Inez Schoenmakers

Symposium on ‘Nutrition and health in children and adolescents’ Session 1: Nutrition in growth and development: Nutrition and bone growth and development

The growth and development of the human skeleton requires an adequate supply of many different nutritional factors. Classical nutrient deficiencies are associated with stunting (e.g. energy, protein, Zn), rickets (e.g. vitamin D) and other bone abnormalities (e.g. Cu, Zn, vitamin C). In recent years there has been interest in the role nutrition may play in bone growth at intakes above those required to prevent classical deficiencies, particularly in relation to optimising peak bone mass and minimising osteoporosis risk. There is evidence to suggest that peak bone mass and later fracture risk are influenced by the pattern of growth in childhood and by nutritional exposures in utero, in infancy and during childhood and adolescence. Of the individual nutrients, particular attention has been paid to Ca, vitamin D, protein and P. There has also been interest in several food groups, particularly dairy products, fruit and vegetables and foods contributing to acid–base balance. However, it is not possible at the present time to define dietary reference values using bone health as a criterion, and the question of what type of diet constitutes the best support for optimal bone growth and development remains open. Prudent recommendations (Department of Health, 1998; World Health Organization/Food and Agriculture Organization, 2003) are the same as those for adults, i.e. to consume a Ca intake close to the reference nutrient intake, optimise vitamin D status through adequate summer sunshine exposure (and diet supplementation where appropriate), be physically active, have a body weight in the healthy range, restrict salt intake and consume plenty of fruit and vegetables.

25(OH)D2 Half-Life Is Shorter Than 25(OH)D3 Half-Life and Is Influenced by DBP Concentration and Genotype

Context: There is uncertainty over the equivalence of vitamins D2 and D3 to maintain plasma 25-hydroxyvitamin D (25(OH)D). Objective: The objective of the study was to compare the plasma half-lives of 25(OH)D2 and 25(OH)D3 in two distinct populations with different dietary calcium intake and 25(OH)D status. Participants: Healthy men (aged 24 and 39 y), resident in The Gambia (n = 18) or the United Kingdom (n = 18) participated in the study. Interventions: The intervention included an oral tracer dose of deuterated-25(OH)D2 and deuterated-25(OH)D3 (both 40 nmol). Blood samples were collected over 33 days. Main Outcome Measures: 25(OH)D2 and 25(OH)D3 plasma half-lives, concentrations of 25(OH)D, and vitamin D binding protein (DBP) and DBP genotypes were measured. Results: 25(OH)D2 half-life [mean (SD)] [13.9 (2.6) d] was shorter than 25(OH)D3 half-life [15.1 (3.1) d; P = .001] for countries combined, and in Gambians [12.8 (2.3) d vs 14.7 (3.5) d; P < .001], but not in the United Kingdom [15.1 (2.4) d vs 15.6 (2.5) d; P = .3]. 25(OH)D concentration was 69 (13) and 29 (11) nmol/L (P < .0001), and the DBP concentration was 259 (33) and 269 (23) mg/L (P = .4) in The Gambia and United Kingdom, respectively. Half-lives were positively associated with plasma DBP concentration for countries combined [25(OH)D2 half-life: regression coefficient (SE) 0.03 (0.01) d per 1 mg/L DBP, P = .03; 25(OH)D3 half-life: 0.04 (0.02) d, P = .02] and in Gambians [25(OH)D2 half-life: 0.04 (0.01) d; P = .02; 25(OH)D3 half-life: 0.06 (0.02) d, P = .01] but not in UK participants. The DBP concentration × country interactions were not significant. DBP Gc1f/1f homozygotes had shorter 25(OH)D2 half-lives compared with other combined genotypes (P = .007) after correction for country. Conclusions: 25(OH)D2 half-life was shorter than 25(OH)D3 half-life, and half-lives were affected by DBP concentration and genotype. The stable isotope 25(OH)D half-life measurements provide a novel tool to investigate vitamin D metabolism and vitamin D expenditure and aid in the assessment of vitamin D requirements.

