By Ted Gansler, MD, MPH
In the course of my routine medical journal reading last year, I came across a short article in The Lancet Oncology about chocolate and cancer prevention. I saved that file on my computer (without reading it), thinking that it might serve as the point of departure for a lighthearted and slightly romantic Valentine's Day essay on this blog.
With that deadline only a few days away, I opened the file and read the article, as well as a few others. The good news is that chocolate does not cause cancer and that moderate consumption of dark chocolate may have a positive impact on heart disease risk. The rest is more complicated.
If you try an Internet search for words like "chocolate prevents cancer," you will find several thoughtful summaries of the available evidence. You will also find some cute but misleading articles implying that eating a lot of chocolate candy prevents cancer. And, you will find a lot of articles with cute headlines and introductions that save their unsweetened facts for the conclusion.
My favorite scientific reviews of cancer and chocolate evaluate information from pre-clinical studies, observational epidemiological studies, and clinical trials separately, and I will follow this approach to get the most thorough view of the topic.
What lab studies can tell us
Most pre-clinical studies are experiments that use lab animals (in this case, mice and rats) or cells growing in lab dishes. The theme of the cell experiments involves adding specific chemicals from chocolate (such as polyphenols, catechins, and proanthocyanidins) and observing what they do to various cell processes that are known to have an effect on cancer formation, growth, spread, etc. Some of the rat and mouse experiments added specific chemicals from chocolate to the animals' food, whereas others used liquid chocolate extracts or unsweetened cocoa powder.
These experiments have found some interesting effects. Some of the animal studies (but not all) found that these substances reduced cancer formation or growth in animals who were exposed to known cancer-causing substances, and some found that cancers that were already there grew more slowly.
Some particularly cynical readers may question the value of such studies, but I should point out that similar studies have led to discovery of important drugs in the past. Once they find out which natural products are useful, chemists can tinker with them, rearranging an atom here or there, often discovering new molecules that are even more useful as drugs.
It's important to recognize that the cells and animals in these pre-clinical studies have not been exposed to chocolate candy products. So, unless you are planning to hand your sweetheart a heart-shaped box containing a mixture of lab rat food pellets and unsweetened cocoa powder (probably a bad idea), these studies have limited relevance to you.
What epidemiologic studies can tell us
Simply stated, observational epidemiologic studies look for statistical links ("correlation") between "exposures" (in this case, eating chocolate) and "outcomes" (like a higher or lower risk of developing cancer). In these studies, the researchers might ask people questions about their intake of various chocolate-containing foods and beverages (dark chocolate, milk chocolate, chocolate milk, etc.) as well as other foods and other exposures and risk factors and then "crunch the numbers."
In the case of chocolate, most studies have focused less on actual amounts of chocolate eaten, and more on total intake of certain types of chemicals that are found in chocolate as well as some other foods and beverages. Results of these studies have been mixed. Some suggest that eating chocolate may lower risk of certain cancers, but other studies do not, and some even noted an increased risk. Although the evidence that eating chocolate may protect against cancer is not very convincing, there is stronger evidence that eating chocolate might help prevent heart disease.
Few clinical trials on chocolate
Clinical trials are essentially human experiments, conducted, of course, under regulated scientific and ethical conditions. Most clinical trials test new drugs, but a few test new diets and combinations of foods. Clinical trials are considered the strongest kind of evidence for guiding health decisions, but, there is almost no trial evidence directly relevant to chocolate and cancer.
One clinical trial found that dark chocolate reduces some kinds of DNA changes that can lead to cancer, and a few others have noticed improvements in other chemical changes related to cancer. There have not been any studies that randomly assign people to consume chocolate or a placebo (sugar pill) and then study the subjects' risk of developing cancer or, if they already have cancer, its growth and their survival. That type of study produces the strongest evidence and is considered the "gold standard" for research.
Many researchers in this field feel that the laboratory results so far are encouraging enough to continue studies in animals and humans. Until we have more concrete answers, here are some points to keep in mind from what we know so far:
- Unlike the rats in the studies I mentioned previously, humans don't consume large amounts of the most promising chemicals that come from chocolate, in a concentrated or purified form. We eat foods and beverages in which chocolate is mixed with varying amounts of fats and carbohydrates, most of which are not especially healthful.
- Used in moderation, unsweetened cocoa powder may be a healthful ingredient in foods and beverages.
- Be careful about the other non-cocoa ingredients in chocolate foods and beverages. Avoid excess sugar, trans fats (anything with "partially hydrogenated" in the list) and saturated fats (milk fat, coconut oil, or palm oil.)
- Consumed in moderation, dark chocolate (which contains high levels of cocoa and lower amounts of other ingredients), may provide some health benefits (mostly related to heart disease).
- All chocolate-based foods are relatively high in calories, and some are very high. These can contribute to overweight and obesity and, in turn, increase risk of several forms of cancer, as well as other health problems.
- In moderation, chocolate-based foods and beverages are neither a major cause of cancer nor a major strategy for avoiding this disease. Such foods and beverages, in moderation and together with a dietary pattern that is otherwise healthful, are fine. If your Valentine would enjoy some chocolate today, don't obsess over the health effects (good or bad) of a moderate quantity.
Dr. Gansler is director of medical content for the American Cancer Society.