Beta-carotene, smoking and cancer: what two trials found

Why were these supplements tested?

Researchers had observed that people eating more fruit and vegetables often had lower rates of lung cancer. That association did not prove beta-carotene caused the difference: diets, smoking and many other factors could differ too. Researchers therefore tested concentrated supplements in randomized trials. A trial can test a particular product and population; it cannot establish that a whole food has the same effect.58

ATBC: what happened among Finnish men who smoked?

ATBC randomly assigned 29,133 men aged 50–69 who smoked to beta-carotene (20 mg a day), vitamin E (50 mg a day), both, or placebo. These were study doses, not recommendations. The beta-carotene comparison combined men assigned it with or without vitamin E and compared them with those not assigned it. Over 5–8 years, lung cancer was diagnosed in 474 of 14,560 assigned beta-carotene (about 3.3%) and 402 of 14,573 not assigned it (about 2.8%). The original report found an 18% higher relative incidence with beta-carotene.

The approximately 0.5 percentage-point difference between those observed proportions is not the same as an 18 percentage-point increase. Follow-up time varied, and these are group results, not an individual forecast.12

CARET: why was the second trial different?

CARET enrolled 18,314 US participants at high lung-cancer risk because of smoking history or asbestos exposure. It randomly assigned a combination of beta-carotene (30 mg a day) and preformed vitamin A as retinyl palmitate (25,000 IU a day), or placebo. These were study doses, not recommendations. After roughly four years on average, the lung-cancer incidence rate was 28% higher relative to placebo, and overall mortality was also higher. An evidence review reports 229 cases among 9,420 assigned the combination (2.4%) and 159 among 8,894 assigned placebo (1.8%); these observed proportions and the relative rate use different calculations. The intervention stopped early.

Because both nutrients were given together, CARET cannot tell us how much of the difference came from either ingredient alone. ATBC provides separate evidence about beta-carotene in men who smoked. Neither number predicts one person’s outcome.752

Does this mean carrots are dangerous?

No. These studies tested concentrated supplements, not carrots, leafy greens or ordinary meals. Beta-carotene in food is part of a food matrix, not a trial capsule; the trial results should not be turned into a warning against vegetables. On the other hand, the observational association that motivated the trials does not prove that any single food prevents cancer. See how plant pigments and preformed vitamin A differ.18

What if I smoke or used to smoke?

In its 2024 summary, the European Food Safety Authority advises people who smoke to avoid beta-carotene supplements; it could not set a tolerable upper intake level for beta-carotene because adequate data were lacking. An absence of a set limit does not establish a safe supplement amount. The US National Eye Institute advises current and former smokers to avoid the original beta-carotene-containing AREDS eye formula. See the difference between AREDS and AREDS2.

If a product was recommended for a diagnosed condition, check its exact ingredients and discuss alternatives with the clinician or pharmacist who advised it. Do not assume these historical trial doses establish a personal safe threshold.

What if I took one in the past? Researchers continued following CARET participants after the study supplements were stopped. Over six years of post-intervention follow-up, the estimated lung-cancer rate remained higher in the group originally assigned the combination, but the uncertainty interval included no difference. This does not establish that the risk disappeared, determine when it might change, or predict an individual outcome. CARET tested beta-carotene and retinyl palmitate together. If you are concerned, bring the product name, ingredients and approximate period of use to a clinician.4910

Can a supplement prevent cancer?

The US Preventive Services Task Force recommends against beta-carotene supplements for prevention of cancer or cardiovascular disease in community-dwelling, nonpregnant adults. This prevention recommendation does not cover treating a known deficiency. Quitting smoking lowers lung-cancer risk over time; a supplement is not a substitute. A healthcare professional can discuss support and, where appropriate, screening.36

What should I check on a label?

Some multivitamins and older eye formulas list beta-carotene as an ingredient or source of vitamin A. Names and formulations differ, so look at the actual ingredient panel and serving size. A regular supplement, a prescribed treatment for a deficiency, and a studied eye-disease formulation answer different questions. Bring the packaging and medicine list when asking for advice. Read how to interpret vitamin A labels or return to the vitamin A overview.49