Sun rays are always shunned, especially in western countries due to their potential to cause skin cancer by too much exposure. Sun is also avoided in tropical countries especially during summer to avoid sun-strokes and consequent fatality. But limited exposure to sun is necessary if humans have to make vitamin D in their body from the precursor 7-dedydrocholesterol present under the skin. Whether it is due to sun rays alone or because of its role in making vitamin D, the amazing finding that pregnant women tend to deliver taller babies during summer compared to those born during winter, is some thing to be examined for confirmation through scientific studies. But the very fact that they found summer born babies, half centimeter taller with wider bodies, is worth probing further. The role of sun light in the formation of Vitamin D3 is well known, the Ultraviolet B rays responsible for the conversion. Vitamin D3 has many functions like promoting bone formation, strengthening of the skeleton, strengthening immune system by promoting phagocytes, helping kidneys re-absorb nutrients, anti-tumor activity etc.
In a cohort study lasting 18 years in UK involving 14000 women and 7000 children of age less than 10 years, a definitive conclusion was drawn regarding the advantages of a few minutes exposure to sun in delivering babies with longer and wider bones. The height growth rate was 2-2.5 times higher in children of mothers who had exposure to sun in the 3rd trimester during the months of March to May, than others who did not had this advantage. In some other studies in countries in Africa and South America, it was consistently found that 65-75% of children born during dry seasons with bright sunlight available in plenty had maximum growth spurt compared to those born during rainy seasons.
The linkage between sunlight and Vitamin D3 formation being undisputed, it is thought that the seasonal variations in the availability of the required UV rays from the sun and the consequence fluctuations in D3 formation, could explain the phenomenon of increased growth during sunny season. Vitamin D3 has critical role in remodeling of skeleton and mineralization of bone tissues. The endocrine system and the hormones involved in growth regulation are influenced by the D3 concentration in the blood. This is confirmed by the findings that use of sun lamps during winter enabled children to gain heights as much as 1.5 cm. Such children had extra 5 inches of bone, because of increased periosteal growth, compared to their counterparts with insignificant exposure to light. Children from 6 months to 4 years can benefit maximum by extra supplementation with Vitamin D3. But such supplementation may be necessary only if adequate exposure to sun cannot be achieved, especially in areas where sunshine is limited.
What is intriguing in all these studies is whether sunshine has really an important role to play in height growth because of the complex metabolic regulatory matrix that works in humans governing the longitudinal and periosteal bone development. If such is the case most of the children born in tropical countries should be taller than those in temperate and cold climates which in reality does not happen. When USA was the most predominant economic power till a few years ago, their population were taller significantly compared to those in Europe. To day newer generation in Netherlands, Norway, Japan and other modern industrial nations boasts of men, a good 1"-2" taller than an average American. Obviously affluence, healthy foods, good physical work out and other favorable developments in the living environment have brought about this significant change. Many factors like gene inheritance, food consumption and others may have more critical influence on height growth than sunlight alone. But Vitamin D3 definitely plays an important role in bone mass development which is reflected more by the thickness or width of the bones.