Older women tend to have low serum 25 hydroxyvitamin D [25(OH)D] levels, low bone mineral density, and high parathyroid hormone (PTH) levels, all harbingers of frailty. Based on new findings, Italian investigators suggest that clinicians should consider fat mass when addressing the bone health of these women.

In a study of 218 fit and healthy women older than 65 (mean age 71 years), fat mass significantly related with serum vitamin D status in agreement with previous research, Caterina Trevisan, MD, of the University of Padova in Italy, and colleagues reported in the Journal of Nutrition, Health and Aging. Higher fat mass correlated with lower 25(OH)D levels, yet higher bone mineral density, including in the lumbar, femoral neck, and total hip regions. Women with higher adiposity had 58% lower risks of secondary hyperparathyroidism (SHPT)o, binary logistic analysis revealed. The average body mass index was 27.1 kg/m2. By contrast, consumption of vitamin D from foods, without dietary supplementation, showed no relationship with 25(OH)D levels, bone mineral density, or SHPT, also in line with previous studies. None of the older women achieved the recommended dietary allowance for vitamin D of 800 IU/day.

“This study demonstrates that fat mass, more than vitamin D intake, may significantly influence serum 25(OH)D levels in fit and healthy older women at or slightly above normal weight,” Dr Trevisan told Renal & Urology News. “These findings confirm that adipose tissue is a factor to be considered when evaluating vitamin D supplementation, since it may influence the achievement of normal values of serum 25(OH)D. At the same time, the protective effect of adipose tissue on bone mineral density suggests that excessive weight loss in such women could be unhealthy for bone mass.”

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Fat can store vitamin D and thereby reduce its bioavailability, the investigators suggested.

“The fact that bone health parameters did not deteriorate despite the low serum 25(OH)D levels might be due to gravitational loading and to fat-mediated endocrine mechanisms likely preserving bone mass,” they wrote.

Physiologic or pathologic changes in fat mass or fat-free mass can significantly influence bone metabolism, Dr Trevisan added. In clinical practice, she recommended using validated methods to assess the body composition of older patients.

For the study, participants completed a 3-day food diary and food frequency questionnaire. They underwent laboratory testing for 25(OH)D and intact PTH. Bone mineral density and body composition were estimated using dual-energy X-ray absorptiometry with fan-beam technology. The investigators defined vitamin D insufficiency as 25(OH)D levels below 50 mmol/L. SHPT was indicated by PTH above 60 pg/mL. Nearly 2 in 3 participants (61.9%) had vitamin D insufficiency. Just 1 in 5 had elevated PTH levels. Osteoporosis developed in 29.8%, and osteopenia in 53.7%. None of the participants had pre-existing kidney disease.

Sunlight exposure also influences serum 25(OH)D levels. As this is difficult to modify, the investigators did not examine it directly. Instead they adjusted results for sunlight exposure using the month of the blood test. Endogenous production of vitamin D is another relevant factor, but it is typically reduced in the elderly.

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1. Trevisan C, Veronese N, Berton L, et al. Factors Influencing Serum-Hydroxivitamin D Levels and Other Bone Metabolism Parameters in Healthy Older Women. J Nutr Health Aging 2017;21(2):131-135. doi: 10.1007/s12603-016-0746-6