A dietary supplement is intended to provide nutrients that may otherwise not be consumed in sufficient quantities.
Supplements as generally understood include vitamins, minerals, fiber, fatty acids, or amino acids, among other substances. U.S. authorities define dietary supplements as foods, while elsewhere they may be classified as drugs or other products.
These products are not intended to prevent or treat any disease and in some circumstances are dangerous, according to the U.S. National Institutes of Health. For those who fail to consume a balanced diet, the agency says that certain supplements "may have value." Effects of most of these products have not been determined in randomized clinical trials and manufacturing is lightly regulated; randomized clinical trials of certain vitamins and antioxidants have found increased mortality rates.
According to scientific evidence, supplements of beta-Carotene and Vitamin E, possibly also Vitamin A, increase mortality. Also supplements of other antioxidants, B vitamins, folic acid or minerals and multivitamin supplements fail to decrease mortality, as well as morbidity to major chronic diseases. However, vitamin D supplements may be useful, but the evidence this far is not conclusive.
Most supplements should be avoided, and usually people should not eat micronutrients except people with clearly shown deficiency. Those people should first consult a doctor. An exception is vitamin D, which is recommended in Nordic countries due to weak sunlight.
Dietary supplements are unnecessary if one eats a balanced diet.
Moreover, many supplements have no practical effect. For example, glucosamine and chondroitin, often thought to relieve joint pain, have been shown to be without benefit. Relief reported for these supplements may be explained by the fact that many people taking them on a doctor's recommendation are also taking other pain relievers. Another common supplement, omega-3 fatty acids, has been similarly shown to be without benefit for heart and cholesterol level of healthy individuals.
Some supplements are harmful. For example, one study funded by the National Institute of Health found that men who consumed vitamin E supplements had a higher rate of prostate cancer than men who did not. Also, it has been found that among patients with coronary heart disease, elevated calcium levels are associated with increased mortality.
Supplements may create harm in several ways, including over-consumption, particularly of minerals and fat-soluble vitamins which can build up in the body. The products may also cause harm related to their rapid absorption in a short period of time, quality issues such as contamination, or by adverse interactions with other foods and medications.
A metastudy showed that daily multivitamin supplements do not prevent the weakening of cognitive performance. Another study showed that multivitamins and omega-3 fatty acids do not alleviate existing mild or medium dementia.
In Dec 17th 2013, the editorial of the scientific Annals of the Internal Medicine journal criticized heavily dietary supplements referring to research surveys. Systematic reviews did not find evidence that vitamin or multivitamin supplements would reduce mortality in general or cardiovascular or cancer mortality. On the contrary, many random clinical trials have shown that beta-carotene, vitamin E and possibly also high doses of vitamin A increase mortality.
JAMA journal published a review on 68 random trials (232,606 participants, 385 publications) showing that supplements of beta-carotene and vitamins A and E seem to add mortality. The effect of vitamin C and selenium was unclear.
Cochrane Database Syst Review published a review of 78 random trials (296,707 participants). In its fixed-effect models antioxidants were shown to increase mortality, particularly beta-carotene and vitamin E.
According to a study on almost 39,000 American women since the 1980s showed that multivitamin, folic acid, Vitamin B5, iron, magnesium, zinc and copper supplements may increase mortality and also other supplements do not seem to decrease common chronic diseases.
Bodybuilding supplements often involve health problems.
For example, the "natural" best-seller Craze, 2012's "New Supplement of the Year" by bodybuilding.com, sold in Walmart, Amazon etc., was found to contain undisclosed amphetamine-like compounds. Also other products by Matt Cahill have contained dangerous substances causing blindness and liver damage, and experts say that Cahill is emblematic for the whole industry.
Natural supplements such as green tea pills may damage liver and cause liver transplants, hepatitis and deaths. Yet people believe that "natural" is healthy or that supplements have been researched.
The catechin contained in green tea is claimed to accelerate metabolism, so they are sold as fat-burners. Green tea pills often contain many times more catechin than a cup of tea and can be toxic to liver, particularly for some people.
The number of incidents of liver damage from dietary supplements has tripled in a decade. Most of the supplements were bodybuilding supplements. Some of the patients require liver transplants and some die. In third of the supplements involved contained unlisted steroids.
FDA reports 50,000 health problems due to supplements each year. It does not have the resources to oversee the market. According to FDA, 70% of the companies do not follow basic quality standards.
BMC Medicine published a study on herbal supplements. Most of the supplements studied were of low quality, a third did not contain the herb claimed at all, e.g., rice power or laxative. A third contained unlisted substances.
Some supplements were contamined by rodent feces and urine.
Only 0.3% of the 55,000 U.S. market dietary supplements were studied so that their common side effects are known.
In early 20th century there were great hopes for supplements, but later research has shown these hopes were unfounded.
"Antioxidant paradox" means the fact that even though fruits and vegetables are related to decreases in mortality, cardiovascular diseases and cancers, antioxidant nutrients do not really seem to help. According to one theory, this is because some other nutrients would be the important ones. Multivitamin pills have neither proved useful but may even increase mortality.
Omega-3 fatty acids and fish oils from food are very healthy, but fish oil supplements are recommended only for those suffering from coronary artery diseases and not eating fish. Latest research has made the benefits of the supplements questionable even for them. Contrary to claims, fish oils do not decrease cholesterol but may even raise the "bad" LDL cholesterol and cause other harms. Also the use of cod liver oil is criticized by scientists.
Alice Lichtenstein, DSc, chairwoman of the American Heart Association (AHA) says that even though omega-3 fatty acids from foods are healthy, the same is not shown in studies on omega-3 supplements. Therefore one should not eat fish oil supplements unless one suffers from heart diseases.
