With people's life and work gradually in high-rise buildings, people have fewer opportunities of exposure to sunlight. A study found that sunlight is a necessary condition for the synthesis of vitamin D, and vitamin D is the key factor to promote calcium absorption. 
Calcium-containing foods in the conceptual structure of modern people are much richer than before.
 some people concluded that modern people are more likely to suffer from diseases caused by calcium deficiency such as osteoporosis.