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Nutrition and breast cancer

Many women diagnosed with breast cancer are concerned about the role that nutrition has during treatment, recovery and recurrence. Studies looking at the role of food, nutrition and physical activity and breast cancer survival are ongoing. There is not yet enough evidence to make specific recommendations.

A diet that promotes healthy, well-balanced eating from a variety of foods, along with regular physical activity and a healthy body weight, likely contribute to overall health while recovering from breast cancer.

Breast cancer survivors can meet their nutritional needs by following Canada’s Food Guide to Healthy Eating. This guide outlines the number of servings required from each of the 4 food groups, as well as the need to eat a variety of foods.

Women with breast cancer should check with their doctor or a registered dietitian before changing their diets or taking dietary supplements (pills or powders).

A woman recovering from breast cancer may have the following concerns about nutrition and breast cancer.

Soy

Soy contains weak estrogen-like phytochemicals called phytoestrogens (specifically isoflavones or isoflavonoids). Soy-based foods include tofu, soy milk, soybeans, soy nuts and miso. Other sources of soy include soy protein powders, pills and other soy supplements.

Some studies have found that soy products may reduce the risk of breast cancer. These studies were done on women from other countries (mostly Asian) whose diets are high in soy, and the results may be due to factors other than soy intake. A recent analysis of the results from 18 studies has suggested a very small decrease in breast cancer risk associated with eating soy foods. These results must be interpreted with caution due to the variation in the amount and types of soy intake among women in the studies.

Many breast cancer survivors are concerned about using soy in their diet because they worry that soy might act in the same way as natural estrogens (estrogenic) and increase the growth of breast cancer. Most of the current evidence suggests that soy is unlikely to be harmful to breast cancer survivors when eaten in similar amounts to a typical Asian diet.

  • Up to 3 servings a day of soy foods, such as tofu or soy milk, may be included in the diet.
  • Use of concentrated sources of soy, such as soy powders or supplements, should be avoided because they may affect the body in similar ways as estrogen.
  • Some doctors may recommend that women on tamoxifen (Nolvadex, Tamofen) avoid soy because it may interfere with the effects of the drug. However, recent studies have suggested that soy may be safe in women on this drug.

It is not known if soy will reduce the risk of breast cancer recurrence or reduce the risk of breast cancer in women who are at high risk of developing the disease.

Dietary and herbal supplements

Dietary supplements, such as vitamin and mineral supplements, or herbal supplements do not take the place of healthy, well-balanced eating. Supplements may be helpful to provide nutrients if a woman cannot get enough through her diet. Check with a doctor or dietitian to find out if a vitamin and mineral supplement is needed.

When considering dietary supplements, be aware that different ages and sexes have different nutrient requirements. The dietary reference intake (DRI) is the average amount of a certain nutrient a person needs to prevent deficiencies and lower the risk of chronic disease.

Taking more of a certain nutrient is not necessarily better or safer. Large doses of certain vitamins and minerals can be harmful or may interact with prescription drugs, over-the-counter medications or cancer treatments. Some herbal supplements can also interfere with medications or cancer treatments.

Breast cancer survivors who are considering taking supplements should discuss the types and amounts of supplements that may be appropriate for their situation with their doctor or registered dietitian. Healthcare professionals involved in the care of a woman with breast cancer should be made aware of all supplements that she is taking, along with the amounts.

Antioxidants

Antioxidants are chemical substances in many foods. Antioxidants protect the body’s cells and tissues from free radicals (unstable molecules made during normal cell metabolism that easily reacts with other molecules). Free radicals are made when our bodies use oxygen. Free radicals can damage cells, which may lead to cancer. Antioxidants are thought to help protect cells by removing free radicals before they cause damage.

Certain vitamins and minerals found in many foods, especially vegetables and fruit, act as antioxidants. Vitamin C, vitamin E, beta carotene and selenium are antioxidants. It is best to choose foods with antioxidants, rather than taking supplements.

Women with breast cancer should check with their doctor about using antioxidants, especially during treatment. Some studies have shown that taking large doses of antioxidant supplements during chemotherapy, radiation therapy, or both may make these treatments less effective. Both treatments work by creating free radicals within cancer cells to damage and destroy them. Because antioxidants remove free radicals, this prevents them from destroying the cancer cells.

Dietary fat

Fat in one’s diet is important for overall health. It provides energy, helps absorb certain vitamins, is necessary for normal growth and development and protects certain organs.

Some research has linked a high-fat diet to a possible increased risk of breast cancer. Too much fat in the diet can lead to being overweight or obese. Being obese (having a body mass index [BMI] of 30 or more) increases a woman’s risk of developing breast cancer, especially after menopause.