Maternal plasma 25-hydroxyvitamin D concentration and birthweight, growth and bone mineral accretion of Gambian infants.

Maternal vitamin D deficiency during pregnancy is a recognized risk factor for rickets and osteomalacia in infancy (1). The circulating plasma concentration of 25-hydroxyvitamin D (25OHD), a long-lived metabolite of vitamin D, is used to judge vitamin D status; values below 25 nmol/L are associated with an increased risk of rickets and osteomalacia (1). There is evidence that a low maternal plasma 25OHD in pregnancy may influence the growth and bone mineral accrual of the offspring during foetal life, infancy and childhood. Positive associations have been reported between maternal vitamin D status in pregnancy and birthweight, birth length, length at 1 year and bone mineral accretion at 9 years (2–6), although evidence is conflicting (7,8). These relationships have been observed at concentrations of 25OHD higher than those associated with rickets and osteomalacia, and there are calls to raise the accepted lower threshold of vitamin D sufficiency for pregnant women, most recently to 80 nmol/L (9). On a population basis, plasma 25OHD concentrations above 80 nmol/L are relatively uncommon in countries at temperate latitudes but are more common among people living in the tropics who have abundant skin sunshine exposure (10). To contribute to the debate on the definition of vitamin D sufficiency in pregnancy, we have investigated the influence of maternal plasma 25OHD concentration on foetal and infant growth in a rural area of The Gambia, West Africa (13°N). In this region, there is tropical sunshine all year, the women are farmers who work out-of-doors for much of each day, and local female dress does not restrict regular sunshine exposure to the face, neck, shoulders, arms and feet, especially during farm work and gardening. The study was a secondary analysis of biochemical, anthropometric and bone data from a subset of 125 women and infants collected during a calcium supplementation study of blood pressure in pregnant Gambian women (International Trial Registry: ISRCTN96502494). No significant benefits for foetal and infant growth of maternal calcium supplementation were identified despite the low customary calcium intake in The Gambia (11). The protocol, methods, maternal characteristics and infant data from the detailed study have been published (11). Briefly, women from the rural villages of Keneba and Manduar, West Kiang, The Gambia were recruited at 20 weeks of pregnancy (P20) and randomized to a daily calcium supplement or a matching placebo tablet until parturition (1500 mg Ca as calcium carbonate and microcellulose-lactose, respectively; Nycomed Pharma AS, Asker, Norway). Fasting, early morning blood was collected and anthropometry performed at P20 and 36 weeks of pregnancy (P36). The mean (± SD) age, weight, height and dietary calcium intake of the women at P20 were 27.4 ± 7.5 years, 56.3 ± 6.7 kg, 1.61 ± 0.05 m and 356 ± 190 mg/day, respectively. The median parity (range) was 3 (0–10). Infant birthweight was measured within 24 h of delivery. Weight, crown-heel length and head circumference were measured at 2, 13 and 52 weeks postpartum. In addition, infant bone mineral content (BMC), bone mineral density (BMD) and bone width (BW) or bone area (BA), were measured by single photon absorptiometry of the midshaft radius (Lunar SP2, Lunar Corporation, Madison, WI, USA) and, for a subset (n = 44, 47 and 52 at 2, 13 and 52 weeks, respectively), by whole-body dual-energy X-ray absorptiometry (Lunar DPX+, software version 4.7b, Lunar Corporation). Plasma 25OHD was measured a using radioimmunometric assay (Diasorin Ltd, Wokingham, Berks, UK), with assay performance monitored through the Vitamin D External Quality Assessment Scheme (DEQAS; Endocrine/Oncology Laboratory, Charing Cross Hospital, London, UK). The intra- and inter-assay coefficients of variation were 4% and 100); possible trends in the data with p = 0.01–0.1 were noted. The analysis was conducted on 123 mother–infant pairs; blood samples from two subjects were not available. Mean ± SD 25OHD (range) was: P20 = 103 ± 25 (53–167) nmol/L; P36 = 111 ± 27 (51–189) nmol/L. No subject had a 25OHD value <50 nmol/L, 20% and 16% had 25OHD <80 nmol/L, at P20 and P36, respectively. There was a high degree of within-subject consistency in 25OHD at P20 and P36 (25OHDP36 = 33.2 + [0.79 ± 0.07]× 25OHDP20, p ≤ 0.001, R2 adjusted 51.5%, n = 121); 11% of women had 25OHD <80 nmol/L at both P20 and P36. The mean birthweight of the infants was 2.99 ± 0.36 kg. The infant anthropometric and bone measures during the first year are given in Table 1. No significant relationships or trends in the data were observed between maternal 25OHD concentration using the values at P20, P36 or the mean of the two and any of the following infant measures: birthweight, infant weight, length, head circumference, BMC, BW (or BA), BMD and size-adjusted BMC of the midshaft radius and whole body at any time postpartum. This is illustrated in Figure 1 for birthweight as a function of maternal 25OHD concentration at P20. Comparing the results for mothers with 25OHD above and below 80 nmol/L did not alter this finding. Table 1 Anthropometric and bone measures of Gambian infants Figure 1 Lack of a significant relationship between infant birthweight and maternal vitamin D status at 20 weeks of pregnancy (p = 0.8). Multiple regression model included season, maternal height, weight, weight gain, supplement group and sex of the infant. No significant interaction between supplement group and maternal 25OHD concentration was observed for any infant variable. Trends in the data were observed in a few instances for a supplement group × 25OHD interaction among the bone measures but no consistent picture emerged and they were considered to have arisen by chance. We conclude that there is no evidence for an influence of vitamin D status during pregnancy on infant growth and bone mineral accrual in the conditions prevailing in The Gambia. The children in this study, as is common in this region of The Gambia (13), were born small, grew well for the first months of life but experienced growth faltering during later infancy compared to Western children (11,14), as demonstrated by their weight and length SDS. The 25OHD concentrations of the women were >50 nmol/L in the second half of pregnancy, and no distinction could be drawn in infant outcomes between mothers with concentrations above or below 80 nmol/L. Thus, our study suggests that, for women with regular, adventitious UVB sunshine exposure and in situations where foetal and infant growth may be constrained by multiple factors, there would be no benefit for foetal and infant growth or bone mineral accrual in aiming to increase the vitamin D status of individual mothers during pregnancy above 50 or 80 nmol/L.

