What are the side effects of too much carotene?

Most people with vitamin A toxicity have a headache and rash.

Consuming too much vitamin A over a long period of time can cause coarse hair, partial loss of hair (including the eyebrows), cracked lips, and dry, rough skin. Chronic consumption of large doses of vitamin A can cause liver damage. It can also cause birth defects in a fetus.

Later symptoms include severe headaches and general weakness. Bone and joint pain are common, especially among children. Fractures may occur easily, especially in older people. Children may lose their appetite and not grow and develop normally. Their skin may itch. The liver and spleen may enlarge.

Consuming very large amounts of vitamin A all at once can cause drowsiness, irritability, headache, nausea, and vomiting within hours, sometimes followed by peeling of the skin. Pressure within the skull is increased, particularly in children, and vomiting occurs. Coma and death may occur unless vitamin A consumption is stopped.

Taking isotretinoin (a vitamin A derivative used to treat severe acne) during pregnancy may cause birth defects. Women who are or who may become pregnant should not consume vitamin A in amounts above the safe upper limit (3,000 micrograms) because birth defects are a risk.

By Fred Decker Updated December 02, 2018

The human body requires vitamin A to operate properly, and like many vitamins it must be drawn from food-based sources. Animal-based foods such as liver supply vitamin A in ready-to-use form, while plant-based foods provide raw materials -- beta carotene and other carotenoids -- that your body uses to manufacture vitamin A. Vitamin A can be dangerous in excess, but beta carotene from dietary sources is essentially harmless. Beta carotene from supplements is more concentrated and can be problematic.

If you eat a diet high in naturally occurring beta carotene from yellow vegetables such as carrots, sweet potatoes and winter squash or from green vegetables such as broccoli and chard, you might experience an odd result. High levels of beta carotene in your diet can give your skin a curiously orange tint, similar to the effect of an artificial-suntan product. It's not harmful, but unless you plan to impersonate a carrot or pumpkin at Halloween, it isn't especially desirable. The effect will subside if you reduce your consumption of beta carotene from food or supplements or both.

Several studies over the years have noted that people eating diets high in beta carotene showed lower incidence of some cancers, especially lung cancers. That doesn't hold true for people taking beta carotene supplements. In fact, the landmark 1996 Carotene and Retinol Efficacy Trial, or CARET -- intended to verify the effectiveness of supplements in preventing cancer -- found that current and former smokers taking beta carotene supplements actually showed an increased risk of contracting the disease.

Although dietary beta carotene is relatively innocuous, the concentrated form found in supplements can have undesirable interactions with specific medications. The University of Maryland Medical Center notes that beta carotene, taken in conjunction with vitamins E and C and the mineral selenium -- all found in multivitamins -- can reduce the effectiveness of some statins. It can also accelerate liver damage when combined with excessive alcohol consumption. Habitual drinkers, or those taking statins, should consult a physician before taking beta carotene supplements.

Your total consumption of vitamin A and beta carotene, from dietary sources or supplements, should be keyed to recommended dietary allowances and kept below the acceptable upper limit, or UL. For adults, the RDA of preformed vitamin A is 3,000 international units and the UL is 10,000 IU. The RDA for vitamin A is equal to 6,000 IU of supplemental beta carotene and 18,000 IU of dietary beta carotene.

Fred Decker is a trained chef and certified food-safety trainer. Decker wrote for the Saint John, New Brunswick Telegraph-Journal, and has been published in Canada's Hospitality and Foodservice magazine. He's held positions selling computers, insurance and mutual funds, and was educated at Memorial University of Newfoundland and the Northern Alberta Institute of Technology.

New research suggests that there could be health hazards associated with consuming excessive amounts of beta-carotene. This antioxidant is a naturally occurring pigment that gives color to foods such as carrots, sweet potatoes and certain greens. It also converts to vitamin A, and foods and supplements are the only sources for this essential nutrient.

But scientists at Ohio State University have found that certain molecules that derive from beta-carotene have an opposite effect in the body: They actually block some actions of vitamin A, which is critical to human vision, bone and skin health, metabolism and immune function.

Because these molecules derive from beta-carotene, researchers predict that a large amount of this antioxidant is accompanied by a larger amount of these anti-vitamin-A molecules, as well.

Vitamin A provides its health benefits by activating hundreds of genes. This means that if compounds contained in a typical source of the vitamin are actually lowering its activity instead of promoting its benefits, too much beta-carotene could paradoxically result in too little vitamin A.

The findings also might explain why, in a decades-old clinical trial, more people who were heavily supplemented with beta-carotene ended up with lung cancer than did research participants who took no beta-carotene at all. The trial was ended early because of that unexpected outcome.

