At dinner parties, Dr Timothy Rebbeck, a cancer specialist at Harvard’s renowned T.H. Chan School of Public Health, is asked one question more often than any other: Can changing the way we eat lower our cancer risk?
“It definitely comes up a lot,” he says. “And my answer is always yes.”
The question is understandable.
Research indicates that roughly 40 percent of Americans will face a cancer diagnosis during their lifetime. Despite major advances in treatment, the disease still claims about 620,000 lives in the US each year.
Concern is also growing over the rise of cancers traditionally associated with older adults among younger Americans, as Our News Outlet reported this week.
A major National Cancer Institute study found increasing rates of 14 cancers in younger people, including colorectal, breast, pancreatic and skin cancers. In some cases, patients are being diagnosed in their 20s and 30s.
Even so, a substantial share of the cancer burden may be preventable.
The American Cancer Society estimates that about two out of every five cancers diagnosed annually could be avoided. For some forms of the disease, the preventable proportion is considerably higher.
Scientists estimate that diet plays a role in around five percent of cancers, or one in 20 cases diagnosed each year
Smoking, alcohol consumption and obesity have long been recognized as major cancer risks.
However, specialists say diet should also be part of the conversation.
Scientists estimate that dietary choices contribute to approximately 5 percent of cancers — equivalent to one in every 20 cases diagnosed each year.
The clearest evidence involves colorectal cancer, the third most common cancer in the US and one of the leading causes of cancer death among people under 50. Diet has also been linked to cancers of the stomach, pancreas and throat.
Newer research is examining possible dietary connections to other cancers as well, including breast cancer.
There is, however, an encouraging message: Dr Rebbeck says it is never too late to make healthier choices.
For people in their 20s and 30s, establishing better eating habits could “significantly” lower their risk of developing cancer over the course of their lives, he says.
Older adults and those in midlife may benefit, too.
“Even if you’ve eaten poorly for decades, switching to a healthier diet will have an immediate impact,” he says. “You will lose weight, reduce inflammation and improve your cancer risk profile.”
So what does the evidence actually show? Which foods belong on our plates more often, and which are best limited?
Why you DON’T need to give up meat
Meat has long been associated with a higher cancer risk when consumed in excess. But does the research really suggest that eliminating it entirely is the healthiest approach?
One of the largest studies ever carried out on diet and cancer suggests the answer is far from straightforward.
The analysis, published this year, followed 1.8 million people across several countries and found that vegetarians were up to 31 percent less likely than meat-eaters to develop several cancers, including pancreatic, prostate, breast and kidney cancer and multiple myeloma.
Yet there were some surprising exceptions.
Vegans had a roughly 40 percent higher risk of colorectal cancer, while vegetarians were more likely than meat-eaters to develop certain other cancers, including esophageal cancer.
The researchers cautioned that relatively few colorectal cancer cases occurred among vegans, meaning the finding needs further investigation.
One possible explanation is that vegans may consume too little calcium, a nutrient found in dairy products that has been linked to a lower risk of colorectal cancer.
Another is that meat-eaters in the study consumed relatively little processed meat, such as bacon, sausages and ham.
The World Health Organization classifies processed meat as carcinogenic, with studies showing that eating 50g a day – roughly one hot dog – is associated with an 18 percent higher risk of colorectal cancer.
Yet average consumption across the study groups was typically just 16g a day. Researchers suggested this might explain why vegetarians weren’t any less likely to develop colorectal cancer.
Dr Rebbeck cautions against drawing sweeping conclusions.
‘I wouldn’t ever hang my hat on one study and say, “Well, it’s bad to be vegan” or “it’s bad to be vegetarian”,’ he says. ‘But I would say that’s telling us something that we need to learn more about.’
So what should you actually eat?
The World Cancer Research Fund recommends eating little, if any, processed meat and limiting red meat – including beef, pork and lamb – to around 350 to 500g cooked weight a week, equivalent to roughly two or three moderate portions.
Dr Rebbeck is stricter, eating processed meat only a couple of times a year and reserving red meat for occasional restaurant meals.
‘We never really have red meat at home. I only have it sparingly, like as a treat when I am at a restaurant,’ he says.
Could a daily glass of milk protect you?
Drinking a glass of milk every day could reduce your risk of colorectal cancer by nearly a fifth, according to a major study.
Researchers at Oxford University tracked more than 542,000 women for an average of 16 years, examining the links between diet and cancer.
They found that consuming an additional 300mg of calcium a day – roughly the amount in a large glass of milk – was associated with a 17 percent lower risk of colorectal cancer.
And every additional 200ml of milk consumed daily – just under a cup – was linked to a 14 percent reduction.
Scientists believe calcium may protect the bowel by binding to harmful substances that could otherwise damage its lining and potentially trigger cancer.
So should you start drinking more milk?
The important finding from the Oxford study is that the apparent protection came largely from calcium, rather than anything unique to milk itself.
Researchers found that calcium from non-dairy foods was also associated with a lower colorectal cancer risk.
That means people who don’t drink milk can obtain the mineral from other sources, including calcium-fortified plant milks, calcium-set tofu and certain green vegetables.
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Change that could cut colon cancer risk
Eating just 10g more fiber a day could reduce your risk of colorectal cancer by around 10 percent, according to research.
