Back in 1994, a study called ATBC with 29,000 male smokers tested whether beta-carotene (an antioxidant) and vitamin A as a supplement could prevent lung cancer. In 1996, another study called CARET gave 18,000 male and female smokers and male asbestos workers beta-carotene supplements for four years to test the same theory. Nearly everyone thought it would work, bu at they were wrong. The studies were halted because lung cancer, heart disease, and death from all causes increased in those who took high doses of beta -carotene. When looking at follow-up data, it was found that in smokers, the higher risk of lung cancer and death continued even after a person stopped taking beta-carotene, suggesting long-term effects from the supplementation.
Perhaps research has found a possible answer to these surprising results that shocked the nutrition world at that time. There are other theories however.
The same effect has not been shown from getting of beta-carotene in your diet from fruits and vegetables, even if you are a smoker. The bottom line: A lifetime of healthy eating is far better than a few years of high-dose vitamins found in supplements during middle age.