Ethnic differences in urinary calcium and phosphate excretion between Gambian and British older adults.

UNLABELLED: Ethnic differences in renal calcium and phosphate excretion exist, which may depend on differences in their dietary intakes and regulatory factors. We report highly significant differences in urinary calcium and phosphate excretion between white British and Gambian adults after statistical adjustment for mineral intakes, indicating an independent effect of ethnicity. INTRODUCTION: Populations vary in their risk of age-related osteoporosis. There are racial or ethnic differences in the metabolism of the bone-forming minerals calcium (Ca) and phosphate (P), with a lower renal Ca and P excretion in African-Americans compared to white counterparts, even at similar intakes and rates of absorption. Also, Africans in The Gambia have a lower Ca excretion compared to white British subjects, groups known to differ in their dietary Ca intake. Here, we report on differences in urinary Ca and P excretion between Gambian and white British adults while allowing for known predictors, including dietary intakes. METHODS: Participants were healthy white British (n = 60) and Gambian (n = 61) men and women aged 60-75 years. Fasting blood and 2-h urine samples were collected. Markers of Ca and P metabolism were analysed. Dietary intake was assessed with country-specific methods. RESULTS: White British older adults had higher creatinine-corrected urinary Ca and P excretion (uCa/uCr, uP/uCr) and lower tubular maximum of Ca and P compared to Gambian counterparts. The predictors of urinary Ca and P differed between groups. Multiple regression analysis showed that dietary Ca and Ca/P were predictors of uCa/uCr and uP/uCr, respectively. Ethnicity remained a significant predictor of uCa/uCr and uP/uCr after adjustment for diet and other factors. CONCLUSIONS: Gambian older adults have higher renal Ca conservation than British counterparts. Dietary mineral intakes were predictors of the differences in urinary Ca and P excretion, but ethnicity remained a highly significant predictor after statistical adjustment. This suggests that ethnicity has an independent effect on renal Ca and P handling.

Ethnic differences in calcium, phosphate and bone metabolism.

The prevalence of osteoporosis and the incidence of age-related fragility fracture vary by ethnicity. There is greater than 10-fold variation in fracture probabilities between countries across the world. Mineral and bone metabolism are intimately interlinked, and both are known to exhibit patterns of daily variation, known as the diurnal rhythm (DR). Ethnic differences are described for Ca and P metabolism. The importance of these differences is described in detail between select ethnic groups, within the USA between African-Americans and White-Americans, between the Gambia and the UK and between China and the UK. Dietary Ca intake is higher in White-Americans compared with African-Americans, and is higher in White-British compared with Gambian and Chinese adults. Differences are observed also for plasma 25-hydroxy vitamin D, related to lifestyle differences, skin pigmentation and skin exposure to UVB-containing sunshine. Higher plasma 1,25-dihydroxy vitamin D and parathyroid hormone are observed in African-American compared with White-American adults. Plasma parathyroid hormone is also higher in Gambian adults and, in winter, in Chinese compared with White-British adults. There may be ethnic differences in the bone resorptive effects of parathyroid hormone, with a relative skeletal resistance to parathyroid hormone observed in some, but not all ethnic groups. Renal mineral excretion is also influenced by ethnicity; urinary Ca (uCa) and urinary P (uP) excretions are lower in African-Americans compared with White-Americans, and in Gambians compared with their White-British counterparts. Little is known about ethnic differences in the DR of Ca and P metabolism, but differences may be expected due to known differences in lifestyle factors, such as dietary intake and sleep/wake pattern. The ethnic-specific DR of Ca and P metabolism may influence the net balance of Ca and P conservation and bone remodelling. These ethnic differences in Ca, P and the bone metabolism may be important factors in the variation in skeletal health.

Bone mineral changes after lactation in Gambian women accustomed to a low calcium intake.