That’s roughly the amount found in two-thirds of a cup of cooked lentils or black beans, or a large avocado.
Yet an estimated 95 percent of Americans fail to consume the recommended 25 to 38g of fiber daily.
Found in beans, whole grains, fruit and vegetables, fiber feeds beneficial bacteria in the gut, which scientists believe may help protect against cancer by reducing inflammation and keeping the cells lining the colon healthy.
Dr Timothy Rebbeck, a researcher at Harvard, said it was possible to eat in a way that reduces cancer risk
Dr Rebbeck considers fiber one of the most important parts of a cancer-prevention diet.
‘I would say that high fiber writ large, regardless of source, will help to reduce your risk of cancer,’ he says.
One easy way to increase your intake is to swap refined grains for whole grains, such as oats, brown rice and whole-wheat bread.
But if Rebbeck had to recommend one particular group of vegetables, it would be cruciferous vegetables, including broccoli, cauliflower, cabbage, kale and Brussels sprouts.
‘If you wanted to pick something that the evidence is pretty strong for that there’s cancer prevention benefits, cruciferous vegetables are probably the one to focus on,’ he says.
A major review of more than 200 studies, published last year, linked eating around five portions a week to a lower risk of several cancers, including colorectal, lung, bladder and kidney cancer.
Their potential benefits go beyond fiber. They contain natural compounds that laboratory studies suggest may help prevent cancer, although the evidence in humans is less certain.
Rebbeck says attempts to reproduce these benefits using supplements have been less successful.
‘That might be because the vegetable and the combination of all that stuff in there is what’s helping to prevent cancer,’ he says.
Two sodas a day could double your risk
Drinking two or more sugary drinks a day could more than double your risk of developing colorectal cancer before the age of 50, according to a major study.
Researchers followed more than 95,000 American women and found that those consuming at least two sugar-sweetened drinks daily had more than twice the risk of early-onset colorectal cancer compared with women who drank fewer than one a week.
Each additional daily serving was associated with a 16 percent increase in risk, while consuming sugary drinks during adolescence was also linked to a higher risk later in life.
The researchers suggested that frequent consumption could contribute to insulin resistance, weight gain and metabolic changes that may encourage cancer development.
But the findings don’t prove that soda causes cancer. Only 109 women developed early-onset colorectal cancer during the study, so the results need further investigation.
Nevertheless, replacing regular soda and other sugary drinks with water or unsweetened alternatives is a simple way to reduce added sugar intake, with potential benefits beyond cancer prevention.
Intriguingly, the researchers also found that replacing just one sugary drink a day with coffee or milk was associated with a lower risk of developing colorectal cancer before 50.
Could your morning coffee be protecting you?
That daily ‘eye-opener’ might be doing more than helping you wake up.
Research backed by the World Cancer Research Fund has linked regular coffee consumption to a lower risk of liver and endometrial cancer – the disease affecting the lining of the uterus.
One of the more surprising findings is that decaffeinated coffee also appears to offer protection, suggesting caffeine may not be responsible.
Scientists believe other compounds in coffee, including antioxidants, could help reduce inflammation and protect cells against damage that may eventually lead to cancer.
But that doesn’t mean drinking more coffee will necessarily prevent cancer. Much of the evidence comes from observational studies, which cannot establish cause and effect.
And there’s no established daily amount that people should drink specifically to reduce their cancer risk.
How much you eat also matters
When it comes to preventing cancer, what you eat isn’t the only thing that matters. How much you eat – and how much body fat you carry – could be just as important.
Excess body fat has been linked to at least 13 types of cancer, including colorectal, pancreatic, kidney and breast cancer after menopause.
Scientists believe one reason is that fat tissue isn’t simply a store of excess energy. It also produces hormones and inflammatory chemicals that can encourage cancer cells to grow.
Being overweight can also increase insulin levels, potentially creating conditions that make it easier for certain tumors to develop.
This has prompted growing interest in whether eating fewer calories – or restricting food intake to certain hours – could help prevent cancer.
A review of animal studies published in Scientific Reports found that cutting calorie intake by 20 to 40 percent reduced the incidence of mammary tumors by as much as 95 percent in some experiments.
But the evidence for intermittent fasting is more mixed. A review published in the American Cancer Society’s journal concluded that there is still insufficient evidence to establish whether fasting itself prevents cancer in humans.
For now, the World Cancer Research Fund recommends maintaining a healthy weight and avoiding weight gain throughout adulthood, rather than following a particular fasting schedule.
Dr Rebbeck, 65, takes a particularly restrained approach to his own diet.
At 5ft 8in and 160lbs, he typically starts the day with two bananas and yogurt, or simply a cup of coffee. He usually skips lunch, occasionally eating a handful of nuts, before having chicken or fish with a cup of cruciferous vegetables for dinner.
He acknowledges that his diet is restrictive and that others may need to eat considerably more, depending on their size and activity levels.
He also walks, hikes and practices tai chi, while rarely drinking alcohol. A steak at a restaurant or a pepperoni pizza is an occasional treat.
‘I have no risk factors that put me at higher risk of cancer, and no one in my family has had it,’ he says.
‘But me and my wife are cancer researchers, so we are mindful of this, and want to do our best to lower our risk.’
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