Colin Campbell's team predisposed mice and rats to cancer by a range of methods, such as dosing them with aflatoxin and breeding them with genes making them prone to cancer. When these mice and rats predisposed to cancer were fed low-casein diets they usually didn't develop cancer. When they were fed high-casein diets, they usually did develop cancer.

Casein is a protein found in abundance in milk and cheese. Like other proteins from animal sources, it is a "complete protein" with the full range of amino acids needed to build mouse, rat or human cells. My hypothesis for what was going on in the experiments is that if there is a superabundance of complete protein available, it makes it easy for cancer cells to grab some of that protein or the complete set of amino acids from that protein to build strong cancer cells. In other words, my hypothesis for the results Colin Campbell's team got is that a diet low in complete protein is like a very safe form of chemotherapy: having low levels of complete protein is harder on cancer cells than it is on normal cells that don't need the materials for growth as much.

A straightforward way to have low levels of complete sets of amino acids, and thereby inhibit the development of any incipient cancers one might have, is to eat relatively little animal protein and to not have too much in the way of complete combinations of proteins for plant proteins beyond that. If you have had a cancer already been detected, it is probably worth talking with one's doctor about cutting down complete protein even further.

I wonder if eating plant proteins in a way that is complete over time, but incomplete on any one day, could hit the sweet spot of providing enough of all the essential amino acids for the normal cells to do their thing, while still making it hard for cancer cells to get all the key amino acids they need to grow quickly.

In addition to the experimental evidence in mice and rats, Colin Campbell and his coauthors have a monograph Diet, Lifestyle and Mortality in China: A Study of the Characteristics of 65 Chinese Counties that provides 65 very interesting data points with rates of incidence for many different types of cancer and other diseases, together with dietary patterns for a county based on samples of individuals from two villages in each county. Because this data was collected relatively soon after the opening of China after the death of Mao, diets in China had not yet Westernized when the data was collected, and were different in different counties. Though it is hard to separate out different possible explanations with only 65 data points, animal protein levels did tend to be positively correlated with cancer incidence, as one should expect of the experiments in rats and mice were an indication of what happens for human beings. (One thing to keep in mind is that this "China Study," by looking at all reported cancers within a county was sensitive to differences in cancer incidence of modest size. Nevertheless, if the correlations are actually indicative of causal effects as in the mice and rat experiments, the effects are much larger than cancer dangers that get hyped a great deal.)

How much animal protein is too much? Experimentally, in the mice and rates the comparison was between something like 5% of calories as animal protein and 20%. Despite starting out in life as an enthusiastic meat, milk and egg eater, Colin Campbell wants everyone to go vegan, but I am not ready to go there, both because

I enjoy animal-based food, and I am not 100% sure a level of complete protein equal to zero is fully safe—and to the extent a vegan diet provides a full set of amino acids it might have the same downsides as an animal protein diet by providing great nutrition for cancer cells.

What I am willing to do personally, is to try to keep animal protein down to 1 calorie per day per pound of body weight, or a number of grams of protein equal to one quarter of my weight in pounds. How do I come up with that as a helpful target? In Chapter 16 of The China Study, "Government: Is It for the People," Colin writes:

Relative to total calorie intake, only 5–6% dietary protein is required to replace the protein regularly excreted by the body (as amino acids). About 9–10% protein, however, is the amount that has been recommended for the past fifty years to be assured that most people at least get their 5–6% “requirement.” This 9–10% recommendation is equivalent to the well-known RDA.

I am thinking of a target of around 7% of calories in animal protein on a typical day, without trying to count up the amount of plant protein. Then the complete animal protein alone would be enough to replace the amino acids that are used up and depleted, with plant protein as extra. The footnote at the end of Colin's passage suggests that 2200 calories a day would be typical for someone weighing 70 kilograms. Since a kilogram is about 2.2 pounds, proportionally that is 2200 calories/(2.2 * 70 kilograms) per day= (100/7) (calories/pound) per day. 7% of that is 1 calorie per pound per day. A gram of protein has about 4 calories, so 1 calorie of animal protein per pound of body weight per day is about 1/4 of a gram of animal protein per pound of body weight per day.

I found by trying it out that for generic animal foods, you can google "how many grams of protein does [animal food x] have" and get a ready answer (with the specification of a measure of weight nice but often optional). For less generic animal foods—for example mozzarella cheese of a particular brand—one can look on the package. To provide some idea of magnitudes, a large egg has about 6 grams of protein, a cup of milk has 7.7 grams of protein, while a quarter-pound hamburger patty has about 29 grams of protein.

Although I think Colin Campbell and his coauthors provide some important data on a possible relationship between animal protein consumption and cancer, I don't think Colin reads the full range of evidence appropriately in coming to his overall viewpoint.

