Our nutrition experts answer the most frequently asked questions about cancer and nutrition.
Frequently Asked Questions About Nutrition
"Does sugar feed cancer?" is one of the most frequent questions we receive as oncology dietitians. While researchers continue to investigate the relationship between sugar intake and cancer, it remains a source of uncertainty and fear for many cancer patients and their caregivers.
Sugar comes in many different forms, but the simplest form is a single molecule called glucose. All cells, including cancer cells, use glucose as their primary fuel. Glucose comes from any food that contains carbohydrates including healthful foods like vegetables, fruits, whole grains and dairy. Glucose also comes from refined carbohydrates and added sugars like white breads, pasta, sweets and sweetened beverages.
The idea that sugar, or glucose, could fuel the growth of cancer cells can lead some people to unnecessarily avoid all carbohydrate containing foods. This approach assumes that if cancer cells need glucose, then cutting it out of one’s diet will stop cancer from growing. Unfortunately, it’s not that simple. All of our healthy cells need glucose to function, and there is no way for our bodies to let healthy cells have the glucose they need, but not give it to the cancer cells. Without adequate carbohydrate intake from foods we eat, our bodies will make glucose from other sources, including protein and fat. Glucose is that critical for our cells to survive and function properly. Not consuming sufficient carbohydrates can lead to the breakdown of protein stores in our body, which can contribute to muscle loss and possibly malnutrition. Following a restricted diet with very low amounts of carbohydrates can also cause unintentional weight loss. This can impact the ability to tolerate cancer treatment. Restricting carbohydrates also eliminates foods that are good sources of fiber, vitamins, minerals and immune supporting phytonutrients.
To date, there are no randomized controlled trials showing sugar causes cancer. There is, however, an indirect link between sugar and cancer. Eating a lot of high sugar foods such as cakes, cookies, and sweetened beverages can contribute to excess caloric intake. This may lead to weight gain and excess body fat. Research has shown that being overweight or obese increases the risk of 11 types of cancers including colorectal, postmenopausal breast, ovarian, and pancreatic cancer.
While it is not necessary to completely avoid sugar, reducing added sugars and consuming nutrient-dense, high fiber carbohydrates may be most effective. Here are some steps you can take to help support your overall health, promote blood glucose control, and maintain a healthy weight.
- Choose whole grains like whole wheat bread, pasta, brown rice, or quinoa over refined grains like white bread, pasta and rice.
- Limit added sugars. The American Heart Association recommends women should have no more than six teaspoons of sugar per day (25 grams) and men should have no more than nine teaspoons of sugar per day (37 grams).
- Balance your plate. Make 50 percent of your plate high fiber vegetables and fruit. Twenty-five percent of your plate should be protein-rich foods and the other 25 percent should be whole grain carbohydrates or starchy vegetables such as peas, corn or potatoes.
- Include a lean protein source with each meal and snack like skinless poultry, fish, eggs, low fat dairy, tofu, beans, nuts or seeds.
- Consume a diet rich in vegetables and fruit which contain fiber, vitamins, minerals and immune supporting phytonutrients. Choose whole fruit over fruit juices and dried fruit.
- Stay well hydrated. Limit sugary beverages such as juice and soda.
Eating soy foods like tofu, edamame and soy milk has been linked to reduced risk of certain cancers including breast cancer, prostate cancer and gastric cancer. Many patients worry, however, that eating soy might be harmful if they have estrogen-receptor positive breast cancer. Let’s clear up confusion about the safety of eating soy foods as it relates to cancer risk.
Soy contains something called phytoestrogens, which are the plant version of estrogen. Here are 3 important things to know:
- Phytoestrogens are structurally different and significantly weaker than human estrogen.
- Phytoestrogens do not turn into estrogen when you eat them.
- Moderate intake of soy, in food form, does not increase cancer growth.
The scientific research to date suggests:
Prostate and breast cancer rates are lower in Asian countries where soy foods are a regular part of an overall healthy diet.
Soy in natural food form such as tofu, edamame and soy milk is safe for consumption, even for people with a cancer diagnosis.
Cancer patients do not need to eliminate all sources of soy food from their diet.