Several studies have looked at dietary fat and survival after breast cancer, but the results have been inconsistent. One recent study suggested that low-fat diets may reduce the risk of breast cancer recurrence, especially in women with early stage, estrogen receptor–negative disease.

Further research is in progress and more studies are needed before definite conclusions can be made about the effect of types and amounts of dietary fat, certain foods or other dietary components on the risk of breast cancer recurring. Women with breast cancer may want to consider eating less fat and more vegetables, fruit and grains as part a healthy diet and as a way to achieve and maintain a healthy body weight.

Some low-fat eating suggestions include:

  • Check that total fat intake is no more than 30% of total daily calories.
  • Choose lower fat milk products, such as 1% or skim milk, yogurt and cheeses.
  • Choose leaner cuts of meat.
  • Limit foods with high amounts of saturated fats and trans fats.
  • Use unsaturated (healthy) fats to prepare foods.
  • Reduce the amount of fat used when preparing food.

Alcohol

Having one or more alcoholic drinks a day is associated with a slight increase in breast cancer risk. It is not yet known if alcohol plays a role in the risk of recurrence, development of a second breast cancer or in breast cancer survival.

Alcohol may cause higher levels of estrogen and may also decrease some essential nutrients that protect against cell damage.

  • Check with the doctor about drinking alcohol during and after breast cancer treatment. Alcohol may interfere with medications, treatments and certain medical conditions.
  • If you choose to drink alcohol, limit it to less than one standard drink per day.

Vegetarian diets

Some women may consider switching to a vegetarian diet after a diagnosis of breast cancer. Vegetarian diets tend to have healthy characteristics, such as being low in saturated fat and high in fibre, vitamins, minerals and phytochemicals. However, there is no evidence that a vegetarian diet can prevent or lower the risk of breast cancer recurrence.

Vegetarian diets that include fish, dairy and eggs usually have enough protein to meet the body’s needs. A vegan diet, which excludes all animal foods, can meet protein needs if legumes, nuts and cereal grains are eaten in the right amount, though vitamin B12 supplements are needed.

Breast cancer survivors who are considering switching to a vegetarian diet should speak with a registered dietitian to ensure that the diet that they follow meets all their nutritional needs.

Meat intake

Meat is a valuable source of several nutrients, including protein, iron, zinc and vitamin B12. Many breast cancer survivors are concerned that eating meat, in particular red meat, will increase the risk of breast cancer recurrence.

There is evidence that people who eat more red and processed meat appear to have an increased risk of cancer, particularly colorectal cancer. Recent evidence also shows that an increased intake of red and processed meat appears to modestly increase the risk of death from cancer, heart disease and other diseases. These studies did not look specifically at breast cancer survivors.

Based on this evidence, breast cancer survivors should try to limit the amount of red meat to 3 servings per week to help reduce the risk dying from cancer. Healthy alternatives to meat include fish, seafood, poultry, dried beans, lentils and tofu.

Flaxseed

Like soy, flaxseed also contains weak estrogen-like phytochemicals called phytoestrogens. Flaxseed contains lignans, a type of phytoestrogen.

A Canadian study on a small number of breast cancer survivors suggested that 2 tablespoons of ground flaxseed taken daily can slow the growth of cancer cells. While further research on mice with human breast cancer showed the same effects, more research is needed on breast cancer survivors to find out if flaxseed can improve survival.

Until more is known, women with breast cancer should talk to their healthcare team if they have questions about flaxseed or flax-based foods and the amount that may be consumed.

Hormones in food

Hormonal growth promoters (HGPs) make animals gain weight faster and increase the growth of lean tissue. The result is a product produced at a lower cost. HGPs are made naturally by animals or can be manufactured (synthetic). HGPs are approved for use in Canada and include estradiol (a type of estrogen), progesterone, testosterone and some other synthetic hormones.

HGPs are only approved for use in beef cattle. Scientific research to date has not shown that food products from animals treated with these HGPs pose a threat to human health. There have been no direct studies done that compare breast cancer risk in women who eat meat from hormone-treated animals to those who eat meat from untreated animals.

Healthy body weight

Maintaining a healthy body weight is an important part of recovery from breast cancer.

Some women may lose weight during treatment for breast cancer because of the side effects of treatments like chemotherapy. Being underweight can increase the risk of complications during treatment and affect a woman’s quality of life.

Other women gain weight during treatment, either because of treatments like hormonal therapy or because a woman is getting less exercise than before.

Being obese increases a woman’s risk of breast cancer. Growing evidence from studies shows that obesity may have a negative effect on breast cancer survival and increase the risk of recurrence.

Maintaining a healthy body weight can be achieved through a combination of diet and physical activity. Breast cancer survivors may want to speak to their healthcare team, a registered dietitian or an exercise specialist for suggestions on how to achieve and maintain a healthy body weight.

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