We all know we should eat lots of fruits and vegetables. I do it because I happen to like fruits and vegetables, but there are good health reasons, too. But my father always said that with diet as with life, "Everything in moderation." A whole lot of fruit and vegetables is not necessarily a whole lot better than just enough. It may not be better at all, and if they take the place of other sources of essential nutrients, maybe worse.
First a disclaimer. I'm a physician. By definition that means I know practically nothing about nutrition or diet and don't even know what I don't know. But I do read about it and one of the things I read lately confirms that moderation is reasonable and immoderation probably not helpful:
Though a low-fat, high-fiber diet rich in fruit and vegetables has been associated with decreased cancer risk, adding more produce to one's diet may not offer a corresponding benefit, new research suggests.Women in the early stages of breast cancer who ate substantially more than five servings a day of fruit and vegetables did not have an advantage over their counterparts who followed the five-a-day plan when it came to progression of the disease.
[snip]
The aim of the study, which was carried out at the University of California, San Diego, was to determine whether a diet very high in veggies, fruit and fiber and low in fat would lower three types of risk among women who had been treated for early stage breast cancer:
- risk of recurrence
- risk of new primary breast cancer
- risk of death from all causes
John P. Pierce, PhD, of UCSD and colleagues conducted the randomized controlled trial with 3,088 women between 18 and 70 years old who were previously treated for early stage breast cancer. (Health Daily)
The intervention group got phone counseling, cooking classes and other encouragement and educational assistance to increase their fruit and vegetable use and in fact ate 65 percent more vegetables, 25 percent more fruit, 30 percent more fiber and 13 percent less fat than the group not receiving these interventions. But it didn't make any difference in death rates in these breast cancer patients, which is a shame on several levels, not only the obvious one of a real benefit but also the loss of an empowering tool for patients who need some way to take control of their disease. But the results seem pretty clear.
Now these were people who already had cancer. Maybe high fiber and lots of fruit would prevent you from getting cancer in the first place. Well of course they will. Everyone knows that. The idea that a high fiber diet might prevent colon cancer is not new. It was suggested many years ago by observations of a low colon cancer risk among South Africa's Zulus, who also ate a lot of fiber. It turns out I once knew one of the anthropologists involved in that work (now deceased) and he told me that it was a very striking observation he felt confident about. Then he added that he didn't think Americans knew exactly what a Zulu high fiber diet meant. Apparently (according to him) the Zulu ate so much fiber they had seven or eight gigantic bowel movements every day. He chuckled and said he didn't think the American public was quite ready for that.
As my Dad used to say, "Everything in moderation."
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I just finished up doing some research for an NIH grant proposal for a client on the use of FV in school interventions to help with obesity. The evidence there is A/B, and better if cojoined with another intervention--say exercise. However, the evidence that FV prevents cancer is weak. Part of that result is due to the fact that it's terribly difficult to create an experimental design that takes account of all the cofounding factors. Moreover, the trial has to run several years to get a reasonable effect size. Then you have the problem of self-reporting and people dropping out or not complying and telling the experimenters.
Marissa: Quite right. We read a lot of nutritional epidemiology (it makes good news stories because we all eat and can relate to the subject matter) but actually doing nutritional epidemiology is another matter. It is incredibly difficult and the results very uncertain. For a disease like cancer which takes many years to develop, doing a case control study (comparing diets of people who get cancer with those who don't) is very difficult because few people can remember with any accuracy what they ate 20 years previously. On the other hand if you ask people what they eat now, you have to wait years and have lots of people to see if there is a difference in cancer between those who eat a lot of fruits and vegetables and those who don't and people change their habits over time as the food supply changes, their incomes and circumstances change and the knowledge and attitudes of the society (and media) change, all of which affects the market, etc.
So instead of following the ups and downs of nutritional epidemiology, I just follow my father's advice: Everything in Moderation.
Speaking without working in this field, diet studies for associations with cancer would seem to be especially subject to uncontrolled confounding.
The literature is full of diet studies where the supposed protective component, when given directly, increased cancer risk. A prominent example is the CARET trial, of smokers and smokers exposed to asbestos, where carotene increased cancer risk
http://jnci.oxfordjournals.org/cgi/content/abstract/88/21/1550
However, the non-intervention arm of the study provided useful data. Asbestos exposure, rather than chest x-ray evidence of asbestos, was a better predictor of future cancer risk.