The dairy industry is smiling because a study published in the prestigious journal, Nature, concluded that 鈥渄airy products help protect against colorectal cancer, and that this is driven largely or wholly by the calcium they contain.鈥� The study triggered headlines about how drinking a glass of milk a day can reduce the risk of colon cancer by 17%. That isn鈥檛 quite correct. It may be true for women who consume less than the recommended 1000 mg of calcium a day, but may not apply to those who already consume the recommended amount. And it certainly does not mean that drinking two glasses of milk will decrease the risk by 34%.
Three years into a seventeen-year-long study in the UK that began in 1996, half a million women were asked 130 questions about the frequency and amounts of foods that they had consumed during the previous seven days. Such surveys are problematic because people have difficulty accurately recalling what they ate and are also prone to including foods they think they should have eaten instead of what they ate. A further issue is the assumption that people continued to eat the same way throughout the whole period of the study as when they originally filled out the questionnaire. This is unlikely because people鈥檚 dietary habits change over time.
Subsequent to the diet survey, the women鈥檚 health was monitored for fourteen years during which time some 12,000 were diagnosed with colon cancer. On average, these were the women who consumed less than the recommended amount of calcium, taking in roughly 800 mg a day. The women who were free of colon cancer averaged around 1100 mg. That鈥檚 a difference of 300 mg, which happens to be the amount of calcium in a glass of milk. This is how the media jumped to the conclusion that a glass of milk reduces the risk of colon cancer by 17%, which was the difference in the cancer incidence between the low and high calcium consumers.
A glass of milk is not the only way to increase calcium intake by 300 mg. About 40 grams of hard cheese, a serving of lasagna, a couple of slices of pizza, two servings of tofu and a glass of calcium enriched soy milk can all supply 300 mg. The study did not look at supplements but there is no reason to believe that these would behave differently. The study also confirmed that alcohol as well as red and processed meats are directly associated with an increased risk of colon cancer. Surprisingly, total sugar intake was found to decrease risk, but that finding was ignored by the media.
Now let鈥檚 crunch a few numbers. The lifetime incidence of colon cancer in women is about 4%, meaning that 40 out of a thousand women will be diagnosed with the disease. If this incidence is reduced by 17%, as found in the study, then only 33 out of 1000 would be diagnosed. This means that 7 out of 1000 women with a below than the recommended intake of calcium will be saved from the disease by drinking a glass of milk a day. That is less than 1 in a hundred. That doesn鈥檛 sound quite as impressive as the headlines make out.
But of course, when considering that millions of people may have a sub-optimal intake, then even 1 in a hundred amounts to many being saved from the dreaded disease. Here we have an example of how statistics are very useful when applied to a specific population, in this case, women who consume less than the recommended amount of calcium. 聽But headlines suggesting that everyone can reduce the risk of colon cancer by drinking a glass of milk a day are misleading. Furthermore, even in low calcium consumers, fewer than one in a hundred will be spared from colon cancer by drinking a glass of milk a day. The same result can be achieved by eliminating alcohol, and that does apply to everyone.
