SCIENCE / MEDICINE : Nutrition Experts Warn Against Vitamin A Overloads

Tales of Vitamin A toxicity are often bizarre, centering on Arctic explorers eating huge amounts of polar bear liver or fitness fanatics gobbling handfuls of vitamin supplements.

But Americans should be aware that more subtle overloads of Vitamin A might pose some serious health risks, particularly to pregnant women, children and the elderly, federal nutrition experts say.

The ideal Vitamin A intake for the average man is about 3,100 to 3,300 international units (IU) daily, according to the National Research Council’s 1989 edition of the Recommended Dietary Allowances (RDAs).

But Dr. John Hathcock of the Food and Drug Administration estimates 3% to 4% of the U.S. population takes vitamin supplements containing 25,000 IU or more of Vitamin A per day.

“That’s a heck of a lot of people. If even 0.1% of the population are taking potentially toxic doses, that’s too high,” Hathcock said, adding that the FDA is still trying to determine at what level the vitamin meets the government’s “generally recognized as safe” standard.

So far, Vitamin A toxicity--which may induce bone abnormalities and life-threatening liver damage--has been conclusively shown only in adults who consistently consume more than 50,000 IU daily of Vitamin A and young children who take more than 20,000 IU daily.

But Hathcock said a growing body of research indicates the Vitamin A safety threshold may be far lower for certain high-risk groups.

“The question is not the upper limits, but rather how little can possibly cause problems,” the nutrition expert told a recent seminar.

Vitamin A is a group of compounds essential to vision, cell growth and immune function.

In nutritional supplements, Vitamin A generally refers to retinol, also known as “preformed Vitamin A.” Food sources of preformed Vitamin A are primarily limited to liver and fish liver oils, and it is rare for toxicity to stem from diet alone, Hathcock said.

The body can also convert precursors called carotenoids into Vitamin A. Carotenoids, which are found in carrots, spinach and many other vegetables, are absorbed through the intestine far less efficiently than retinoids. But they are not known to be toxic even when taken in large doses over long periods of time.

Much of the worry over Vitamin A’s safety arose in the mid-1980s after it became evident that the anti-acne drug Accutane, which is a powerful analog of retinol, caused birth defects if taken during the early weeks of pregnancy.

Hathcock said evidence is now “close to compelling” that daily Vitamin A consumption exceeding 100,000 IU by pregnant women can cause miscarriages and birth defects, mainly of the face and skull, in their fetuses.

For daily intakes between 20,000 and 100,000 IU, Hathcock said “modest evidence” for Vitamin A-related birth defects exists. The FDA researcher said that, in his opinion, problems likely arise “in the 25,000 IU bracket, maybe as low as 20,000 IU.”

There currently is no persuasive evidence to link birth defects to daily consumption below 20,000 IU, Hathcock said.

The FDA researcher advised pregnant women to take low- to moderate-potency vitamin supplements with Vitamin A levels in the 4,000 to 5,000 IU range. And he warned that it would be “imprudent” to take formulas that contain several times the recommended daily dosage of Vitamin A.

The Vitamin A supplements most commonly on the market range in potency from 2,500 to 20,000 IU, but there are some 25,000 IU products on the market, according to the Council for Responsible Nutrition, a trade group for the supplement industry.

The council recommends supplements formulated for use by pregnant women contain no more than 8,000 IU of Vitamin A per day. In addition, any woman who could become pregnant should not take more than 10,000 IU per day, except on the advice of a doctor, the group said.

Another recent study indicated the elderly also may be at increased risk for lower-level Vitamin A toxicity, perhaps experiencing problems from long-term use in the 5,000 to 10,000 IU per day range, Hathcock said.

But the FDA official cautioned that the liver damage found in the older people may not be a direct result of Vitamin A overload. He said Vitamin A may be only unmasking “silent,” pre-existing liver damage, such as that caused by viral hepatitis. Other factors, including alcohol and the antibiotic tetracycline, may also impair the body’s ability to handle Vitamin A, he said.

Children appear more susceptible than adults to overzealous use of Vitamin A, Hathcock said. Although safety limits for children are still up in the air, toxic effects have been reported with intakes as low as 1,500 IU per 2.2 pounds of body weight per day, the FDA said.

Vitamins designed for children generally have appropriate levels of Vitamin A, Hathcock said. But he warned that some parents may be creating problems for their youngsters by giving them adult supplements or feeding them diets with very high levels of liver.

VITAMINS ESSENTIAL TO HUMAN HEALTH Vitamin: A (retinol) What It Does and Sources: Maintains skin, eyes, urinary tract, and lining of the nervous, respiratory and digestive systems. Sources: Sweet potatoes, milk, liver, fish liver oils, butter, green and yellow vegetables. Vitamin: B-1 (thiamine) What It Does and Sources: Needed for carbohydrate metabolism and release of energy from food. Helps heart and nervous system function properly. Sources: Yeast, meat, whole-grain and enriched breads and cereals, nuts, peas, potatoes, most vegetables. Vitamin: B-2 (riboflavin) What It Does and Sources: Helps body cells use oxygen. Promotes tissue repair, healthy skin. Sources: Milk, cheese, liver, fish, poultry, green vegetables. Vitamin: Niacin What It Does and Sources: Essential for cell metabolism and absorption of carbohydrates. Sources: Liver, yeast, lean meat, whole-grain and enriched breads and cereals. Vitamin: B-6 (pyridoxine) What It Does and Sources: Needed for healthy teeth and gums, blood vessels, nervous system and red blood cells. Sources: Yeast, whole-grain cereals, meat, poultry, fish, most vegetables. Vitamin: Pantothenic acid What It Does and Sources: Helps the body convert carbohydrates, fats and proteins into energy. Sources: Egg yolk, meat, nuts, whole-grain cereals. Vitamin: Biotin What It Does and Sources: Needed for healthy circulatory system and healthy skin. Sources: Egg yolk, nuts, liver, kidney, most fresh vegetables; made by intestinal bacteria. Vitamin: Folic acid What It Does and Sources: Needed for production of red blood cells. Sources: Green leafy vegetables, yeast, meat, poultry, fish. Vitamin: C (ascorbic acid) What It Does and Sources: Helps bones and teeth. Needed for tissue metabolism, wound healing. Sources: Citrus fruits, tomatoes, raw cabbage, potatoes, strawberries. Vitamin: D What It Does and Sources: Essential for calcium and phosphorus metabolism. Sources: Fish liver oils, fortified milk, eggs, tuna, salmon, sunlight. Vitamin: E (tocopherol) What It Does and Sources: Helps prevent the oxidation of polyunsaturated fatty acids in cell membranes and other body structures. Sources: Whole-grain cereals, lettuce, vegetable oils. Vitamin: K What It Does and Sources: Needed for normal blood clotting. Sources: Leafy vegetables; made by intestinal bacteria. Source: Food and Nutrition Board, National Academy of Sciences