Key Point

In lieu of smoking cessation, smokers can protect themselves from lung cancer by smoking less and maintaining a high dietary intake of phytoestrogens.

HOUSTON AND COPENHAGEN—For smokers, kicking the habit is and always will be the best way to minimize the risk of lung cancer. But for those who cannot or will not give up cigarettes, there are other things they can do that may have positive effects on their overall health.

Cutting back is one alternative. In a large population-based study, heavy smokers who reduced their cigarette consumption by at least half were 27% less likely to develop lung cancer during the mean follow-up period of 18 years.¹

A second strategy that is gaining credence in the medical literature is a high dietary intake of phytoestrogens—plant-derived nonsteroidal compounds with weak estrogen-like activity. These compounds include phytosterols, isoflavones, cumestrans, and lignans and are present in soy products and in many fruits and vegetables. There are already a few investigations to suggest that phytoestrogens protect against lung cancer and other solid tumors.

The most recent of these was an ongoing study of 1,674 lung cancer cases and 1,735 matched controls by Schabath et al.² “Our main findings were that patients with lung cancer tended to consume lower amounts of phytoestrogens than controls, that there were sex-specific differences both in intake and in protective effects, and that the apparent benefits were evident in both never and current smokers but less so in former smokers,” the study authors related.

FEWER CIGARETTES, LESS LUNG CANCER

To determine the effect of reduced cigarette use on lung cancer incidence, Nina S. Godtfredsen, MD, PhD, and colleagues performed a population-based cohort study of 11,151 men and 8,563 women ages 20 to 93 years. Based on self-reports obtained during two examinations five to 10 years apart, these individuals were assigned to one of six groups: continued heavy smokers (those who smoked 15 or more cigarettes per day), reducers (heavy smokers who cut their cigarette use by 50% or more without quitting), continued light smokers (those who smoked one to 14 cigarettes daily), quitters (those who quit smoking between the two examinations), continued ex-smokers, and never smokers.

Eight hundred sixty-four primary lung cancers were diagnosed during follow-up: 360 in women and 504 in men. There were 229 squamous cell carcinomas, 234 adenocarcinomas, 179 small cell lung cancers, and 222 unspecified lung cancers.

Among reducers, who cut their cigarette consumption by a mean of 62% between examinations, the hazard ratio for lung cancer during follow-up was 0.73 relative to continued heavy smokers. For continued light smokers, quitters, continued ex-smokers, and never smokers, the hazard ratios were 0.44, 0.50, 0.17, and 0.09, respectively.

“Omitting the first two years after follow-up in the analyses to control for any possible ill-quitter or ill-reducer effect did not significantly change results,” the authors pointed out. “[N]either did omission of participants according to a chronic baseline disease index based on self-reported chronic respiratory conditions or hospital admission for lung disease before study entrance.”

Excluding users of noncigarette tobacco products from data analyses did not significantly change the study findings either. In a multivariate analysis, inhaling cigarette smoke, longer duration of smoking, and male gender were independent risk factors for lung cancer.

Compensatory smoking provides the most likely explanation for the 35% difference between the mean decline in cigarette use and the reduction in lung cancer risk among reducers. “There was not an equal reduction in lung cancer because these individuals probably smoked more intensely,” suggested Dr. Godtfredsen, Senior Researcher at the Copenhagen Centre for Prospective Population Studies. “I still advise people who smoke to stop completely,” she told Pulmonary Reviews, “since smoking reduction has not been shown to lessen the risks of chronic lung diseases such as COPD or of myocardial infarction.”

PHYTOESTROGEN AND LUNG CANCER

Reports suggesting that hormone therapy protects against lung cancer in women prompted Schabath and colleagues to explore the possibility of preventing such cancer with the use of phytoestrogens. For their study, records of the dietary intake of 12 different phytoestrogens were collected from participants by questionnaire and analyzed over the course of an eight-year period.

“Overall, consumption of phytoestrogens was statistically significantly higher in controls than in cases, attributed largely to variation in intake for men,” the study authors observed. “In women, only intake of total phytoestrogens from food sources was statistically significantly higher in controls than in cases.”

Phytoestrogen-related protection against lung cancer increased with each quartile of phytoestrogen intake. For those in the highest quartiles of total phytosterol, isoflavone, and lignan consumption, the odds ratios for lung cancer were 0.79, 0.68, and 0.72, respectively. The odds ratios were 0.76 and 0.54, respectively, in the highest quartiles of total phytoestrogen intake from all sources and from food sources only.

PHYTOSTEROLS BENEFIT BOTH MEN AND WOMEN

As far as affording protection against lung cancer was concerned, the beneficial effect of phytosterols and isoflavones was significant for both genders, although for phytosterols the effect achieved only borderline significance. The protection afforded by lignans was significant only for men.

“For current smokers, the highest quartiles of intake for each of the phytoestrogen groups were associated with significant reductions in risk ranging from 31% to 58%,” the authors added. “For former smokers, the protective effects were generally attenuated, and statistically significant trends were observed only for total phytoestrogens from food sources and for soy-derived isoflavones. Protective effects were evident for never smokers in all analyses, although only statistically significant trends were observed for total intake of phytoestrogens from food sources.”

Together, hormone therapy and the highest quartiles for intake of enterolactone and enterodiol (lignans) produced a 50% reduction in lung cancer risk in women versus separate risk reductions of 26% and 27%, respectively. A similar but lesser association was observed between hormone therapy and the isoflavones daidzein and genistein.

STOP SMOKING AND EAT YOUR FRUITS AND VEGETABLES

In an editorial,³ Dacey and Johnstone advocated smoking cessation as the best and most effective way to reduce harm from tobacco. “Nonetheless, because only a minority of smokers are able to quit smoking completely, it is important to inform them that the more they can reduce the number of cigarettes they smoke, the more they will decrease their risk of lung cancer,” the two physicians said. “[P]atients should be informed that they may further reduce their risk of developing cancer by adopting a diet rich in fruits and vegetables,” they added.

—Timothy Begany

References
1. Godtfredsen NS, Prescott E, Osler M. Effect of smoking reduction on lung cancer risk. JAMA. 2005;294:1505-1510.
2. Schabath MB, Hernandez LM, Wu X, et al. Dietary phytoestrogens and lung cancer risk. JAMA. 2005;294:1493-1504.
3. Dacey LJ, Johnstone DW. Reducing the risk of lung cancer. JAMA. 2005;294:1550-1551.

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