In the War Against Cancer, the Latest Weapons Are Fruits and Vegetables
Can cancer be traced to diet?
Recently two respected British researchers suggested that diet might be responsible for 35 percent of all cancers. The number was a guesstimate; they said it might be as low as 10 percent or as high as 70 percent. We can't improve on those numbers. But it looks like diet can influence most cancers, simply by delaying the appearance of the tumor.
What factors are involved?
We don't know exactly how much the cancers can be delayed. But there are some very important correlations. When fat intake goes up, cancers of the colon, breast and prostate go up. Of all the dietary components we studied, the evidence for a causal relationship between fat intake and cancer risk was most persuasive.
"Delaying the appearance of the tumor" sounds fatalistic.
In a sense, it is. Many now believe, and I among them, that most of us already have enough cancer cells in our body to give us cancer someday, though for many of us not in our normal lifetime. Roughly speaking, cancer has two phases of onset: initiation and promotion.
How do these phases work?
Initiation can happen in days, hours or even minutes when a carcinogen comes in and binds itself to the DNA of a cell, mutating it. This infant cancer cell doesn't do us any damage. It can sit for years before it multiplies to where a pathologist says you've got cancer. We're beginning to identify things that inhibit this promotion process and others that enhance it. This is where nutrients come into play.
Is this a new turn in cancer research?
Yes. If you go back 20 or 30 years, much work was on identifying carcinogens. Lots was done to see if viruses caused cancers, but that hasn't been too significant of late. With nutrition involved in such a major way, how many infant cancer cells we have may matter less than what we do to them.
How do fats promote cancer?
There are various hypotheses. We don't know enough about any one of them. For colon cancer we do know that higher fat intake produces a higher level of certain bile acids in the gut. It turns out a couple of these bile acids are pretty good tumor promoters. If you give them to lab animals that you've given some carcinogens to, their chance of getting cancer is quite a bit greater.
The report suggests polyunsaturates are not as harmless as people think.
When polyunsaturates are consumed as part of a high total fat intake—which I would suggest is anything above 20 percent of total calories—there's evidence they can turn on the cancer promotion process. To lower their serum cholesterol levels, people have been substituting polyunsaturates without paying much attention to their total fat intake. They might be lowering their risk of heart disease to some extent, but they're increasing their risk of cancer. A safer bet is just to eat less fat.
How do you respond to the American Meat Institute, which called the NAS report "misleading," or the National Cattlemen's Association, which called it "inconclusive and premature"?
First, evidence in science is never conclusive. Biology is far too complex. If we waited for all the information, we'd be sitting around here for another 500 years. But the relationship between diet and cancer, in my opinion, is now more persuasively established than the one between diet and heart disease, a connection that has been suggested for 25 years or more.
How did the committee get its guidelines for lowering fat consumption?
We decided to come up with a reasonable, practical number, something the whole population might work toward. So we recommended a reduction in fat intake from a current 40 percent of total calories to 30 percent, although I would suggest getting it down to about 20 percent. In China, where I was in June, it's only 9 percent. So you can go down to 20 percent and not experience problems.
How can people cut down on fat?
People can drop from 40 to 30 percent probably just by eating less fried food, less salad oil, less butter, trimming fat off meat, stuff like that. But people who are at 45 to 55 percent are going to have to make some basic changes in their food selection.
Why does the report urge people to eat more fruits and vegetables?
There are about 18 studies that show that as intake of a substance called beta-carotene goes up, risk of various cancers goes down. That even includes lung cancer.
What is beta-carotene?
Beta-carotene is one of a group of yellow-colored compounds called carotenoids found in carrots, sweet potatoes, deep yellow and dark green vegetables and members of the cabbage family such as broccoli, cauliflower, brussels sprouts and kale.
How does beta-carotene work?
After it is ingested, it is converted into vitamin A, which can inhibit the cancer process. There's also evidence that this is true for food containing vitamin C, which is found in citrus fruits and vegetables like tomatoes and peppers. Also, members of the cabbage family may contain some anticancer substances other than beta-carotene.
Why, then, did the report discourage taking vitamin supplements?
Vitamin A is a relatively toxic compound. Beta-carotene is not. Second, in taking supplements we may miss the multitude of actions provided by a correct choice of food.
You're not claiming this diet, or any of its components, can guarantee you won't get cancer, or cure it, are you?
No. I don't even come close to understanding the complexities of nature. I worry about emphasizing any individual nutrient. The message here is collective nutrient action.
Why are whole grains recommended?
They are a good source of nonprotein, fat-free energy, and the fiber they contain may prove beneficial.
Why did the committee make no specific recommendation about dietary fiber?
It's been a popular notion inside and outside the scientific community that fiber was associated with a lower risk for colon cancer and maybe some other cancers. But when we examined that literature closely, we weren't convinced the data were all that conclusive. Not that dietary fiber doesn't work the way it is alleged to work—that is, possibly, by diluting carcinogens in the bowel, insulating them from bowel tissues, or just speeding the stool along. Rather, dietary fiber is a complex mixture, and different components seem to have different capabilities. We know so little about them.
Why was there no warning in the report about food additives?
The same British report that said diet could be responsible for up to 70 percent of cancers suggested that less than 1 percent of all cancers were related to food additives. We couldn't find any evidence that additives have anything to do with cancer in people.
Is the report's warning against salt-cured and salt-pickled foods an indictment of salt itself?
No. When we say salt-cured, we primarily mean potassium nitrate, or saltpeter, which has been used for centuries to keep down bacterial growth. In the stomach, some of the nitrate is reduced to nitrite by bacteria under certain conditions. The nitrites may combine with amines—prevalent chemicals in food—to form nitrosamines, which cause stomach cancer. In the U.S., the incidence of stomach cancer in the last 40 years has come down dramatically. We don't really know why, but the hypothesis is that a generation ago we were consuming a lot more nitrate-cured meats because refrigeration was less widespread.
What do you and your family eat?
Our consumption of animal products is relatively low. We tend to emphasize fruits, vegetables and whole grains, and have for some time. That doesn't mean we avoid anything. It's okay to put butter on your baked potatoes, just use it modestly.
So this report needn't panic people?
Right. In the last 20 years we've had too many scare stories, and many of them don't mean a diddly squat, quite frankly. But this report is very positive. It's saying the big factors that affect cancer risk largely involve nutrients. You're out there on a limb. By shifting these food choices, not only for cancer but for general good health, you can come back in a little bit.
On Newsstands Now
- Kim's Delivery Room Drama!
- Katie: A Year After Split
- Princess Kate: Palace's Baby Plan Revealed
Pick up your copy on newsstands
Click here for instant access to the Digital Magazine