Researchers say that lack of vitamin D could be involved in the increase in asthma and allergies. They believe the increase in the two conditions has come at the same time as a decline in exposure to the sun, the main source of vitamin D, Forbes reported.
The theory is that as people have become more prosperous over the past 50 years, more time is spent indoors travelling in cars rather than walking, for example, which has resulted in less exposure to sunlight.
"Coupled with inadequate intake from foods and supplements, this then leads to vitamin D deficiency," say the researchers.
"Our studies show that higher vitamin D intake by pregnant mothers reduces asthma risk by as much as 40 percent in children aged three to five." They suggest that low levels of vitamin D may affect the development of foetal lungs and immune system, leading to a higher risk of asthma and allergies.
The researchers say the theory explains the geographic pattern for cases of asthma and allergies.
They add: "We believe these patterns can be explained by a decrease in exposure to the sun and the limited sources of vitamin D in the diet to compensate for this decrease in sun exposure, leading to vitamin D deficiency. More than five million people in the UK are being treated for asthma and more than 15 million people suffer with an allergy of some kind.
Vitamin D is essential for the processing of calcium. Unlike other vitamins, it can be made in our bodies as a result of exposure to sunlight. Low levels of vitamin D have also been associated with osteoporosis, hypertension, diabetes and cancer.