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. 
A well-known example is vitamin C, which can effectively fight viral infections, prevent or reverse disease caused by bacteria, and help the body detoxify organic and inorganic toxins.  Vitamin C also reduces the risk for cancer, strengthens connective tissues (collagen), and counteracts stress by increasing the adrenal´s production of cortisol. The dose required is set according to the body's need. Nobel Price Laureate Linus Pauling suggested that an optimal daily intake of vitamin C could vary from at least 250 mg up to 20 grams per day.  Because unabsorbed vitamin C attracts water into the gut, some people may experience loose stools, gas and/or diarrhea by ingesting only 1-2 grams at a time, while others with a higher level of stress may tolerate 5-6 grams or more. The dose that causes loose stools is called the "bowel tolerance" for vitamin C.  To avoid the laxative effect of high doses, it is best to take vitamin C throughout the day in smaller divided doses.
"Studies show that omega-3 fatty acids are cardio-protective and the basis for the AHA recommendation to consume fatty fish twice weekly," she says. "But the studies do not show that popping a supplement can have the same benefit in healthy individuals." She adds, "There is also a misperception that fish oils can lower cholesterol, but this is not true."
As the table above shows, some of the best sources of calcium are dairy products. However, dairy products such as whole milk, cheese, and yogurt also tend to contain high levels of saturated fat. The USDA recommends limiting your saturated fat intake to no more than 10% of your daily calories, meaning you can enjoy whole milk dairy in moderation and opt for no- or low-fat dairy products when possible. Just be aware that reduced fat dairy products often contain lots of added sugar, which can have negative effects on both your health and waistline.
Try a lower-calorie version. Use lower-calorie ingredients or prepare food differently. For example, if your macaroni and cheese recipe uses whole milk, butter, and full-fat cheese, try remaking it with non-fat milk, less butter, light cream cheese, fresh spinach and tomatoes. Just remember to not increase your portion size. For more ideas on how to cut back on calories, see Eat More Weigh Less.
^ Wilson, RD; Committee, Genetics; Wilson, RD; Audibert, F; Brock, JA; Carroll, J; Cartier, L; Gagnon, A; Johnson, J. A; Langlois, S; Murphy-Kaulbeck, L; Okun, N; Pastuck, M (2015). "Pre-conception Folic Acid and Multivitamin Supplementation for the Primary and Secondary Prevention of Neural Tube Defects and Other Folic Acid-Sensitive Congenital Anomalies" (PDF). Journal of Obstetrics and Gynaecology Canada. 37 (6): 534–552. PMID 26334606.
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.