Medicine companies follow FDA regulations. Some makers of dietary supplements follow the U.S. Pharmacopeial (USP) Convention quality standards. This means they volunteer to have their products tested. An outside company will check them for quality and purity before they are sold. These supplements display additional credentials on their labels. Look for them to say “USP Verified” or “ConsumerLab.com Approved Quality.”
It is possible to ingest too much of certain vitamins and minerals (vitamin A, calcium, iron, copper, selenium) which may exacerbate an existing imbalance or lack of another mineral (magnesium, zinc). It is also important to balance intake of fatty acids in the omega-6 and omega-3 series, as most people get too much omega-6 and not enough omega-3. Small children can be overdosed with adult doses of for example vitamin A or iron, and pills may be dangerous for babies or young children because they can get stuck in the throat. Therefore, I recommend consulting a doctor or nutritionist educated in orthomolecular medicine. Most people are likely to benefit from taking a broad-spectrum multivitamin/mineral supplement as a basic insurance against deficiencies.
A person's caloric requirement depends on his body size and exercise level. Sedentary people of both genders will keep their weight stable by taking in about 13 calories per pound of body weight each day. Moderate physical activity boosts this requirement to 16 calories a pound, and vigorous exercise calls for about 18 calories a pound. On average, a moderately active 125-pound woman needs 2,000 calories a day; a 175-pound guy with a similar exercise pattern needs 2,800 calories. And like women, men will lose weight only if they burn more calories than they take in.
For example, while increased consumption of fruits and vegetables are related to decreases in mortality, cardiovascular diseases and cancers, supplementation with key factors found in fruits and vegetable, like antioxidants, vitamins, or minerals, do not help and some have been found to be harmful in some cases. In general as of 2016, robust clinical data is lacking, that shows that any kind of dietary supplementation does more good than harm for people who are healthy and eating a reasonable diet but there is clear data showing that dietary pattern and lifestyle choices are associated with health outcomes.