-aid from the Australian Government Cooperative Research Centre (CRC) for Pork and Australian Pork Ltd. S.S. designed research; J.O.Mc. and S.S. conducted research; J.O.Mc., H.T., P.P. and S.S. analyzed data and wrote the paper. S.S. had primary responsibility for final content. All authors read and approved the final manuscript.

Conflicts of Interest

The authors declare no conflict of interest.


  1. Bartley, K.A.; Underwood, B.A.; Deckelbaum, R.J. A lifecycle micronutrient perspective for women’s health. Am. J. Clin. Nutr. 2005, 81, 1188–1193. [Google Scholar]
  2. Commonwealth Department of Health and Ageing Australia; Ministry of Health New Zealand; National Health and Medical Research Council. Nutrient Reference Values for Australia and New Zealand including Recommended Dietary Intakes. Available online: (accessed on 9 August 2013).
  3. Truswell, A.S. The B Vitamins. In Essentials of Human Nutrition, 4th ed.; Mann, T., Ed.; Oxford University Press: New York, NY, USA, 2012; pp. 217–235. [Google Scholar]
  4. Spina Bifida and Anencephaly before and after Folic Acid Mandate—Unite States, 1995–1996 and 1999–2000. Available online: (accessed on 9 August 2013).
  5. Honein, M.A.; Paulozzi, L.J.; Mathews, T.J.; Erickson, J.D.; Wong, L.C. Impact of folic acid fortification of the US food supply on the occurrence of neural tube defects. J. Am. Med. Assoc. 2001, 285, 2981–2996. [Google Scholar] [CrossRef]
  6. O’Leary, F.; Samman, S. Vitamin B12 in health and disease. Nutrients 2010, 2, 299–316. [Google Scholar] [CrossRef]
  7. Kimura, N.; Fukuwatari, T.; Sasaki, R.; Hayakawa, F.; Shibata, K. Vitamin intake in Japanese women college students. J. Nutr. Sci. Vitaminol. 2010, 49, 149–155. [Google Scholar]
  8. Tucker, K.; Rich, S.; Rosenberg, I.; Acques, P.; Dallal, G.; Wilson, P.W.F.; Selhub, J. Plasma vitamin B12 concentrations relate to intake source in the framingham offspring study. Am. J. Clin. Nutr. 2000, 71, 514–522. [Google Scholar]
  9. McArthur, J.O.; Petocz, P.; Caterson, I.D.; Samman, S. A randomized controlled trial in young women of the effects of consuming pork meat or iron supplements on nutritional status and feeling of well-being. J. Am. Coll. Nutr. 2012, 31, 175–184. [Google Scholar] [CrossRef]
  10. Worsley, A.; Skizypiec, G. Teenage vegetarianism: Prevalence, social and cognitive contexts. Appetite 1998, 30, 151–170. [Google Scholar] [CrossRef]
  11. Combs, G.F., Jr. Vitamin B6. In The Vitamins: Fundamental Aspects in Nutrition and Health, 3rd ed.; Elsevier Academic Press: Burlington, VT, USA, 2008; pp. 313–329. [Google Scholar]
  12. Sydney South West Pathology Service Laboratory Handbook. Available online: (accessed on 9 August 2013).
  13. Fayet, F.; Flood, V.; Petocz, P.; Samman, S. Relative and biomarker-based validity of a food frequency questionnaire that measures the intakes of vitamin B12, folate, iron, and zinc in young womem. Nutr. Res. 2011, 31, 14–20. [Google Scholar] [CrossRef]
  14. Craig, C.L.; Marshall, A.F.; Sjöström, M.; Bauman, A.E.; Booth, M.L.; Ainsworth, B.E.; Pratt, M.; Ekelund, U.; Yngve, A.; Sallis, J.F.; et al. International physical activity questionnaire: 12 Country reliability and validity. Med. Sci. Sports Exerc. 2003, 35, 1381–1395. [Google Scholar] [CrossRef]
  15. Fraser, C.G. Reference change values. Clin. Chem. Lab. Med. 2012, 50, 807–812. [Google Scholar]
  16. Minitab Statistical Software, version 16. Minitab Inc. State College, PA, USA, 2010.
  17. IBM SPSS Statistics for Windows, version 19.0. IBM Corp, Armonk, NY, USA, 2010.
  18. Shibata, K.; Fukuwatari, T.; Watanabe, T.; Nishimuta, M. Intra- and inter-individual variations of blood and urinary water soluble vitamins in Japanese young adults consuming a semi-purified diet for 7 days. J. Nutr. Sci. Vitaminol. 2009, 55, 459–470. [Google Scholar] [CrossRef]
