Rickets is a childhood deficiency of vitamin D, which is called osteomalacia in adults. It causes bone weakness (causing fractures), bone pains and can cause muscle weakness and joint inflamation.
It is widely thought that sunlight on our skin provides us with a mechanism to produce vitamin D but this is largely untrue; you have to be outdoors every day at the right time, in order to achieve this, and in northern climates the required wavelengths of light are simply absent for four months at winter. Most vitamin D is, and always will be, sourced from our diets. In particular, vitamin D comes from animal fat and fish. Some plants contain a form of vitamin D denoted D2 (ergocalciferol) but this is not utilized very well in animals.
“Rickets and/or low vitamin D levels has been well-documented in many vegetarians and vegans (26), since animal fats are either lacking or deficient in vegetarian diets (as well as those of the general Western public who routinely try to cut their animal fat intake), since sunlight is only a source of vitamin D at certain times and at certain latitudes, and since current dietary recommendations for vitamin D are too low, this emphasizes the need to have reliable and abundant sources of this nutrient in our daily diets. Good sources include cod liver oil, lard from pigs that were exposed to sunlight, shrimp, wild salmon, sardines, butter, full-fat dairy products, and eggs from properly fed chickens.”
Dr Stephen Byrnes, 2000
Reference 26 is duplicated at the end of this page.