Two registered dietitians from Harvard-affiliated hospitals tout the benefits of fruits, vegetables, and whole grains
“What can I eat to reduce my risk of developing prostate cancer?” That’s one of the most common questions physicians hear from men concerned about prostate health. Undoubtedly, many hope to hear their doctor rattle off a few foods guaranteed to shield them from disease. Although some foods have been linked with reduced risk of prostate cancer, the proof is lacking, at least for now.
What dietitians, physicians, and researchers can say — and there’s plenty of evidence to support this statement — is that a balanced, healthy diet rich in fruits, vegetables, and whole grains remains your best bet. In fact, given that most men with prostate cancer now die with their disease, not from it, and that heart disease ranks as the nation’s No. 1 killer, sticking to a heart-healthy diet can help increase survival. Admittedly, you might want to tweak your eating plan to suit your particular tastes and medical concerns (allergies, for example), but Mom was right: fruits, vegetables, and that loaf of whole-wheat bread are good for you.
But just how much broccoli do you need to put on your plate? Do you have to buy organic fruits? What’s wrong with white bread? And why can’t you simply take a supplement to get the vitamins and minerals your body needs? To find out, Harvard editors invited two registered dietitians from Harvard-affiliated hospitals to share their expertise:
Stacy Kennedy, M.P.H., R.D., L.D.N., senior clinical nutritionist at Dana-Farber Cancer Institute. She is most interested in weight management, physical activity, integrative therapies, and plant-based diets.
Kathy McManus, M.S., R.D., L.D.N., director of the Department of Nutrition at Brigham and Women’s Hospital. During her career, she has been a co-investigator on a number of obesity research trials, presented her research in national and international forums, and written numerous publications and book chapters.
When people talk about eating for prostate health, what do they mean? Is there a specific “prostate diet,” or should people simply stick to a healthy diet in general?
KENNEDY: There is not a prostate diet per se. But there is a healthy diet. We can talk about diet for a whole host of diseases — heart disease, cancer, diabetes, metabolic syndrome, high blood pressure, and other chronic diseases — but there is a general healthy way of eating. That’s what we’re trying to promote. There can be some nuances. Evidence might show this, that, or the other may help decrease the risk of prostate cancer and other prostate disorders, but there really isn’t one thing or one prescribed diet. Patients should pay attention to their overall eating pattern, not a few particular foods.
Don’t fruits and vegetables help protect against disease?
KENNEDY: A plant-based diet, which includes not only fruits and vegetables, but also other things like nuts and seeds, whole grains, spices, and seasonings, has been shown to help decrease the risk of developing cancer when consumed consistently. Can we say that eating fruits and vegetables is certainly going to prevent an individual from getting prostate cancer? No. But it is one way of helping to reduce the risk. Fruits and vegetables are also beneficial because they contain lots of nutrients that are important for overall health and wellness.
How many servings of fruits and vegetables should someone eat in a day?
KENNEDY: Our recommendation is five to 10 servings a day. [See “What counts as a serving?”]
Ten servings? That sounds like a tremendous amount.
KENNEDY: It does. It really does.
McMANUS: Well, let’s explain what we mean by a “serving.” A serving is a cup of leafy greens. So if you have a salad, you might have three cups, or three servings, of vegetables right there. A serving is also a half-cup of cooked vegetables or a medium-sized piece of fruit or a cup of berries.
KENNEDY: A medium-sized banana is actually a serving, too. So if you make a smoothie in the blender and put in a banana and a cup of blueberries, you’ve got two servings right there. Then you make a salad with two cups of greens, throw on a half-cup of carrots and a half-cup of peppers, and the number of servings is quickly going up.
McMANUS: The other important message here is that color is an indicator of phytonutrient content.
What are phytonutrients?
KENNEDY: Phytonutrients are plant-based nutrients. It’s really an umbrella term for all of the good things that are found in plant foods.
McMANUS: The deeper the color, the more vibrant the color, supposedly, the higher the phytonutrient content. For example, lettuce that’s deep green has more nutrients than iceberg lettuce.
KENNEDY: People have probably seen fancy words like “flavonoids” and “lycopene” when they read about fruits and vegetables. These are considered phytonutrients.
What counts as a serving?
Many organizations and government agencies publish guidelines on what constitutes a serving. The USDA guidelines are the most widely accepted. Here’s how they define serving sizes for the food groups in this article:
1 slice of bread
1 cup of ready-to-eat cereal
½ cup of cooked cereal, rice, or pasta
1 medium apple, orange, or banana
½ cup of chopped, cooked, or canned fruit
¾ cup of fruit juice.
Should people eat specific types of fruits and vegetables? Or does that not matter much beyond substituting a darker lettuce for iceberg lettuce?