BACKGROUND/OBJECTIVES: Previous studies in Gambian women with a low calcium intake have described decreases in whole-body and regional bone mineral content (BMC) and areal bone mineral density (aBMD) during the first year of lactation. The aim of this study was to examine whether these effects are reversed after lactation. SUBJECTS/METHODS: Thirty-three Gambian women who had a previous dual-energy X-ray absorptiometry (DXA) scan at 52 weeks lactation (L52) were invited to participate in a follow-up study when neither pregnant nor lactating (NPNL) for ≥3 months and/or when 52 weeks postpartum in a subsequent lactation (F52). Whole body, lumbar spine and hip bone mineral were measured by DXA. Anthropometry and dietary assessments were also conducted. Repeated-measures analysis of covariance was used to determine differences from L52 at NPNL and F52. RESULTS: Twenty-eight women were scanned at NPNL and 20 at F52. The mean±s.d. calcium intake of the 33 women at NPNL and F52 was 360±168 mg/day. BMC, aBMD and size-adjusted BMC (SA-BMC) at all sites were higher at NPNL than L52. Percent increases in SA-BMC (mean±s.e.m.) were significant (P<0.0001): whole body=2.7±0.4%; lumbar spine=4.9±1.0%; total hip=3.7±1.0%. There were no significant differences in any measurements between the two lactation time points (L52 and F52). CONCLUSIONS: This study of Gambian women with low calcium intakes demonstrates that bone mineral mobilised during lactation is recovered after lactation. Successive periods of long lactation are not associated with progressive skeletal depletion.

Acute response to oral calcium loading in pregnant and lactating women with a low calcium intake: a pilot study.

UNLABELLED: This pilot study in women from The Gambia with low habitual calcium intakes showed differences in calciotropic hormones between pregnant, lactating and non-pregnant, non-lactating women similar to those in Western women. The response to oral calcium loading indicates a high degree of calcium conservation independent of reproductive status. INTRODUCTION: In pregnancy and early lactation, parathyroid hormone (PTH) concentrations may be suppressed. Uncertainty exists about how calcium metabolism is regulated, particularly when calcium intake is low. METHODS: We investigated fasting markers of calcium metabolism and the acute calcemic and calciuric responses after an oral calcium load in 30 pregnant, lactating or non-pregnant, non-lactating (NPNL) Gambian women with low habitual calcium intakes. Women received 1 g elemental calcium (CaCO3) at 0 min. Blood was collected at -30 and 180 min. Urine was collected from -60 to 0, 0-120 and 120-240 min. Samples were analysed (blood: ionized calcium (iCa); plasma (p): total calcium (tCa), phosphate (P), creatinine (Cr), PTH, 1,25-dihydroxyvitamin D (1,25(OH)2D), osteocalcin (OC), ß C-terminal cross-linked telopeptide of type 1 collagen (ßCTX), cyclic adenosine monophosphate (cAMP); urine (u): Ca, P, Cr, cAMP). RESULTS: Pre-loading, groups did not differ significantly in iCa, pP, uCa/Cr and uP/Cr. pOC concentrations were significantly lower and NcAMP and p1,25(OH)2D higher in pregnant women; pPTH and pßCTX in lactating women were higher than in NPNL women. Post-loading, iCa, ptCa and uCa/Cr concentrations increased; pPTH, NcAMP, ßCTX and uP/Cr decreased in all groups, but the magnitude of change did not differ significantly between groups. CONCLUSION: Differences between pregnant, lactating and NPNL Gambian women in pPTH, NcAMP and p1,25(OH)2D and bone markers were similar to Western women. However, the response to calcium loading indicates that there may be no differences in renal and intestinal calcium economy associated with reproductive status, potentially due to a high degree of calcium conservation associated with low intakes.

Calcium intake of rural Gambian infants: a quantitative study of the relative contributions of breast milk and complementary foods at 3 and 12 months of age.

