If you dig in to the scientific literature on the role of diet in cancer prevention and treatment, it can be frustrating at times. While lab and animal testing tell us a lot about how certain compounds in plant foods can help us fight cancer, when you look at research using actual living, breathing humans, the results can be all over the map. Why is that?
One reason is that human studies that look at diet and disease use free-living humans.
- Epidemiological studies survey subjects periodically about what they eat using some sort of a dietary assessment tool, and then they watch and wait to see who develops disease and who doesn’t, and determine if there are common dietary patterns between the subjects who stay healthy and the subjects who don’t.
- Randomized control trials tell one group of people (the experimental group) to eat a certain way, and another group (the control group) to just keep on eating their usual diet. Which group a subject is assigned to is random. It also may not be possible to have people stick to their assigned diet for long enough to really get good answers, because disease like cancer can take years or decades to develop.
It isn’t possible to get a 100 percent accurate picture with either type of study. People may not remember exactly what they ate. They may fudge a bit so it looks like they eat better than they really do. The experimental subjects may not adhere to their plan. The control subjects may improve their diets (even though they were told not to) because they are aware they are being studied.
Basically, unless you keep your human subjects locked in a lab and totally control everything they eat (and how much they sleep, exercise, stress out, smoke, drink alcohol, etc.), there is an element of chance in even the most carefully designed study.
A second reason is our genes. While our genetic predisposition to cancer or other diseases isn’t necessarily our destiny (that’s where diet and lifestyle can tip the scales), it does play a role. (That’s epigenetics for you.) People who have no genetic predisposition to cancer and live a healthy lifestyle are going to be less likely to develop cancer than people who do have that predisposition but live a healthy lifestyle.
Also, our genes may affect how much benefit we get from powerhouse foods like broccoli and berries. Our genes contain the codes for building specific proteins, and some of those proteins (enzymes are one example) are responsible for making use of the various elements of the food we eat. That means you may benefit more from broccoli than your neighbor, or vice versa.
A third reason is our gut microbiome. There are certain parts of our food (especially plant foods), that our bodies don’t digest by themselves. That’s the job of the bacteria that live in our intestine. There are certain types of fiber, for example, that don’t benefit us directly. Rather, it’s the byproduct of digestion by our friendly bacteria that is beneficial to health.
The composition of our microbiomes vary depending on many factors, including diet, medication use, stress, and so on. I know of research going on right here in Seattle that is looking at how people metabolize flax seeds. It may turn out that some people benefit from putting flax on their oatmeal and in their smoothies, while others don’t. Wouldn’t it be nice to know which camp you fall into? I don’t know anyone who actually enjoys flax…it’s just kind of neutral.
All of this uncertainty and individual diversity partially explains why we are encouraged to eat a variety of vegetables, fruits, whole grains, beans and other healthy foods. Eating a varied diet ensures that our bodies get the full spectrum of vitamins, minerals, fiber and phytochemicals. While this is important generally, it may also be directly important if it turns out that you personally benefit from broccoli but not berries, and you’d been avoiding broccoli but eating berries every day.
The field of nutrigenomics (how our genes interact with the foods we eat) is very exciting, and still very new. The hope is that continued research will make personalized nutrition more than just a dream, that we will be able to truly eat for our genes if we choose to. Until then, I’m hedging my bets by eating mostly plants, and a wide variety of them.
Bottom photo used with permission by the Academy of Nutrition and Dietetics.