In the name of deregulation, the Dietary Supplement Health and Education Act of 1994 restricted the Food and Drug Administration from exerting authority over supplements as long as manufacturers made no claims about preventing or treating disease. As a result, the FDA currently regulates dietary supplements as a category of food, and not pharmaceutical drugs.
In contrast with pharmaceutical manufacturers — who must demonstrate their products are effective as well as safe — supplement manufacturers are not required to demonstrate efficacy.
Supplement manufacturers must, however, indicate a product is safe prior to introduction. The product cannot be marketed for 75 days following filing of this information with the FDA. Listing the information, however, does not mean the FDA regards the product as necessarily safe.
In practice, the FDA has said it has lacked funds to determine whether a given supplement should be considered "hazardous" and, thus, removed from the market. In one situation where this standard was reached (Ephedra), the agency faced significant opposition from the supplement industry and the U.S. Congress, and thus limited itself to making announcements about Ephedra's problematic safety records on the FDA website.
In 2007 the FDA implemented a "good manufacturing practices" policy to ensure dietary supplements "are produced in a quality manner, do not contain contaminants or impurities, and are accurately labeled."  Also in that year, the FDA implemented a rule requiring that supplement manufacturers submit reports of serious adverse events involving their products.
Because of the 1994 legislation, the FDA must demonstrate that individual supplements are unsafe using its adverse events reporting system, which may capture only 1 to 10 percent of all adverse events linked to supplements.
In 2012 the director of the FDA’s Dietary Supplements Program, called the level of non-compliance with regulations on dietary supplements “astonishing." Based on audits completed by the FDA’s compliance division in 2011 and 2012, the official, Dan Fabricant, said it was estimated that nearly 70% of dietary supplement manufacturers are currently not compliant with rules governing "good manufacturing practices."
Fabricant also indicated that FDA had concerns regarding under-reporting of adverse events.
According to Consumer Reports, the 1994 law "has left consumers without the protections surrounding the manufacture and marketing of over-the-counter or prescription medications" and it became the FDA's responsibility to prove that a supplement is not safe.
Similarly, Time described the 1994 legislation as "ill-conceived and reprehensible", that "gives the industry virtually free reign [sic] to market products defined as dietary supplements, while severely limiting the FDA's ability to regulate them".
FDA rules require that supplements meet specifications for "purity, strength, and composition and has been manufactured, packaged, labeled, and held under conditions to prevent adulteration."  FDA inspectors can look at a company's records upon request. However, enforcement is difficult given the number of supplement manufacturers and the 16% decline in FDA investigators from 2003 to 2006. Suppliers provide certificates of analysis stating that they have tested the material.
In the U.S., contamination and false labeling are "not uncommon". The United States Pharmacopeia offers a "seal" that may indicate that the product has been tested for integrity and safe manufacturing, and it is the only certification program that conducts random off-the-shelf testing. In 2008 ConsumerLab.com criticized the USP for proposing a 10 microgram per daily serving limit on lead in dietary supplements and drugs. It noted that under the FDA's 2006 guidance on lead in candy, only 0.2 micrograms of lead per serving are allowed. Consumerlab randomly tests some dietary supplements and makes the results available to subscribers. It has reported that 25% of the supplements it tests have problems, and half of the multivitamins category had similar problems. NSF International, HFL Sport Science, and the Natural Products Association also have dietary supplement certification programs.
The FDA says that if a product sold in the U.S. as a dietary supplement is promoted or claimed as a treatment, prevention or cure for disease, it is considered an unapproved — and thus illegal — drug. In addition, all supplements must, in the U.S., include on the label a statement that all claims by the seller "have not been evaluated by the Food and Drug Administration," and that the product "is not intended to diagnose, treat, cure or prevent any disease."
Supplement makers are permitted, however, to claim their product supports the structure or function of the body (e.g., "glucosamine helps support healthy joints" or "the hormone melatonin helps establish normal sleep patterns").
The FDA must be notified of these claims, which must be substantiated according to FDA rules. In reality, however, inaccurate claims are very common. For example, the compound hydrazine sulfate is promoted as a treatment for cancer, despite no significant evidence that it is either safe or effective.
In the U.S., a supplement may include vitamins, minerals herbs, amino acids, or any substance historically used as a diet supplement. Products must also be for ingestion as a pill, capsule, tablet, powder or liquid and not represented for use as a conventional food or as the sole item of a meal or diet. Finally, the products must be labeled as a "dietary supplement".
The European Union's Food Supplements Directive of 2002 requires that supplements be demonstrated to be safe, both in dosages and in purity. Only those supplements that have been proven to be safe may be sold in the bloc without prescription. As a category of food, food supplements cannot be labeled with drug claims but can bear health claims and nutrition claims.
The dietary supplements industry in the United Kingdom (UK), one of the 28 countries in the bloc, strongly opposed the Directive. In addition, a large number of consumers throughout Europe, including over one million in the UK, and various doctors and scientists, had signed petitions by 2005 against what are viewed by the petitioners as unjustified restrictions of consumer choice.
In 2004, along with two British trade associations, the Alliance for Natural Health (ANH) had a legal challenge to the Food Supplements Directive referred to the European Court of Justice by the High Court in London.
Although the European Court of Justice's Advocate General subsequently said that the bloc's plan to tighten rules on the sale of vitamins and food supplements should be scrapped, he was eventually overruled by the European Court, which decided that the measures in question were necessary and appropriate for the purpose of protecting public health. ANH, however, interpreted the ban as applying only to synthetically produced supplements—and not to vitamins and minerals normally found in or consumed as part of the diet.
Nevertheless, the European judges acknowledged the Advocate General's concerns, stating that there must be clear procedures to allow substances to be added to the permitted list based on scientific evidence. They also said that any refusal to add the product to the list must be open to challenge in the courts.