9.4: Cancer

Although heart disease is our leading cause of death, it has fallen so much that it may fall below cancer deaths, elevating cancer to our leading cause of death (see Fig. 9-5). However, the number of people 85 and older has been increasing. In this age group, deaths from heart disease increases dramatically to more than twice that of cancer.

Cancer occurs more often as we get older, so cancer rates rise as the average age of the population rises. When one adjusts for an aging population and excludes lung cancer, the death rate from all other cancers combined has decreased over the past 68 years (see Fig. 9-5).

Figure 9-7: Cancer deaths (estimated % of total cancer deaths in 2021)¹⁰

In looking at death rates from various causes, keep in mind that if we don’t die of one thing, we eventually die of something else. When fewer people die of heart attacks, other causes fill the gap. For example, most prostate cancers are diagnosed in older men. Fewer middle-age men dying of heart disease can be partially responsible for greater numbers of men surviving to old age—and being diagnosed with prostate cancer.

Lung cancer is the leading cause of cancer death in both men and women (see Fig. 9-7). About 40% of all cancers is attributed to exposure to tobacco products (it’s said that tobacco use can cause at least 12 kinds of cancer).¹¹ Smoking cessation lowers the risk of both cancer and heart disease.

About 18% of cancer cases is attributed to being overweight, physical inactivity, and an unhealthy diet (including excess alcohol).¹² Cancer of the colon, prostate, and breast is associated with diets high in animal fats, but studies have not shown fat itself to be the culprit.

Diets high in animal fats tend to be low in plant foods and fiber, and high in protein, saturated fat and calories. Animal studies support the relationship of this general dietary pattern to cancer. For example, animal studies have shown that: (a) tumors are more common in animals fed high-fat diets, (b) diets low in protein and calories suppress the development of tumors, (c) certain plant constituents protect against cancer.

Obesity is also more common in populations with high-fat diets. Thus, obesity represents another variable along with the various dietary factors. Many variables are clustered, however. A diet high in fat and calories, for example, coupled with inactivity leads to excess calories and overweight.

Figure 9-8: Cancer Death Rate in 2019 by Age, Gender, Ethnicity¹

Breast Cancer

Breast cancer is more common than lung cancer, but fewer women in the U.S. die of breast cancer—it’s detected earlier and is less fatal than lung cancer; 90% of breast cancer patients (but only 19% of lung cancer patients) live 5 or more years after diagnosis.¹⁰

The causes of breast cancer aren’t known, although there are some known risk factors. Genetic susceptibility is a factor (inheriting “breast cancer genes” BRCA-1 and 2 account for 5-10% of breast cancers). Women with mothers or sisters with premenopausal breast cancer have a higher risk.

A woman’s reproductive history also affects her risk, presumably through the timing and duration of various female hormone levels. This includes first menstruation before age 12, prolonged use of birth-control pills, no children or having the first child after age 30, menopause after age 55, prolonged hormone-replacement therapy after menopause—all involve long stretches of uninterrupted high-estrogen levels.

Diet-related risk factors include being overweight after menopause,¹² and alcohol intake (more risk with more alcohol).¹³ Alcohol raises estrogen levels, as does excess body fat in postmenopausal women. (Estrogen falls at menopause, making body fat a major source of estrogen production.)

There’s a wide variation in the frequency of breast cancer throughout the world. Countries with higher fat diets have higher rates of breast cancer. When women move from countries with low-fat diets and low breast cancer rates, to countries with high-fat diets and high breast-cancer rates, their rate of breast cancer rises. In animal studies, a high-fat diet produces more mammary (breast) tumors.

Because of the number of variables present in population studies, however, it’s hard to determine whether a specific dietary component (e.g., animal fat) promotes breast cancer. To get groups of women that are comparable in terms of race, socioeconomic level, etc., the control and experimental groups are best drawn from the same population. But when this is done, the diets are usually fairly similar. Also, diets assessed at the time of the study may not be as important as the diet in earlier years.

We’ve been left to rely on international comparisons to get large differences in diet. Studies comparing breast cancer rates in Japan with rates in the United States are noteworthy, because the countries are comparable in industrialization, yet have dramatically different rates of breast cancer.

Age-adjusted death rates for breast cancer are much lower in Japan than the U.S. The estimated age-adjusted occurrence rate (cases/100,000 women) in 2018 for breast cancer was 85 in the U.S. and 46 in Japan.¹⁴ If genetics were a determining factor, Japanese-Americans would be expected to have rates similar to their racial counterparts in Japan. Japanese-Americans, however, have higher rates (though not as high as Caucasian-Americans). As Japanese in Japan and in the U.S. have become more “Americanized,” their breast cancer rates have gone up.

It’s important to emphasize that proof of a link isn’t proof of cause-and-effect. As noted earlier, a high-fat diet tends to be high in calories, etc., so it’s hard to fault one dietary component. There are also protective dietary factors to consider. For example, substances in soybeans (prominent in the Japanese diet) might affect the risk of breast cancer. There are also non-dietary factors to consider. For example, breast-feeding is more common in Japan than the U.S., and the use of birth-control pills and post-menopausal hormone-replacement therapy is much less common.

Japan’s changing social factors and dietary patterns are giving us more clues. Fish, rice, and soy are staples in the traditional Japanese diet. As Japan’s diet has become more Americanized—more meat, fat, fast-foods, sweets, etc.—breast cancer and obesity have become more common.