KENNEDY: I think there are two key points here. One is quantity. We definitely want fruits and vegetables to be more prominent in our diet — in our meals and in our snacks. In addition, eat “by the rainbow.” That means choosing things that are red, orange, yellow, dark green, blue, and purple, as well as things that are white such as garlic, onions, and cauliflower. You want to have the entire color spectrum because each color represents different and important nutrients for reducing disease risk. There are other groups of vegetables that aren’t sorted according to color. For example, there’s some preliminary research showing that cruciferous vegetables — broccoli, cabbage, cauliflower, Brussels sprouts, radishes, bok choy, and kale — may help to decrease the risk of aggressive prostate cancer.
What about tomatoes? Some studies have shown that tomatoes, which contain the nutrient lycopene, might help prevent cancer; others have come to the opposite conclusion. What do you recommend to patients?
KENNEDY: Lycopene may or may not turn out to be a nutrient that is particularly important when it comes to prostate cancer. But there are many beneficial nutrients in tomatoes besides lycopene. You can get folic acid, you can get vitamin C. There are lots of good things in tomatoes. Because we know tomatoes are a healthy food for several reasons, I think it would make sense to try to include some cooked tomatoes and tomato sauce in your diet if you’re at risk for prostate cancer or if you’re a prostate cancer survivor. I don’t think we can conclusively say, “You’re definitely going to benefit from eating tomatoes,” but we can say that eating more tomatoes isn’t going to be harmful.
Should the tomatoes be cooked? In general, is it better to eat cooked or raw fruits and vegetables?
KENNEDY: That’s a great question. With tomatoes, the body absorbs more lycopene if they are cooked in the presence of a healthy fat. But in general, you’ll absorb or get different nutrients depending on whether a vegetable is cooked or eaten raw. We certainly don’t want people overcooking vegetables, or fruits for that matter, because that degrades the nutrients. Eating fruits and vegetables lightly cooked or eating them raw are both good choices, but for different reasons.
McMANUS: I don’t think we want people to get too hung up on whether vegetables should be eaten raw or cooked. Our main goal is to get people to eat more fruits and vegetables. How they’re prepared is less important. Mix it up. Salad is obviously raw vegetables, but you could have some roasted vegetables, or you could stir-fry them in a little bit of olive oil and garlic. Make them appealing. Make them taste good. That will really help people eat more of them. That’s our main message.
What strategies can our readers use to work more fruits and vegetables into their diet?
McMANUS: Well, part of it is taking small steps. When you tell people to get five to 10 servings a day, they can feel overwhelmed. So you have to think about it in simple ways. Add extra diced vegetables into soups, casseroles, and spaghetti sauce. That’s an easy way to do it. If you’ve got something cooking in the oven, throw in some vegetables. If you’re roasting a chicken, throw in some carrots, sweet potatoes, peppers, and onions. That’s quick and easy. Try a stir-fry. Make vegetable stews and veggie wraps. Those are great ways to really pack in extra vegetables without a whole lot of extra effort.
People always talk about having something sweet for dessert. A great way to do that is to choose fruit — berries, melon, or clementines. Think about fruit as a great way to end a meal.
In between meals, pick some favorite vegetables and have them with hummus, black bean dip, salsa, or light dressing. That can be a good way to get in some extra vegetables. Try the same thing with fruit. Slice up some fruit and dunk it in yogurt or put a little bit of peanut butter or almond butter on it. Smoothies are a great way to eat on the run. If you want to have a quick and healthy breakfast, you could blend some yogurt and fruit.
Is there any difference between organic and conventional produce? Is there any benefit to eating organic produce?
KENNEDY: Well, the research on that is still very new. We cannot say conclusively that choosing organic necessarily translates into any health benefit. The evidence certainly supports eating more fruits and vegetables — the average American eats two to two-and-a-half servings a day; we’re recommending five to 10. The studies that have been done haven’t really looked at organic versus conventional produce; they’ve looked at fruits and vegetables over all. Certainly if you have access to organic foods, can afford them, and believe that they have some benefit, we would be supportive of that choice. It’s really a personal decision.
The “dirty dozen”
If you’re worried about pesticide residue on produce but can’t afford to purchase every organically grown food, limit your organic buying to these 12 fruits and vegetables. When grown conventionally, these items tend to have the highest levels of pesticides on them:
bell peppers (red and green)
What about the pesticides used to grow conventional produce?
KENNEDY: There’s a list that’s referred to as the dirty dozen, the 12 fruits and vegetables that tend to carry the most pesticides. [See “The dirty dozen.”] If you eat some of the fruits and vegetables every day that are on the list, you might want to look at organic options. So if you’re eating conventional spinach and strawberries all the time and you’d like to switch to organic without spending a lot, you can try frozen varieties.
