Deciphering the Diet-Cancer Connection

The discourse about the link between diet and cancer can often be as convoluted as a tangled mess of spaghetti. If you were expecting definitive answers in the realm of nutritional science, the TH Chan School of Public Health discussion on "Reducing Cancer Risk Through Nutrition" will disappoint.


“So just to get started, we know that an overall healthy dietary pattern has the potential to lower cancer risk by 10% to 20%, which seems like a lot to me.”

Those were the introductory words of Gabrielle Emanuel, Senior reporter at WBUR and the moderator of Harvard’s TH Chan School of Public Health discussion of “Reducing Cancer Risk Through Nutrition.”

Ms. Emmanuel went on to ask questions of the three panel members, Edward Giovannucci: Professor of Nutrition and Epidemiology and Timothy Rebbeck: Vincent L. Gregory, Jr. Professor of Cancer Prevention, both of T.H. Chan, and Eliza Leone: Registered Dietitian and Instructor in Harvard’s Culinary Medicine and Nutrition Course.

Protein, fats, and carbohydrates came under scrutiny, with a nod toward lean meats, plant-based protein, plant fats, and whole grains. Yet, amidst the dietary labyrinth, causality remains elusive, and hand waving is more prevalent than concrete conclusions.

Here are the highlights

What constitutes a healthy diet?

  • Protein: Prioritize lean meats and plant-based protein sources like beans and lentils.
  • Fats: Emphasize plant-based fats such as nuts, avocados, olive oil, and minimize animal-based fats.
  • Carbohydrates: Choose whole grains, vegetables, and fruits over processed carbohydrates.

What is the dietary link to cancer?

Most of the study of disease and nutrition has centered around diabetes and cardiovascular disease; less has been done in the area of cancer. However, two key risk factors mentioned by the panelists were associated with all three diseases.

“The inflammation is not cancer, but it may set up the environment in which cancer can arise.”

– Dr. Timothy Rebbeck, Professor of Cancer Prevention

Chronic inflammation a prolonged and dysregulated immune response is known to play a crucial role in both the initiation and progression of cancer. By creating a microenvironment that supports the survival and proliferation of cancer cells releasing reactive oxygen and nitrogen species, which, can damage DNA and promote genetic mutations. Dr. Giovannucci offered as an example the significant relationship between hepatitis, a chronic inflammation of the liver, and subsequent liver cancer. Dietary inflammation is more subtle and effects the whole body.

“Just by having a lot of cells dividing, you have a bigger chance of getting a mutation that eventually will lead to a cancer.”  

- Dr. Edward Giovannucci, Professor of Nutrition and Epidemiology

Obesity is a noted risk factor for cancer and has obvious connections to our dietary intake. More specifically obesity is often accompanied by insulin resistance leading to elevated levels of insulin and insulin-like growth factor-1 (IGF-1) promoting the growth of cancer cells. Additionally, obesity is associated with increased production of estrogen in postmenopausal women, and elevated estrogen levels are linked to certain types of cancer, including breast and endometrial cancer. Finally, adipose tissue is biologically active producing and releasing pro-inflammatory molecules such as tumor necrosis factor-alpha (TNF-α), interleukin-6 (IL-6), and adipokines like leptin. These substances act as signaling molecules, triggering and sustaining inflammation within the adipose tissue and beyond.

Unhealthy dietary patterns act through these two mechanisms.

What are the roles of Vitamins and Supplements?

Vitamins and supplements, touted by many as potential saviors, offer a mixed bag of results. There are no magic bullets among the vitamins, and caution was advised against relying solely on supplements.

"There have been some long-term trials [of vitamins], randomized trials, and they don't see any harm. And after about 10 years of use, you start seeing at least a hint of a benefit. So there is some evidence with long-term use. … But I think it's probably enough evidence to say it doesn't hurt and it may help."

- Edward Giovannucci: Professor of Nutrition and Epidemiology

The two compounds that might offer some protection were Vitamin D, where a Harvard study showed a reduction in cancer mortality but not a change in incidence, and a protective role for calcium in colon cancer. Ms. Leone in addition to pointing out that there are no ‘magic bullets’ among the vitamins and supplements went on to say

“our bodies are able to absorb those nutrients more easily through food than we are through supplements. So any time we're taking any supplements-- nutritional supplements-- these are meant to help fill in the gaps of what we may be missing.”

What is the role of alcohol? 

“I mean, [alcohol] probably does slightly increase your risk of cancer, particularly in women because breast cancer is affected by alcohol. But there could potentially be some benefits on diabetes and cardiovascular disease.”

Alcohol is a carcinogen, and cancers have been associated with “patterns of alcohol consumption that are high.” People find this particular message difficult because it interferes with their lifestyles. But just as driving is a risk that can be mitigated by a seat belt, “you can make smart choices or limited choices about alcohol consumption.” For those who will not abstain, the panel fell back on the general US recommendation, One glass a day for women, two for men.

Does Exercise impact our risk of cancer?

Exercise emerged as a potential counterweight to a less-than-ideal diet. While not a carte blanche to indulge in unhealthy eating habits, exercise can offset some negative effects, emphasizing the importance of energy balance.

“So I think exercise can-- in a sense, ...offset some of the potentially bad effects of a bad diet. Now, I'm not saying to have a bad diet and exercise. But the other way to think of it is like the worst thing you can do is not exercise and have a bad diet. So at least do one. It's better to do both. … I think it comes down to energy balance.”

- Edward Giovannucci: Professor of Nutrition and Epidemiology

Are there any myths you would like to debunk?

Dr. Rebbeck spoke to the belief that eating refined sugar feeds cancers. He noted that dysfunction of the mTOR metabolic pathway in cancers is driven by glucose in cells. However,  interpreting that to mean eating refined sugar feeds the tumor is wrong. 

Ms. Leone pointed out that intermittent fasting lacks substantial evidence supporting its role in cancer prevention.

Do you have any practical advice as to using your diet to reduce the risk of cancer?

   - Learn to cook simple, nutritious meals at home to control ingredients and portion sizes. Make use of calorie density, eat less caloric foods that are of a greater volume and therefore more filling

   - Prioritize plant-forward diets to emphasize a higher intake of plant-based foods, including a variety of fruits, vegetables, and whole grains, but do not completely exclude animal products.

   - Make gradual, achievable, sustainable changes to dietary habits and lifestyle to reduce cancer risk.

Fun Fact: Dr. Rebbeck noted that he and his wife used smaller plates to make their portions seem larger and presumptively more filling – small plates, small glasses, to make use of the psychological effect of a small plate making a serving seem larger. That work, by Dr. Brian Wansink of Cornell, has been called into question, and 15 of Dr. Wansink’s papers along these psychological lines have been retracted.

The only constant in the intricate dance between diet and cancer seems to be the lack of clear-cut answers. The journey through this nutritional maze demands a nuanced understanding, emphasizing moderation, whole foods, and a mindful approach to lifestyle choices.

You can find a summary of evidence on nutrition and cancer here. Watch the video of the discussion here. And you might find this article of more than passing interest. Is everything we eat associated with cancer? A systematic cookbook review  American Journal of Clinical Nutrition