Girls are stronger with higher levels of vitamin D, but the association was not found in boys. These are the results from a new large study from the Odense Child Cohort, recently published in the Journal of Clinical Endocrinology and Metabolism.

According to the study's first author, medical student Rada Faris Al-Jwadi, girls with low vitamin D have a 70 percent increased risk of being among the lowest 10 percent in a test for muscle strength.

We also found that girls were stronger if their Vitamin D level was more than 50 nmol/L. The most surprising finding was that this difference was only evident in girls and not in boys.

The study shows no association with vitamin D levels in mothers during pregnancy or in the umbilical cord at birth. This leads to the conclusion that there is no prenatal programming effect of muscle strength. We are talking about a more immediate effect of Vitamin D, says Rada Faris Al-Jwadi.

According to Henrik Thybo Christesen, Professor at H.C. Andersen children's hospital, Odense University Hospital and University of Southern Denmark, the study offers no explanation for the difference between boys and girls.

But other studies on children and adults have shown that vitamin D increases the levels of IGF-I, which is a growth factor that increases muscle strength.

Also, the IGF-I level is different in boys and girls which could be part of the explanation. We can't, based on our data, conclude that girls will get stronger muscles if they got more vitamin D through their food, as supplement pills or because of more sun exposure which are some of the most important sources of Vitamin D. Even though, our association could mean exactly that.

In the study, 881 5-year-old children in Odense Child Cohort got their muscle strength measured with a standardized test for hand grip strength meant for children. For 499 of the children, Vitamin D status analyses were done. Low Vitamin D levels were defined as serum 25OH-Vitamin D below 50 nmol/L. The statistical analyses were adjusted for height, weight and body fat percentage and were statistically highly significant. This means that the association wasn't due to being overweight and thereby having lower Vitamin D and lower muscle strength. It also means that it wasn't because girls liked to be more inside and were less physically active. The body fat percentage was calculated based on skin fold measurements.