The scientists aren't recommending against eating foods high in beta-carotene, and they are continuing their studies to determine what environmental and biological conditions are most likely to lead to these molecules' production.

"We determined that these compounds are in foods, they're present under normal circumstances, and they're pretty routinely found in blood in humans, and therefore they may represent a dark side of beta-carotene," said Earl Harrison, Dean's Distinguished Professor of Human Nutrition at Ohio State and lead author of the study. "These materials definitely have anti-vitamin-A properties, and they could basically disrupt or at least affect the whole body metabolism and action of vitamin A. But we have to study them further to know for sure."

The study is scheduled for publication in the May 4, 2012, issue of the Journal of Biological Chemistry.

Previous research has already established that when beta-carotene is metabolized, it is broken in half by an enzyme, which produces two vitamin A molecules.

In this new study, the Ohio State researchers showed that some of these molecules are produced when beta-carotene is broken in a different place by processes that are not yet fully understood and act to antagonize vitamin A.

Harrison is an expert in the study of antioxidants called carotenoids, which give certain fruits and vegetables their distinctive colors. Carotenoids' antioxidant properties are associated with protecting cells and regulating cell growth and death, all of which play a role in multiple disease processes.

For this work, he joined forces with co-authors Robert Curley, professor of medicinal chemistry and pharmacognosy, and Steven Schwartz, professor of food science and technology, both at Ohio State. Curley specializes in producing synthetic molecules in the pursuit of drug development, and Schwartz is an expert at carotenoid analysis.

Curley manufactured a series of beta-carotene-derived molecules in the lab that match those that exist in nature. The researchers then exposed these molecules to conditions mimicking their metabolism and action in the body.

Of the 11 synthetic molecules produced, five appeared to function as inhibitors of vitamin A action based on how they interacted with receptors that would normally launch the function of vitamin A molecules.

"The original idea was that maybe these compounds work the way vitamin A works, by activating what are called retinoic acid receptors. What we found was they don't activate those receptors. Instead, they inhibit activation of the receptor by retinoic acid," Curley said. "From a drug point of view, vitamin A would be called an agonist that activates a particular pathway, and these are antagonists. They compete for the site where the agonist binds, but they don't activate the site. They inhibit the activation that would normally be expected to occur."

Once that role was defined, the researchers sought to determine how prevalent these molecular components might be in the human body. Analyzing blood samples obtained from six healthy human volunteers, the scientists in the Schwartz lab found that some of these anti-vitamin-A molecules were present in every sample studied, suggesting that they are a common product of beta-carotene metabolism.

The compounds also have been found previously in cantaloupe and other orange-fleshed melons, suggesting humans might even absorb these molecules directly from their diet.

Harrison noted that the findings might explain the outcome of a well-known clinical trial that has left scientists puzzled for years. In that trial, people at high risk for lung cancer -- smokers and asbestos workers -- were given massive doses of beta-carotene over a long period of time in an attempt to lower that risk. The trial ended early because more supplemented participants developed cancer than did those who received no beta-carotene. This outcome was reinforced by results of a follow-up animal study.

"Those trials are still sending shockwaves 20 years later to the scientific community," said Harrison, also an investigator in Ohio State's Comprehensive Cancer Center. "What we found provides a plausible explanation of why larger amounts of beta-carotene might have led to unexpected effects in these trials."

The research also has implications for efforts to bio-engineer staple crops in developing countries so they contain excess beta-carotene, which is considered a sustainable way to provide these populations with pro-vitamin A. Existing projects include production of golden rice in Asia, golden maize in South America and cassava in Africa.

"A concern is that if you engineer these crops to have unusually high levels of beta-carotene, they might also have high levels of these compounds," Harrison said.

The researchers are continuing to study these compounds, including whether food processing or specific biological processes affect their prevalence. Previous studies have suggested that oxidative stress, which can result from smoking and air pollution exposure, can lead to higher production of these anti-vitamin-A molecules, Harrison noted.

This research was supported by the National Institutes of Health and the Ohio Agricultural Research and Development Center.

Additional co-authors include Abdulkerim Eroglu, Carlo dela Sena and Sureshbabu Narayanasamy of the Department of Human Nutrition; Damian Hruszkewycz of the College of Pharmacy; and Ken Riedl and Rachel Kopec of the Department of Food Science and Technology, all at Ohio State. Harrison, Curley, Eroglu and dela Sena also are affiliated with Ohio State's Biochemistry Program.

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Materials provided by Ohio State University. Note: Content may be edited for style and length.