Vitamin D levels are lower among adults with Prader-Willi syndrome (PWS) that in other obese individuals, mainly because PWS patients tend to consume less vitamin D through their diet, a study suggests.
The study, “The Sun’s Vitamin in Adult Patients Affected by Prader–Willi Syndrome,” was published in the journal Nutrients.
Vitamin D is produced through exposed to sunlight, but can also be obtained in supplements or in foods such as salmon, tuna, sardines, eggs, mushrooms, and dairy products.
Because vitamin D is involved in calcium absorption, people with insufficient levels of the vitamin often have thin or misshapen bones, and are more prone to develop osteoporosis.
Obese patients frequently have low vitamin D levels, with some studies suggesting that a greater fat mass predicts lower levels of this vitamin and others pointing to a low intake of vitamin D in diet.
Due to excessive hunger and food intake, most people with Prader-Willi syndrome grow to become morbidly obese. Studies suggest they have more fat mass than others with a comparable body mass index (BMI).
If this were true, and levels of vitamin D were dependent on fat mass, those with PWS would have lower levels of the vitamin than other obese patients.
To investigate the association between fat mass and vitamin D levels, researchers in Italy examined 15 adults with PWS (mean age 28) and 15 individuals matched to patients in terms of sex, age, and BMI serving as controls. All were from same geographical Naples metropolitan area, meaning they had similar sunlight exposure.
Participants were asked to complete an interview about their demographic data, medical history, vitamin D supplement use, treatments, and smoking habits. They also reported their food intake for a period of three days to determine how much vitamin D they were obtaining from diet.
Measurements of body fat, BMI, and waist circumference were collected. Vitamin D levels were assessed via blood levels of 25-hydroxy vitamin D (25OHD), a metabolite of the vitamin that reflects vitamin D produced in the skin and obtained from food and supplements.
None of the participants reported any sun exposure at the time of enrollment or used vitamin D supplements. Also, all PWS patients had received recombinant human growth hormone (rhGH) treatment during childhood, which was stopped at least two years before the study was initiated.
Patients and controls were similar in terms of obesity, waist circumference, and fat mass. But while both groups consumed less vitamin D than recommended, dietary intake was significantly lower among those with PWS, the researchers reported. Patients also had significantly less 25OHD than controls.
The team then divided participants according to their BMI and fat mass. They found that patients whose BMI or fat mass were above the median had significantly lower blood levels of 25OHD than those below the median.
In contrast, controls showed no significant differences in 25OHD levels comparing high and low levels of fat mass and BMI. This suggested that vitamin D levels dropped with higher measures of body fat in PWS patients only.
A first analysis to investigate which factors were associated with vitamin D levels in circulation showed that BMI, waist circumference, fat mass, and dietary intake were all significantly associated with 25OHD levels.
But a more consistent analysis then demonstrated that only the amount of vitamin D obtained in diet was significantly associated with 25OHD levels, explaining about 84% of its variability across patients.
BMI values of 42 kg/m2 or greater and fat mass of 41 kg (about 90 lbs) or higher both accurately predicted the lowest levels of 25OHD, the team also found. This may help identify patients more in need of vitamin D supplements.
“In our study, dietary vitamin D intake was significantly lower in PWS adults than in the control group, and, consequently, 25OHD levels were lower among PWS adults across all categories of BMI and fat mass compared with their obese counterpart,” the researchers wrote.
“The results of the present study do not lend support to the thesis that the extent of adipose [fat] tissue is a main determinant of low vitamin D status in PWS adults, while it is evidenced that the dietary vitamin D intake is the major determinant of 25OHD levels,” they concluded.
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