That's true about many chronic diseases. Hallmarks of diabetes for example -- insulin resistance, high fasting and post-meal blood glucose, high or low blood insulin, fatty acid pathology -- emerge from a confluence of factors, both genetic and environmental. ("Environmental" includes how someone eats.)
Cancer and diabetes aren't caused by eating one particular food either. While eating carbohydrates can result in high blood glucose in people with diabetes, eating carbohydrates by no means causes diabetes. (Some types of carbohydrates are more prone to raising blood glucose (BG) than others, some have no effect on BG at all, some actually lower BG.)
Similarly, a diet high in meat does not cause colon cancer, although it appears to contribute to the disease in predisposed individuals. But then, carbohydrates, at least certain ones, appear to contribute to cancer too. Likewise diets high in meat and fat have been shown to contribute to diabetes.1
When it comes to food, it's likely the pattern of eating, rather than the contribution of a single nutrient, that plays the larger role in cancer and diabetes risk.
I thought the Discussion section in this study laid out the link between cancer and diabetes nicely (and drew attention to a risk-laden style of eating along the way):
Dietary Risk Factors for Colon Cancer in a Low-risk Population, American Journal of Epidemiology, 1998
First I'll summarize its results.
It was a 6-year prospective study of over 32,000 Seventh-day Adventists living in California, a unique group that enjoy very little alcohol, tobacco, and meat. They also enjoy a low incidence of colon cancer. Were their vegetarian diets protective? Possibly. After adjustment for alcohol, smoking, and a few other things, here's how their diets affected their risk:
"The authors found a positive association with total meat intake (risk ratio (RR) for ≥ 1 time/week vs. no meat intake = 1.85)."So, there was a 1.85 greater probability of colon cancer for those who consumed meat once a week or more compared to not at all. That's an 85% increased risk. Pretty high.
"Among meat eaters ... positive associations with red meat intake (RR for ≥ 1 time/week vs. no red meat intake = 1.90)."A 90% increased risk for red meat alone. Even higher.
"... and white meat intake (RR for ≥ 1 time/week vs. no red meat intake = 3.29)."And here, an incredible 229% increased risk for eating white meat at least once a week. You just don't see RRs that high. I was surprised that white meat was associated with a higher risk than red meat. The iron in red meat, its ability to oxidize, is thought to contribute to colon abnormalities. Something stronger must be working here. In this case, white meat included poultry and fish.
"An inverse association with legume intake (RR for >2 times/week vs. 1 time/week = 0.53)."Here we have a very strong protective effect for legumes - beans, lentils, and peas. It just about cut cancer risk in half for those who ate legumes a mere twice a week.
"Among men, a positive association with body mass index was observed (relative to the RR for tertile III (>25.6 kg/m2) vs. tertile I (22.5 kg/m2) = 2.63)."Another strong association, 163% increased risk for men with a BMI greater than 25.6 (You can calculate your BMI here.)
Here I saw the first clue (in this study at least, the hypothesis is several years old) of colon cancer's link to diabetes. Men typically carry fat in an android fashion, that is, centrally. Women carry fat in a gynoid fashion, that is, peripherally - hips and butt. Central fat is more metabolically active, we add to it and take away from it more easily than fat on our butt.
Having lots of metabolically active fat (abdominal fat) is linked to insulin resistance, and so, to diabetes. (Some say obesity causes insulin resistance, but we're finding it may be the other way around.) Insulin resistance in turn is linked to higher levels of circulating insulin. Insulin is a growth hormone, high levels of which are associated with cancer growth. (People with diabetes have a higher risk for many cancers, including colon cancer.)
The final finding:
"A complex relation was identified whereby subjects exhibiting a high red meat intake, a low legume intake, and a high body mass experienced a more than threefold elevation in risk relative to all other patterns based on these variables."Could we get much higher risks than those above? Apparently so, more than threefold in some cases, when several factors are combined.
So, the interplay of all these variables led to an even higher risk for cancer than any one of them separately. If you were...
- A man,
- who ate red meat more than once a week and
- ate legumes less than once a week and
- had a BMI greater than 25,
The study's graphs provide good visual impact. (RR is Relative Risk, anything over 1.0 is an increased risk.)
Notice in the first one below, if your diet included meat your increased risk for colon cancer was considerably lessened if you also ate legumes more than twice a week.
Although, as you can see from the next 2 graphs, if the meat you were eating was white, eating legumes didn't help at all. (Go ahead and speculate ... maybe it's that white meat, chicken and fish, are more often served breaded and fried? In easily oxidized omega-6 fat? The authors didn't say.)
As to the link between colon cancer and diabetes, the authors said, "many of the dietary risk factors associated with colon cancer may reflect the effect of hyperinsulinemia." And high insulin may, in turn, reflect a certain pattern of eating:
"... the characteristically low polyunsaturated/saturated fat ratio from a higher red meat intake contributed to insulin resistance2, while the low intake of legumes, a food of characteristically low glycemic response, contributed to a heavier glycemic load."In this case, the combination of meat and refined carbs (biscuits & gravy? burger on a bun?) was ominous.
More in this vein to follow...
1 Dietary Patterns and Risk for Diabetes, The Multiethnic Cohort, Diabetes Care, March, 2010
2 The degree of saturation in the fat we eat affects insulin resistance, one example: Substituting Dietary Saturated For Monounsaturated Fat Impairs Insulin Sensitivity In Healthy Men And Women: The KANWU Study, Diabetologia, 2001
I think the relative risks in this study were higher than usual because there was a large group who ate no meat that could be used for comparison. Other studies compare groups where everyone eats meat, low-consumers vs. high-consumers, so the distinctions are less. Also, 6 years is a fairly long time, it allows simmering cancers to emerge.