Free 25-Hydroxyvitamin D: Impact of Vitamin D Binding Protein Assays on Racial-Genotypic Associations.

Context: Total 25-hydroxyvitamin D (25OHD) is a marker of vitamin D status and is lower in African Americans than in whites. Whether this difference holds for free 25OHOD (f25OHD) is unclear, considering reported genetic-racial differences in vitamin D binding protein (DBP) used to calculate f25OHD. Objectives: Our objective was to assess racial-geographic differences in f25OHD and to understand inconsistencies in racial associations with DBP and calculated f25OHD. Design: This study used a cross-sectional design. Setting: The general community in the United States, United Kingdom, and The Gambia were included in this study. Participants: Men in Osteoporotic Fractures in Men and Medical Research Council studies (N = 1057) were included. Exposures: Total 25OHD concentration, race, and DBP (GC) genotype exposures were included. Outcome Measures: Directly measured f25OHD, DBP assessed by proteomics, monoclonal and polyclonal immunoassays, and calculated f25OHD were the outcome measures. Results: Total 25OHD correlated strongly with directly measured f25OHD (Spearman r = 0.84). Measured by monoclonal assay, mean DBP in African-ancestry subjects was approximately 50% lower than in whites, whereas DBP measured by polyclonal DBP antibodies or proteomic methods was not lower in African-ancestry. Calculated f25OHD (using polyclonal DBP assays) correlated strongly with directly measured f25OHD (r = 0.80–0.83). Free 25OHD, measured or calculated from polyclonal DBP assays, reflected total 25OHD concentration irrespective of race and was lower in African Americans than in US whites. Conclusions: Previously reported racial differences in DBP concentration are likely from monoclonal assay bias, as there was no racial difference in DBP concentration by other methods. This confirms the poor vitamin D status of many African-Americans and the utility of total 25OHD in assessing vitamin D in the general population.

