By Real World Health Care Editorial Staff  |  Sep 6, 2023

The Role of Dietary Supplements during Cancer Treatment

According to the American Cancer Society (ACS), there is growing evidence that diet, physical activity, and related factors can help some cancer survivors live longer, lower their risk of the cancer returning (or a new cancer developing), and limit some side effects of treatment. It can also lower their risk of developing some other serious diseases and improve their overall health and wellness.

The ACS has been studying the impact of these lifestyle factors for decades and last year published a detailed update of its Nutrition and Physical Activity Guideline for Cancer Survivors. The Guideline, reviewed and updated every five to seven years as new evidence accumulates, offers tips for diet and nutrition during and after treatment, including for those who aren’t having problems with malnutrition or nutrition-related side effects.

One of the guideline’s authors is Marjorie (Marji) McCullough, ScD, RD, senior scientific director within the Epidemiology Research Group at ACS. Dr. McCullough has also served as co-principal investigator of the ACS’s Cancer Prevention Study-II (CPS-II) cohort and the CPS-II Nutrition Survey cohort, giving her unique insight into the role of diet and nutrition on cancer incidence and mortality.

Marjorie McCullough

Marjorie McCullough

“The American Cancer Society conducts some of the largest population-based prospective cohort studies to help us identify risk factors for cancer development and identify ways to both prevent cancer and improve outcomes after a cancer diagnosis,” she said. “The evidence from these and other studies clearly shows that poor dietary patterns are associated with higher risk of mortality after a diagnosis of some types of cancer.

Evaluating Nutritional Deficiencies

The ACS recommends that nutrition screening, assessment, and counseling begin as soon as possible after cancer is diagnosed.

“The goal of such an assessment is to prevent or resolve nutrient deficiencies, help patients maintain an ideal body weight, and preserve muscle mass, all of which may help to improve treatment tolerance,” Dr. McCullough said. “The assessment should also screen for food insecurity and other social health barriers that can lead to poor adherence with nutrition guidance.”

While Dr. McCullough noted that there is no single “magic bullet” diet for cancer patients, she pointed to several healthy eating patterns that can help patients maintain adequate nutritional intake, including the ACS Guideline, the Mediterranean diet, a vegetarian or mostly plant-based diet, the Dietary Approaches to Stop Hypertension (DASH) plan, the Alternate Healthy Eating Index, and the US Dietary Guidelines for Americans. At the core of each is eating a variety of vegetables, fiber-rich legumes, whole fruits, and whole grains while avoiding or limiting red and processed meats, sugar-sweetened beverages, and highly processed foods.

When Nutritional Needs Can’t Be Met by Diet Alone

Ideally, said Dr. McCullough, a patient should get all their nutrients from whole foods, but for those at risk of or showing deficiencies, nutritional or dietary supplements may be needed

The term “dietary supplement” refers to a wide range of products, including vitamins and minerals, herbs and other botanicals, amino acids, enzymes, and more. They come in a variety of forms including pills, gummies, powders, liquids, teas, and bars.

Unlike medicines (drugs), dietary supplements are not intended to treat, diagnose, prevent, or cure diseases, and they do not have the same strict safety and effectiveness requirements as FDA-regulated drugs. Taking supplements can have risks, especially for people who are getting cancer treatment, according to the ACS, which cautions:

  • Some dietary supplements can cause skin sensitivity and severe reactions when taken during radiation treatment.
  • For people who need surgery, some supplements may react with medicines used during and after surgery or might increase the risk of certain side effects such as bleeding and infection. Others might affect the way the heart and brain work, leading to complications during and after surgery.
  • People getting chemotherapy may be at higher risk for side effects if they take some dietary supplements. Some supplements can interfere with how the body breaks down chemotherapy, which can make it less effective or increase side effects.

According to Dr. McCullough, in studies of people without cancer, high-dose individual supplements were found to increase the risk of certain cancers. For example, beta-carotene has been linked to an increased risk of lung cancer among high-risk populations.

“For these reasons and more, it is imperative for health care providers to screen their patients for all medications and supplements currently used and for patients to be honest with their care team about the supplements they are taking or considering,” Dr. McCullough said.

The ACS encourages people to look beyond anecdotal evidence, vague testimonials, and product packaging claims for supplements and instead search out peer-reviewed research through organizations such as the ACS, National Cancer Institute, National Institutes of Health, and the US Food and Drug Administration. Talk with your oncology care team, including a registered dietitian/nutritionist (RDN), before taking a new supplement. Additional tips from the ACS for choosing and using dietary supplements safely can be found here.

Dr. McCullough regularly conducts peer-reviewed research and has a special interest in studying the role of vitamin D on cancer risk and mortality. For example, she and colleagues found that vitamin D deficiency is associated with a higher risk of colorectal cancer. In another study she conducted, vitamin D was not related to subsequent incidence of invasive breast cancer.

“Inconclusive or seemingly discordant study results about vitamin D can be hard for the general public to decipher,” Dr. McCullough concluded. “Fortunately, this essential nutrient can be found in fatty fish, certain mushrooms, and eggs, as well as in fortified foods like dairy products, cereals, orange juice, and yogurt. Patients who are deficient in vitamin D can talk to their health care provider about correcting the deficiency with a supplement.”

Have questions about nutrition and physical activity during and after cancer treatment?  Get answers to common questions here.

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