BACKGROUND/OBJECTIVES: There is a paucity of information from developing countries on total calcium intake during infancy, and potential consequences for growth and bone development. DESIGN: Observational longitudinal study of rural Gambian infants (13 males and 17 females) at 3 and 12 months of age. SUBJECTS/METHODS: Breast-milk intake and calcium concentration, weighed dietary intake, anthropometry, midshaft radius bone mineral content (BMC) and bone width (BW). RESULTS: At 3 and 12 months (mean ± s.d.) calcium intake from breast milk was 179 ± 53 and 117 ± 38, and from other foods 12 ± 38 and 73 ± 105 mg/day. There was no difference in total calcium intake; 94% and 62% of calcium came from breast milk. At 3 and 12 months, weight s.d.-scores were -0.441 ± 1.07 and -1.967 ± 1.06; length s.d.-scores were -0.511 ± 1.04 and -1.469 ± 1.13. Breast-milk calcium intake positively predicted weight (P = 0.0002, P ≤ 0.0001) and length (P = 0.056, P = 0.001). These relationships were not independent of breast-milk intake, which positively predicted weight (P ≤ 0.002) and length (P = 0.06, P = 0.004). At 3, but not 12 months, weight and length correlated with total calcium intake. There were no relationships between total calcium intake and breast-milk intake with BW or BMC. CONCLUSION: The combination of low calcium intake from breast milk and complementary foods resulted in a low total calcium intake close to the estimated biological requirement for bone mineral accretion. Relationships between calcium intake and growth were largely accounted for by breast-milk intake, suggesting that low calcium intake per se was not the limiting factor in the poor growth. These findings have potential implications for deriving calcium requirements in developing countries.

Interrelation of parathyroid hormone and vitamin D metabolites in adolescents from the UK and The Gambia.

Parathyroid hormone (PTH) is used as a marker of vitamin D (VD) status. However, PTH depends on many other factors. The 24,25-dihydroxy VD (24,25VD) concentration may be a sensitive marker because its production is reduced in VD deficiency. The relationship between VD metabolites, their ratio and PTH was investigated in adolescents from the UK and The Gambia with different calcium intakes and VD status. In the UK, there was a significant positive (+ve) association between 25VD and both 1,25-dihydroxy VD (1,25VD) and 24,25VD and a negative (-ve) association with PTH. The 24,25:25VD ratio was consistent across the 25VD concentration range. There was a +ve association between PTH and 1,25:25VD, (1,25+24,25):25VD or 1,25:24,25VD, a -ve association with 24,25VD and none with 1,25VD or 24,25:25VD. Using LnPTH and 1,25:25VD ratio (but not 1,25VD:24,25VD or 25VD:24,25VD) increased uniformity between groups and strength of relationships compared to PTH and 1,25 or 25VD alone. In The Gambia, there was a significant -ve relationship between 25VD and PTH and none with 1,25VD. There was a +ve association between 1,25VD or 1,25:25VD and PTH. The more uniform prediction of PTH by the 1,25VD:25VD ratio may be because this better reflects the extent to which PTH-induced 1,25VD production can be met by VD supply. Further validation is needed.

Use of bioelectrical impedance analysis to assess body composition in rural Gambian children.

OBJECTIVE: To validate the Tanita BC-418MA Segmental Body Composition Analyser and four-site skinfold measurements for the prediction of total body water (TBW), percentage fat-free mass (%FFM) and percentage body fat (%BF) in a population of rural Gambian children. SUBJECTS/METHODS: One hundred and thirty-three healthy Gambian children (65 males and 68 females). FFM estimated by the inbuilt equations supplied with the Tanita system was assessed by comparison with deuterium oxide dilution and novel prediction equations were produced. Deuterium oxide dilution was also used to develop equations for %BF based on four-site skinfolds (biceps, triceps, subscapular and suprailiac). RESULTS: The inbuilt equations underestimated FFM compared to deuterium oxide dilution in all the sex and age categories (P<0.003), with greater accuracy in younger children and in males. The best prediction of %FFM was obtained from the variables height, weight, sex, impedance, age and four skinfold thickness measurements (adjusted R(2)=0.84, root mean square error (MSE)=2.07%). CONCLUSIONS: These data suggest that the Tanita instrument may be a reliable field assessment technique in African children, when using population and gender-specific equations to convert impedance measurements into estimates of FFM.