  19. Talwar, D.K.; Azharuddin, M.K.; Williamson, C.; Teoh, Y.P.; McMillan, D.C.; St. J. O’Reilly, D. Biological variation of vitamins in blood of healthy individuals. Clin. Chem. 2005, 51, 2145–2150. [Google Scholar] [CrossRef]
  20. McKinley, M.C.; Strain, J.J.; McParlin, J.; Scott, J.M.; McNulty, H. Plasma homocysteine is not subject to seasonal variation. Clin. Chem. 2001, 47, 1430–1436. [Google Scholar]
  21. Harris, E.K. Effects of intra- and inter-individual variation on the appropriate use of normal ranges. Clin. Chem. 1974, 20, 1535–1542. [Google Scholar]
  22. Manore, M.M. Vitamin B6 and exercise. Int. J. Sport Nutr. 1994, 4, 89–103. [Google Scholar]
  23. Kiley, J.; Hammond, C. Combined oral contraceptives: A comprehensive review. Clin. Obstet. Gynecol. 2007, 50, 868–877. [Google Scholar] [CrossRef]
  24. Prasad, A.S.; Oberleas, D.; Moghissi, K.S.; Stryker, J.C.; Lei, K.Y. Effect of oral contraceptive agents on nutrients: II. Vitamins. Am. J. Clin. Nutr. 1975, 28, 385–391. [Google Scholar]
  25. Lussana, F.; Zighetti, M.L.; Bucciarelli, P.; Cugno, M.; Cattaneo, M. Blood levels of homocysteine, folate, vitamin B6 and B12 in women using oral contraceptives compared to non-users. Thromb. Res. 2003, 112, 37–41. [Google Scholar] [CrossRef]
  26. Sütterlin, M.W.; Bussen, S.S.; Rieger, L.; Dietl, J.; Steck, T. Serum folate and vitamin B12 levels in women using modern oral contraceptives containing 20 mg ethinyl estradiol. Eur. J. Obstet. Gynecol. Reprod. Biol. 2003, 107, 57–61. [Google Scholar] [CrossRef]
  27. Green, T.J.; Houghton, L.A.; Donovan, U.; Gibson, R.S.; O’Connor, D.L. Oral contraceptives did not affect biochemical folate indexes and homocysteine concentrations in adolescent females. J. Am. Diet. Assoc. 1998, 98, 49–55. [Google Scholar] [CrossRef]
  28. Wilson, S.M.C.; Bivins, B.N.; Russell, K.A.; Bailey, L.B. Oral contraceptive use: Impact on folate, vitamin B6, and vitamin B12 status. Nutr. Rev. 2011, 69, 572–583. [Google Scholar] [CrossRef]
  29. Riedel, B.; Bjorke Monsen, A.L.; Ueland, P.M.; Schneede, J. Effects of oral contraceptives and hormone replacement therapy on markers of cobalamin status. Clin. Chem. 2005, 51, 778–781. [Google Scholar] [CrossRef]
  30. Shojania, A.M.; Wylie, B. The effect of oral contraceptives on vitamin B12 metabolism. Am. J. Obstet. Gynecol. 1979, 135, 129–134. [Google Scholar]
  31. Gardyn, J.; Mittelman, M.; Zlotnik, J.; Sela, B.A.; Cohen, A.M. Oral contraceptives can cause falsely low vitamin B12 levels. Acta Haematol. 2000, 104, 22–24. [Google Scholar] [CrossRef]
  32. Georgiou, C.C.; Betts, N.M.; Hoerr, S.I.; Keim, K.; Peters, P.K.; Stewart, B.; Voichick, J. Among young adults, college students and graduates practiced more healthful habits and made more healthful food choices than did non students. J. Am. Diet. Assoc. 1997, 97, 754–759. [Google Scholar] [CrossRef]
  33. Burrows, T.; Pursey, K.; Neve, M.; Stanwell, P. What are the health implications associated with the consumption of energy drinks? A systematic review. Nutr. Rev. 2013, 7, 135–148. [Google Scholar]
  34. Fayet, F.; Flood, V.; Petocz, P; Samman, S. Avoidance of meat and poultry decreases intakes of omega-3 fatty acids, vitamin B12, selenium and zinc in young women. J. Hum. Nutr. Diet. 2013. [Google Scholar] [CrossRef]
  35. Fayet, F.; Petocz, P.; Samman, S. Prevalence and correlates of dieting in college women: A cross sectional study. Int. J. Women’s Health 2012, 4, 405–411. [Google Scholar]
Nutrients EISSN 2072-6643 Published by MDPI AG, Basel, Switzerland RSS E-Mail Table of Contents Alert