For more detail on specific categories of soy products, please see this excerpt previously published in the Cancer Nutrition Consortium Newsletter.
It can be helpful to think of soy products in three distinct categories:
- Soy foods like edamame, tofu and unsweetened soy milk.
- Soy protein supplements like protein powder or nutritional bars made with soy protein isolate.
- Soy condiments or by products such as soy sauce, soybean oil and soy lecithin.
Current research supports including soy foods in the diet of cancer survivors and does not suggest harmful effects, even for those experiencing estrogen-receptor positive breast cancer. In fact, research in patients with breast cancer patients suggests possible benefit to overall survival with consuming moderate amounts of soy foods, or 1-2 servings per day. One serving of soy is equivalent to ½ cup of edamame, 1 cup of soy milk or ¼ cup of tofu. The bottom line is that soy foods like edamame, tofu and unsweetened soy milk can safely be included as an alternative protein or dairy source, even for those going through cancer treatment.
Soy protein supplements
The effect of soy protein supplements and soy derived protein powders on cancer growth is less clearly understood. This type of powder is typically used to make a smoothie or shake but can also be the source of protein in nutrition bars, certain pre-packaged frozen veggie burgers and vegetarian/vegan meat alternatives. Research is less clear on the effects of consuming these more concentrated sources of soy. Theoretically these products could provide higher levels of phytoestrogens if taken consistently, due to their concentrated nature. While consensus on clinical guidelines for soy do not yet exist, many healthcare providers favor minimizing intake of soy protein powder supplements (soy protein isolate) in the diets of patients with hormone sensitive cancer.
Soy sauce, soybean oil and soy lecithin are examples of soy products that do not contain significant levels of phytoestrogens. This means they do not pose risk in terms of fueling cancer growth. At the same time soy sauce and soybean oil have other nutritional drawbacks. Soy sauce is high in sodium and soybean oil is comprised mostly of polyunsaturated fat, which makes it less desirable than other oils such as olive oil. Soybean oil is an ingredient in many packaged foods such as crackers, cookies, breads and salad dressings. Unfortunately, soybean oil added in the manufacturing of these foods is often partially hydrogenated. This form of soybean oil is known as "trans fat," and should be avoided for general health purposes. Even though soybean oil isn’t likely to exacerbate cancer growth it is not considered a healthy fat overall. Soy lecithin is an emulsifier, meaning it is used to help keep things like salad dressing stable in the bottle. Soy lecithin does not contain phytoestrogens and has no documented association with cancer risk.
For those who do include soy in their diet, additional questions about genetic modification (GMO) and conventional versus organic options sometimes arise. Undeniably, soy is a crop that undergoes genetic modification in U.S. agriculture. The short and long-term effects of genetic modification as it pertains to soy and cancer risk have not been well studied and are subject to much debate. Regardless, those seeking to avoid GMO in soy foods can elect to purchase organic options.
The take home message regarding soy and cancer is that eating tofu stir-fry, an edamame appetizer or having unsweetened soy milk as a replacement for dairy is safe for cancer survivors. This is true for women with estrogen-receptor positive breast cancer as well as men and children. Those undergoing treatment for ER+ breast cancer may want to avoid soy protein isolate in powdered form as well as soy protein enriched nutrition bars or vegetarian meat replacements.
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How do you decide if you should buy organic food?
When discussing ways to add more fruits and vegetables to the diet, the topic of conversation frequently turns to whether it is better to buy organic produce.
When navigating the supermarket, you will generally see two types of produce displayed, either 'conventional' or 'organic.' Conventional refers to produce that has been farmed with the application of pesticides. According to the USDA "organic is a labeling term that indicates that food or other agricultural products have been produced through approved methods. These methods integrate cultural, biological and mechanical practices that foster cycling of resources, promote ecological balance and conserve biodiversity. Synthetic fertilizers, sewage sludge, irradiation and genetic engineering may not be used."
The scientific research on organic verses conventional foods and cancer risk is not conclusive enough for specific guidelines to be determined. At this time there is no clear research to suggest that organic foods can prevent cancer or other diseases or that organic food is consistently higher in vitamins, minerals and other nutrients. Therefore, the decision to choose organic or conventional produce is ultimately a personal health choice.