Calcium economy in human pregnancy and lactation

Pregnancy and lactation are times of additional demand for Ca. Ca is transferred across the placenta for fetal skeletal mineralisation, and supplied to the mammary gland for secretion into breast milk. In theory, these additional maternal requirements could be met through mobilisation of Ca from the skeleton, increased intestinal Ca absorption efficiency, enhanced renal Ca retention or greater dietary Ca intake. The extent to which any or all of these apply, the underpinning biological mechanisms and the possible consequences for maternal and infant bone health in the short and long term are the focus of the present review. The complexities in the methodological aspects of interpreting the literature in this area are highlighted and the inter-individual variation in the response to pregnancy and lactation is reviewed. In summary, human pregnancy and lactation are associated with changes in Ca and bone metabolism that support the transfer of Ca between mother and child. The changes generally appear to be independent of maternal Ca supply in populations where Ca intakes are close to current recommendations. Evidence suggests that the processes are physiological in humans and that they provide sufficient Ca for fetal growth and breast-milk production, without relying on an increase in dietary Ca intake or compromising long-term maternal bone health. Further research is needed to determine the limitations of the maternal response to the Ca demands of pregnancy and lactation, especially among mothers with marginal and low dietary Ca intake, and to define vitamin D adequacy for reproductive women.

MAVIDOS Maternal Vitamin D Osteoporosis Study: study protocol for a randomized controlled trial. The MAVIDOS Study Group

MAVIDOS is a randomised, double-blind, placebo-controlled trial (ISRCTN82927713, registered 2008 Apr 11), funded by Arthritis Research UK, MRC, Bupa Foundation and NIHR.BackgroundOsteoporosis is a major public health problem as a result of associated fragility fractures. Skeletal strength increases from birth to a peak in early adulthood. This peak predicts osteoporosis risk in later life. Vitamin D insufficiency in pregnancy is common (31% in a recent Southampton cohort) and predicts reduced bone mass in the offspring. In this study we aim to test whether offspring of mothers supplemented with vitamin D in pregnancy have higher bone mass at birth than those whose mothers were not supplemented.Methods/DesignWomen have their vitamin D status assessed after ultrasound scanning in the twelfth week of pregnancy at 3 trial centres (Southampton, Sheffield, Oxford). Women with circulating 25(OH)-vitamin D levels 25-100 nmol/l are randomised in a double-blind design to either oral vitamin D supplement (1000 IU cholecalciferol/day, n = 477) or placebo at 14 weeks (n = 477). Questionnaire data include parity, sunlight exposure, dietary information, and cigarette and alcohol consumption. At 19 and 34 weeks maternal anthropometry is assessed and blood samples taken to measure 25(OH)-vitamin D, PTH and biochemistry. At delivery venous umbilical cord blood is collected, together with umbilical cord and placental tissue. The babies undergo DXA assessment of bone mass within the first 14 days after birth, with the primary outcome being whole body bone mineral content adjusted for gestational age and age. Children are then followed up with yearly assessment of health, diet, physical activity and anthropometric measures, with repeat assessment of bone mass by DXA at age 4 years.DiscussionAs far as we are aware, this randomised trial is one of the first ever tests of the early life origins hypothesis in human participants and has the potential to inform public health policy regarding vitamin D supplementation in pregnancy. It will also provide a valuable resource in which to study the influence of maternal vitamin D status on other childhood outcomes such as glucose tolerance, blood pressure, cardiovascular function, IQ and immunology.

