The relationship between bone mineral density (basically a measurement of how much minerals like calcium you have in your bones) and vitamin D levels in young children has never been determined.
Recently, Doctors Xiao Dan Yu and colleagues of the Shanghai Jiao Tong University School of Medicine in China measured 25(OH)D levels and bone mineral density (BMD) in children using ultrasound.
Two hundred and three young (age 0-6 year old) Chinese children had both their 25(OH)D levels and their bone mineral density measured. The authors found that the prevalence of low BMD was 47% in children with 25(OH)D lower than 20 ng/ml, much higher than 17% in children with 25(OH)D over 20 ng/ml (p<0.05).
Even more, they found a remarkable non-liner association between BMD and 25(OH)D, essentially showing the higher your vitamin D level, the less likely you are to have low BMD. Once levels reached above 20 ng/ml, there wasn’t much more protection having levels around 40 and 50 ng/ml than having levels just at 20 ng/ml.
The reason the cutoff is so low is that the amount of calcium in your bones is crucial and not just for bone health. The first function of vitamin D is to maintain serum calcium in your blood. If there is not enough calcium in your diet, then activated vitamin D and parathyroid hormone take calcium from your bones to maintain blood calcium. If you get enough calcium in your diet, then activated vitamin D will control both itself and parathyroid hormone and tell the body to stop pulling calcium from the bones and take it from the intestine instead.
That is why the amount of calcium in your bones is associated with lower cutoff values of 25(OH)D then are chronic diseases. The body’s first responsibility is to keep you alive tomorrow, so it sacrifices long term benefits for short-term survival. That is, when 25(OH)D levels are low, the body triages 25(OH)D for the life-saving “maintain serum calcium” function, which is intimately involved with bone calcium. So 25(OH)D levels only need to be above 20 ng/ml in young children to protect their bone mineral density, but they probably need to maintain even higher 25(OH)D levels to protect against the myriad of childhood diseases now associated with vitamin D deficiency.
The authors concluded,
“In conclusion, low serum 25(OH)D concentrations are very common in the young children. Quantitative ultrasound BMD was increased with the increase of serum 25(OH)D. Low BMD in young children is likely to decrease when serum 25(OH)D is raised over at least 20 ng/ml. A threshold for 25(OH)D of 20 ng/ml existed for low BMD confirming the concentration to define vitamin D deficiency for infants and children.”
Lastly, I’d like to remind that bone mineral density testing is not a test of the quality of bone, only the density of the mineral content. It does not tell you if you have enough magnesium in your bones, or enough vitamin K2, or enough boron or silica. It also tells you nothing about the quality of the protein of bone.