Smoking

At present in the UK about 25% of adults still smoke.  Tobacco smoke is a major cause of lung disease and lung cancer.  Over 25 years ago it was observed that the risk of lung cancer was lower in those with higher intakes and blood levels of certain nutrients. Attempts to lower the risk of cancer developing in smokers by use of high dose supplements have been unsuccessful and in some instances have resulted in an increase in the risk of developing cancer.

Beta-carotene
Several large trials using very high doses, 20 to 30 mg per day, of beta-carotene in smokers showed either no reduction or an increase in the risk of cancer developing.  As a result of these trials the Expert Committee on Vitamins and Minerals advises that “.. as a matter of prudence, smokers or those exposed to asbestos should not take beta-carotene supplements.”  
A recent report from the World Cancer Research Foundation and the American Institute for Cancer Research found that the evidence linking beta-carotene to cancer in smokers is “convincing”. As a result the Centre for Science in the Public Interest in December 2007 asked the Food and Drug Administration in the US to require manufacturers of dietary supplements that contain large doses of beta-carotene to warn smokers or people exposed to asbestos of an increased risk of lung cancer if they take these supplements.  A number of US manufacturers, some of whom have a presence in the UK, were named and shamed.
www.cspinet.org/new/200712051.html
www.dietandcancerreport.org/?p=recommendations
Cancer Research UK www.cancerhelp.org.uk/help/default.asp?page=9973 and
http://info.cancerresearchuk.org/cancerstats/types/lung/riskfactors/

Vitamin A
Supplements of retinol were not associated with an increased risk of cancer in the trials involving smokers but in other trials there does appear to have been an increase in cancer rate overall and thus it would be prudent for smokers to not exceed the Safe Upper Level of 1500 ug per total intake, which typically means not exceeding 800 ug per day from supplements.