Antioxidants have long been considered a key element in strategies to reduce cancer risk, but scientific understanding of how they work, where we get them and implications for healthy eating have evolved over the years.
Antioxidants are the antidote to oxidative stress, which occurs when highly reactive molecules — commonly called “free radicals” — rise to unhealthy levels. As levels of these reactive molecules rise, they create cell damage that increases susceptibility to cancer and other chronic diseases.
What We’ve Learned
The goal is balance — not overkill. Free radicals can come from your exposure to environmental hazards like pollution and second-hand smoke. But they also form as part of everyday metabolic processes. And the goal for health is not to eliminate them all. It’s rising levels of free radicals that trigger your body to amp up its complex antioxidant system of enzymes and other compounds and turn on other cancer-protective defenses. The health risk comes when the level of free radicals overwhelms antioxidant defenses. Your body’s internal antioxidant defense system alone isn’t enough, so a healthy diet provides critical support.
It takes more than vitamins. Early research focused on vitamins C and E as essential antioxidants. That led to focusing on foods to maximize consumption of foods (and potentially supplements) particularly high in those specific nutrients. But then research found that phytochemicals – naturally-occurring compounds in plant foods – showed antioxidant effects in laboratory studies, too. Tests of how well compounds could “quench” free radicals in laboratory studies resulted in identifying foods like walnuts, tea and coffee that were not as high as other foods in the traditional “antioxidant” nutrients as having potential to fight oxidative stress.
Antioxidants need to get to where your body needs them. As the quality of research has progressed, with more availability of studies that follow people over many years and periodically ask about how their eating may change over time, links to vegetable and fruit consumption have become less clear than when we relied on research with weaker methodology. These studies do show a modest link between people who report eating more produce and lower cancer risk. But it is a challenge in research to get an accurate picture of what people eat. Besides, the amount of antioxidants reaching the body cells depends on how much is lost in cooking and how well compounds are absorbed. Studies that evaluate blood levels of carotenoids (like beta-carotene and its chemical “cousins”) more strongly and consistently show lower risk of cancer overall and some specific cancers.
Protection is a symphony, not a solo performance. Blood levels of carotenoids are widely recognized as a marker of vegetable and fruit consumption. So although there’s good evidence from studies in cells and animals that carotenoids — and other antioxidant nutrients and compounds — could help thwart cancer development, human studies show that it’s most likely that they work together in various pathways. That could be part of the reason why supplements supplying just one or a few antioxidants have not shown benefits for cancer prevention. For example, studies suggest that a wide range of phytochemicals from plant foods also have potential to trigger enzymes that inactivate carcinogens and influence gene expression and cell growth signaling.
Phytochemicals get over-simplified. We still have a lot to learn about these potential protectors in our foods. Laboratory studies have identified many steps in the process of cancer development that could be derailed by phytochemicals found in food. However, these compounds are broken down to other compounds as food is digested. So tests on the compounds found in foods don’t necessarily reflect the compounds that actually reach our body cells. Moreover, laboratory studies don’t necessarily use amounts of a compound that would reflect the exposure we’d get from food.
The gut microbiome complicates the picture. Gut bacteria play a role in breaking down the large phytochemicals into compounds that can be absorbed. And since people differ in digestive tract bacteria, people may differ in the protection they get from particular phytochemicals. It’s also important to note that not only do the gut bacteria affect phytochemicals, but also that phytochemicals (like certain polyphenols) seem to nurture healthy gut microbes. This provides another route through which foods rich in phytochemicals may reduce cancer risk.
Metabolic health plays a role. As exciting as the research is suggesting multi-faceted ways that antioxidant nutrients and phytochemicals could protect against cancer development, all those mechanisms are acting within an even more complex system. For example, levels of free radicals can increase from chronic low-grade inflammation and insulin resistance. These conditions, which also promote cancer through additional paths, can develop with excess body fat.
A big picture view shows your whole lifestyle matters. As you choose strategies you want to use to reduce your risk of cancer, and as you read about the mixed results of antioxidants in some studies, it’s important to recognize that cancer risk involves a wide range of choices. For example, smoking increases exposure to free radicals and multiple other hazards. Your choices about processed meat and alcohol also affect the balance between risks and protectors. On the other hand, physical activity triggers the body’s antioxidant defenses and cancer-protective pathways.
As we step back and look at a bigger and bigger picture of influences on cancer risk, you can see why a narrow focus on antioxidants alone does not show up in research as a simple solution. That doesn’t mean that antioxidant-rich foods aren’t helpful. It just means that they need to be one part of a larger strategy.
- Focus on eating a variety of relatively unprocessed plant foods. This includes vegetables, fruits, whole grains, pulses (like dried beans and lentils) and nuts. As knowledge of the thousands of phytochemicals in these plant foods has grown, recommended strategies now focus on variety. So be sure to include plenty of dark green, deep orange and vitamin C-rich choices, but don’t make them your only choices.
- Use these healthy food choices to fuel a healthy lifestyle. Frozen and canned plant foods can count as relatively unprocessed, too. The key is that they should be relatively whole so they include the parts that are highest in protective compounds. And they shouldn’t have lots of added sugars and fats that make them so concentrated in calories that they promote unintended weight gain. As you make these foods the biggest portion of your eating choices, it will help you limit foods and drinks low in nutrients and high in calories. And remember that all of these healthy food choices can fuel your daily physical activity, which plays a vital role in cancer prevention.