People use dietary supplements for a wide assortment of reasons. Some seek to compensate for diets, medical conditions, or eating habits that limit the intake of essential vitamins and nutrients. Other people look to them to boost energy or to get a good night's sleep. Postmenopausal women consider using them to counter a sudden drop in estrogen levels.
A recent study predicts that global warming may reduce the nutrient density in many foods worldwide.  Atmospheric CO2 is estimated to surpass 550 ppm in the next 30-80 years, leading to larger crops with lower content of protein, iron and zinc per energy unit. Assuming that diets remain constant, while excluding other climate impacts on food production, the researchers estimated that elevated CO2 could cause an additional 175 million people to be zinc deficient and an additional 122 million people to be protein deficient in 2050. Anemia would increase significantly if crops lose even a small amount of iron. The highest risk regions - South and Southeast Asia, Africa, and the Middle East - are especially vulnerable, since they do not have the means and access to compensate using nutritional supplements.
Among other things, you need calcium to build healthy bones and teeth, keep them strong as you age, regulate the heart’s rhythm, and ensure your nervous system functions properly. Calcium deficiency can lead to, or exacerbate, mood problems such as irritability, anxiety, depression, and sleep difficulties. If you don’t get enough calcium in your diet, your body will take calcium from your bones to ensure normal cell function, which can lead to weakened bones or osteoporosis. Women are at a greater risk than men of developing osteoporosis, so it’s important to get plenty of calcium, in combination with magnesium and vitamin D, to support your bone health.
Recent research shows that differences in the content of the selenium in the soil can cause major differences in the concentration of selenium in meat.  For instance, since the soil in Finland is poor in selenium, the authorities decided in the early 1980s to add selenate to commercial fertilizers. A survey of selenium status among 108 healthy young people showed an increase in the blood selenium level of about 50 percent after four years. 
What's a man to do? Fortunately, he does not have to choose between his bones and his prostate. The solution is moderation. The Baltimore Longitudinal Study of Aging, for example, found no link between a moderate consumption of calcium (about 800 mg a day, two-thirds of the RDA) and prostate cancer. In addition, a randomized clinical trial of calcium supplements of 1,200 mg a day found no effect on the prostate, but only 327 men were in the calcium group, and the supplementation lasted just four years. Finally, the Harvard scientists speculate that a high consumption of vitamin D may offset the possible risks of calcium, so a daily multivitamin may also help.
For instance, foods rich in vitamin E and beta-carotene are healthy and can help reduce cancer risk. However, the U.S. Preventive Services Task Force (USPSTF) and the American Academy of Family Physicians (AAFP) recommend against taking vitamin E or beta-carotene for the prevention of cardiovascular disease. People who smoke or are at risk for lung cancer also should avoid beta-carotene. It can increase their risk of lung cancer.