That having been said, there’s certainly no evidence that we need to choose organic. When you look at the evidence, maintaining a healthy weight, engaging in physical activity, and eating a plant-based diet far outweigh the theoretical concerns about organic versus conventional foods.
Can’t people just take supplements to get the nutrients they need?
KENNEDY: Research from the World Cancer Research Fund and the American Institute for Cancer Research supports getting nutrients primarily from foods, rather than looking to supplements to prevent cancer. Of course, there are reasons why an individual might want to take a supplement. For example, a person with a low blood level of vitamin D might want to take vitamin D to help correct the deficiency. That’s different than grabbing supplements as a panacea and taking them in place of eating a healthy diet.
Eat the W-H-O-L-E grain
What’s the difference between refined and whole grains?
McMANUS: Refined grains and carbohydrates are easily and rapidly absorbed into the bloodstream, which leads to a quick rise in blood sugar. At the same time, there’s a rise in insulin, which can actually drive the blood sugar, or blood glucose, too low. When that happens, the gut and brain start sending out hunger signals, and the inclination is to grab more food and potentially overeat. In the long run, we think this constant barrage of spikes in blood sugar and insulin production can lead to type 2 diabetes. Whole grains help limit the dramatic peaks and valleys in blood sugar, so insulin production is steadier [see Figure 1].
Fiber is another benefit of whole grains. There are good data from both the Nurses’ Health Study and the Health Professionals Follow-up Study showing that men and women who ate the most cereal fiber from grains — about 7.5 grams a day, which is equivalent to a bowl of oatmeal and a couple of pieces of whole-grain bread — were 30% less likely to develop type 2 diabetes than those who didn’t eat much fiber. Other data show that those who eat the most whole grain foods, an average of two-and-a-half servings a day, are 30% less likely to develop heart disease than those who eat few whole-grain foods. So you can see that whole grains are beneficial. We also know that whole grains and high-fiber foods can help improve gastrointestinal health and help prevent constipation. Data linking whole grains and cancer prevention aren’t really there, but we’re looking at the entire diet and how it can help prevent chronic disease in general. That’s extremely important.
Figure 1: Digestion and glucose absorption
Type 2 diabetes develops when the normal process of glucose metabolism goes awry, preventing cells from receiving the energy they need to function properly.
Energy for the body’s cells is provided by the food you eat.
Carbohydrates in food are converted to simple sugars, such as glucose, in the intestines.
Glucose is absorbed into the bloodstream and carried to cells to provide energy.
The pancreas produces insulin, which helps cells use glucose.
Normal glucose absorption
Insulin binds to receptors on the cell membrane, much as a key fits into a lock, signaling glucose transporters.
Glucose transporters move to the cell membrane and bind to glucose.
Glucose enters the cell.
Type 2 diabetes
Cells develop a resistance to insulin (some insulin “locks” on the cell no longer accept insulin “keys”). Glucose builds up in the bloodstream.
What makes whole grains so effective at controlling blood sugar?
McMANUS: They haven’t been stripped of the intact bran. When grain gets refined, that gets stripped away. With the intact bran, whole grains take longer to digest, keep you feeling full longer, and control spikes in blood sugar.
What should people look for on the label?
McMANUS: The one thing that you want to look for is actually the word whole, W-H-O-L-E, with the first ingredient. Men should also know how much fiber they need. The fiber goal for men 50 years or younger is 38 grams a day. For men older than 50, it’s 30 grams a day. The key is for people to not fall for the “come-ons” on the packages, even though they may sound enticing, because the fiber content may not be high enough. The FDA has some specific guidelines for what labels can claim, based on nutrient content. For instance, a product that’s labeled as “a good source of fiber” must contribute 10% of what they call the daily value, the amount that one should consume daily.
Figure 2: It’s all in the label
That doesn’t seem like very much.
McMANUS: Exactly. If you consume 2,000 calories a day, the daily value for fiber is 25 grams. If a food has 5 grams of fiber per serving, it contains 20% of the daily value for fiber. At 20%, the manufacturer can claim that the food is “high in fiber,” “rich in fiber,” or “an excellent source of fiber,” and yet you’re only getting 5 grams.
If you’re trying to get 35 grams of fiber a day, eating foods like fruits and vegetables — foods that don’t have labels — can be very helpful. A plant-based diet is going to get you to your 35-gram goal faster than the traditional Western diet, which is loaded with red meat and dairy, foods that are very low in fiber.
The message here is to set a goal and work at it slowly over time. And when you’re looking at the Nutrition Facts label, look at the number of grams of fiber per serving [see Figure 2].
When it comes to making all of these healthy changes, how can our readers up their chances of success?
McMANUS: Start with small steps. Making too many changes at once can be difficult. Make one change and stick with it for a while. Then take another step. You’ll be more likely to succeed.