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. 
Osteoporosis patients can be relieved or cured by getting more sun.
 some people concluded that modern people are more likely to suffer from diseases caused by calcium deficiency such as osteoporosis.