Vitamins are uniformly safe and beneficial in any dose - more is better.

The postwar vitamin boom had its own logic. Nutritional science had just spent two decades identifying the molecular causes of deficiency diseases, scurvy, rickets, beriberi, pellagra, and in every case, the answer was a specific vitamin. Give the vitamin, cure the disease. The pharmaceutical and health food industries were selling something even simpler: if some is good, more is better.

By the 1950s, multivitamin advertising made expansive claims about energy, vitality, and health. Grocery stores sold fortified everything. The implicit model was that vitamins were pure goods that the body would use what it needed and excrete the rest. That model is approximately correct for water-soluble vitamins under normal conditions. It is substantially wrong for fat-soluble vitamins and for any vitamin at extreme doses.

Vitamins A, D, E, and K dissolve in fat, not water. They accumulate in fatty tissue and the liver rather than being flushed through the kidneys. Vitamin A toxicity, hypervitaminosis A, causes headaches, nausea, bone pain, and in severe cases, liver damage and death. Arctic explorers in the early 20th century learned this the hard way: eating polar bear liver, which concentrates extraordinary amounts of vitamin A, produced violent illness. The clinical literature on vitamin A toxicity was well documented by the 1940s, but that knowledge did not translate into popular understanding.

The pivotal moment for public awareness was Linus Pauling's megavitamin advocacy starting in 1970. Pauling, whose scientific stature was enormous, argued for vitamin C supplementation at 1,000 to 10,000 milligrams daily and later extended his enthusiasm to other vitamins. The implicit message, that more vitamins could only help, filtered into the wellness culture of the 1970s and 1980s.

The harms emerged incrementally. Vitamin B6 neuropathy, nerve damage from excess pyridoxine, was reported in 1983 among patients taking doses far below what might seem dangerous. A 1994 clinical trial testing beta-carotene supplements in smokers, expecting a cancer-preventive effect, found instead that supplemented smokers had significantly higher lung cancer rates than the placebo group. The intervention had to be stopped early.

Modern dietary supplement regulation in the United States still does not require proof of safety before sale. The Dietary Supplement Health and Education Act of 1994 placed the burden of demonstrating harm on the FDA rather than proof of safety on the manufacturer. The infrastructure of trust that Pauling and decades of vitamin marketing built remains largely intact in consumer culture, even as the clinical evidence has become considerably more complicated.