For those who decide to purchase organic produce for lower pesticide residues you may find the Environmental Working Groups "Clean fifteen" and "Dirty Dozen" to be a helpful resource. This is an annual shoppers guide that outlines 15 lowest incidence of pesticide residue when grown conventionally and the 12 produce with the highest incidence. This can be a practical way to prioritize which produce you choose to purchase organically versus conventionally to stretch your food dollar the furthest.
If your decision to choose organic is driven by environmental concerns you may also want to consider the benefits of purchasing local produce through co-op, farm shares or farmers markets. The USDA controls the labelling of organic produce, which can be a costly process for small farms, but when you shop small you can talk to the grower directly about their farming practices. You also have the added benefit that local produce has the potential for higher nutrients as it generally is picked fresh and sold directly to the consumer as opposed to the produce at your supermarket that may have travelled much further before reaching your kitchen.
The bottom line is that the benefits of eating fresh fruits and vegetables far outweigh the risks associated with pesticide residue. Phytonutrients found in fruits and vegetables (organic or conventional) promote immune support and detoxification in the body and are excellent sources of disease-fighting nutrients.
Fasting and calorie restriction (CR) have been areas of interest in both prevention of cancer, as well as reducing side effects from cancer treatments. There are limited studies looking at the impact of fasting/CR and incidence of cancer, and caution must be used in interpreting these results given the studies vary in type and quality.
Fasting during the window around chemotherapy has gained attention as a possible way to reduce common side effects of treatment. The thinking is that, when nutrient intake is significantly reduced or withheld, healthy cells respond to this stress and can slow growth and shift towards maintenance, whereas cancer cells cannot. This may cause cancer cells, but not healthy cells, to experience the damaging effects of treatment.
While a few small trials show some benefit in reducing side effects of treatment, larger scale trials are needed to further assess the safety and effectiveness of this type of dietary intervention. The need for more data is a particularly important considering many patients already find it challenging to meet nutrition needs during cancer treatment.
The term superfoods has become synonymous with ultra-healthy, nutrient-packed, plant-based foods and supplements. They are advertised as having exceptional powers for fighting or preventing common health conditions, like cancer or diabetes, and promoting everything from energy and vitality to hormonal balance. New superfoods are constantly being introduced, adding to the saturated health-food marketplace and contributing to consumer confusion. Many patients ask us, "What are superfoods?" and "Do we need to eat these unusual foods in order to be healthy?"
Many superfoods are simply colorful, phytonutrient rich fruits, vegetables, herbs, nuts, seeds or legumes from around the globe. Superfoods, just like regular plant foods, provide fiber, vitamins, minerals, antioxidants, anti-inflammatory and immune supportive nutrients.
While "Superfoods" are often marketed as a panacea or magical elixir that can improve upon or enhance one’s ability to prevent or reduce cancer, the scientific evidence is sparse. It's important to note that current studies are preliminary. They are conducted on cells or animals in the lab, not humans. Even though they may not always live up to their hype, superfoods can be a part of a well-balanced diet and offer new and interesting options for delicious plant-based meals.
Here are ideas for how to include superfoods in your everyday diet:
- Drink green tea once or twice a day
- Add goji, acai, sea buckthorn, currant or other berries to oatmeal, cereal, salads, baking or smoothies.
- Add chia, hemp or ground flax seeds to salads, smoothies, baked goods, hot or cold cereals.
- Include seaweed in soups or as a snack paired with avocado.
- Drink Kefir or Kombucha for a fun treat.
- Add cacao powder to smoothies, waffle or pancake mix, oats, muffins, breads, desserts or other baking for chocolate flavor without added sugars.
- Swap brown rice with teff in stir-fry, bowls or side dishes
What we do know: a balanced diet plentiful in plant-based foods, such as fruits, vegetables, nuts, seeds and whole grains, along with healthy fats, and lean proteins like fish, peas, lentils and other lean proteins, in combination with regular physical activity and weight management, may help reduce the risk for developing certain cancers and may help promote survivorship.
To learn more about superfoods and cancer, listen to the Dana-Farber podcast Cancer Mythbusters: Superfoods and Cancer featuring nutritionist Stacy Kennedy.