Vitamin D Deficiency and Its Health Consequences in Africa

Africa is heterogeneous in latitude, geography, climate, food availability, religious and cultural practices, and skin pigmentation. It is expected, therefore, that prevalence of vitamin D deficiency varies widely, in line with influences on skin exposure to UVB sunshine. Furthermore, low calcium intakes and heavy burden of infectious disease common in many countries may increase vitamin D utilization and turnover. Studies of plasma 25OHD concentration indicate a spectrum from clinical deficiency to values at the high end of the physiological range; however, data are limited. Representative studies of status in different countries, using comparable analytical techniques, and of relationships between vitamin D status and risk of infectious and chronic diseases relevant to the African context are needed. Public health measures to secure vitamin D adequacy cannot encompass the whole continent and need to be developed locally.

Role of Assay Type in Determining Free 25-Hydroxyvitamin D Levels in Diverse Populations

The choice of a vitamin D–binding protein assay is key in calculating free 25-hydroxyvitamin D levels. The results of this analysis support the use of total 25-hydroxyvitamin D as a marker of vitamin D status, regardless of race or GC genotype.

Free 25-hydroxyvitamin D is low in obesity, but there are no adverse associations with bone health

BACKGROUND The mechanism and clinical significance of low circulating 25-hydroxyvitamin D [25(OH)D] in obese people are unknown. Low total 25(OH)D may be due to low vitamin D-binding proteins (DBPs) or faster metabolic clearance. However, obese people have a higher bone mineral density (BMD), which suggests that low 25(OH)D may not be associated with adverse consequences for bone. OBJECTIVE We sought to determine whether 1) vitamin D metabolism and 2) its association with bone health differ by body weight. DESIGN We conducted a cross-sectional observational study of 223 normal-weight, overweight, and obese men and women aged 25-75 y in South Yorkshire, United Kingdom, in the fall and spring. A subgroup of 106 subjects was also assessed in the winter. We used novel techniques, including an immunoassay for free 25(OH)D, a stable isotope for the 25(OH)D3 half-life, and high-resolution quantitative tomography, to make a detailed assessment of vitamin D physiology and bone health. RESULTS Serum total 25(OH)D was lower in obese and overweight subjects than in normal-weight subjects in the fall and spring (geometric means: 45.0 and 40.8 compared with 58.6 nmol/L, respectively; P < 0.001) but not in the winter. Serum 25(OH)D was inversely correlated with body mass index (BMI) in the fall and spring and in the winter. Free 25(OH)D and 1,25-dihydroxyvitamin D [1,25(OH)2D] were lower in obese subjects. DBP, the DBP genotype, and the 25(OH)D3 half-life did not differ between BMI groups. Bone turnover was lower, and bone density was higher, in obese people. CONCLUSIONS Total and free 25(OH)D and 1,25(OH)2D are lower at higher BMI, which cannot be explained by lower DBP or the shorter half-life of 25(OH)D3 We speculate that low 25(OH)D in obesity is due to a greater pool of distribution. Lower 25(OH)D may not reflect at-risk skeletal health in obese people, and BMI should be considered when interpreting serum 25(OH)D as a marker of vitamin D status.

Abundant sunshine and vitamin D deficiency.

In this issue of the British Journal of Nutrition, Islam et al. (1) and Woo et al. (2) present data from Bangladesh and China showing a high prevalence of poor vitamin D status in women of childbearing age combined with a concurrent elevation in plasma parathyroid hormone (PTH) concentration. These reports add to the growing evidence of a high prevalence of vitamin D insufficiency in countries with abundant sunshine(3-5). Vitamin D can be acquired by cutaneous synthesis following exposure to sunlight and from the diet(3-5). Vitamin D supply varies considerably worldwide, across population groups and between individuals, mainly because of differences in skin exposure to UVB radiation, the efficiency of cutaneous synthesis (for example, due to ageing and skin pigmentation) and in food fortification practices and supplement use(3,4). Vitamin D is an important determinant of bone health at all ages; deficiency causes rickets in children and osteomalacia in children and adults. A low vitamin D status has also been associated with increased bone loss and osteoporotic fracture risk in older people(6,7).