In particular, given the evidence he has to worry about animal protein (by my hypothesis, because of its completeness), there is no evidence to speak of against animal fat. Any worrisome correlation that seems to make animal fat suspect could easily be due to negative effects of the animal protein that tends to go along with animal fat. And the US and other advanced countries have done a massive experiment of trying to improve health by subtracting fat—mostly animal fat—from food products. That subtraction of animal fat has done nothing to improve health, as large studies such as the Women's Health Study have indicated. Colin Campbell talks about interactions among the components of whole foods being the key, but only the nutritional interaction among the amino acids in animal proteins is necessary to explain things. Subtracting animal fat doesn't do any good because it is the animal protein that is the problem, not the animal fat!

For more on dietary fat, see "Jason Fung: Dietary Fat is Innocent of the Charges Leveled Against It."

Also, I am quite skeptical of Colin Campbell's view that animal protein is key to the main axis of "Western Diseases." The reason I am skeptical is that most "Western Diseases" or "diseases of affluence" are correlated with obesity, and there is reason to think that, to a first approximation, "Obesity Is Always and Everywhere an Insulin Phenomenon." Disregulation of the insulin system has to do with things that cause insulin to spike. In "Obesity Is Always and Everywhere an Insulin Phenomenon" I give a rule of thumb that a Food Insulin Index over 40 is high. Sugar and refined carbs typically exceed that level of 40 by quite a bit. Animal foods, not so much. Let me reproduce my animal food selections from a food insulin index table that I gave in Obesity Is Always and Everywhere an Insulin Phenomenon" here:

Meat and Eggs

eggs (poached) 23± 8

steak 37 ± 24

skinless roast chicken 17± 8

white fish 43± 26

hot dog 16± 6

bacon 9± 4

tuna packed in water 26± 8

tuna packed in oil (drained) 16± 4

Dairy

strawberry lowfat yogurt . 64 ± 12

skim milk 60 ± 26

1% milk 34 ± 8

whole milk 24 ± 6

cream cheese 18 ± 12

butter 2 ± 2

Skim milk and lowfat yogurt look bad, and whitefish is a little high, but it doesn't look like most animal foods cause big insulin spikes.

In other words, meat, dairy and eggs, by providing an abundance of complete proteins may help build strong cancer cells, but there is no good evidence that they make people fat. And if meat, dairy and eggs don't make you fat, the variance of cancer they would be available to help explain would be the (still substantial) variance of cancer that is not explained by variance in obesity. Statistically, I would be especially interested in multiple regressions of cancer on obesity and animal protein consumption (with measurement error correction for obesity to make sure that one wasn't undercontrolling for obesity).

As I mentioned above, personally, I am convinced enough by the evidence to marginally cut back on my animal protein consumption. If I were diagnosed with cancer, I would cut back further. There are some extra issues with dairy proteins that Colin Campbell points to that worry me some—grist for a future post. There is no evidence whatsoever to convince me to reduce my animal fat or plant fat consumption.

Despite disagreements, it is important to point to areas of agreement. Colin Campbell, Gary Taubes and Jason Fung all argue quite strongly that sugar and refined carbs are very bad. You should cut them out of your diet.

Everyone agrees that non-starchy vegetables are great. Because fruit has both sugar (bad) and antioxidants + other valuable phytochemicals (good), Colin Campbell and Gary Taubes differ on how much fruit to eat. But everyone agrees that whole fruits are much healthier than fruit juice. (The fiber in the whole fruit slows down the digestion of the sugar to moderate the insulin spike.)

More research is needed to definitely answer the key questions. Unfortunately, much of the research effort in the last few decades has been barking up the wrong tree, thinking that fat in general, or animal fat in particular, was a key dietary villain. In my view, research should focus on (a) "How much can be explained by insulin levels?" and (b) "Controlling for insulin levels, what are the effects of animal protein (or complete sets of amino acids)?" If there are some baleful effects, then I would want to know "Is there some level of animal protein consumption that is safe?" I also want to know whether infrequent (say once a week or once every other week) high levels of animal protein from going out to a restaurant are dangerous or not. The part of this question that relates to my hypothesis is the question of if cancer growth depends on a regular superabundance of complete protein or (as I hope is not so) if cancer growth can proceed vigorously even with only an occasional superabundance of complete protein.

In "Obesity Is Always and Everywhere an Insulin Phenomenon," I make the analogy that trying to lose weight without skipping meals is like trying to reduce inflation without a recession. Given the crucial importance of combining a low-refined-carb diet with fasting for weight loss and maintaining a lower weight, research should focus on (c) investigating the effects of fasting. The direct effect of fasting is to lower insulin levels. But "How often can fasting lower insulin resistance?" And "To what extent can relatively frequent 18-20 hour fasts or 42-44 hour fasts mitigate any baleful effects of animal proteins that might be discovered under research agenda (b)?"

Don’t miss my other posts on diet and health:

I. The Basics

II. Sugar as a Slow Poison

III. Anti-Cancer Eating

IV. Eating Tips

V. Calories In/Calories Out

VI. Other Health Issues

VII. Wonkish

VIII. Debates about Particular Foods and about Exercise

IX. Gary Taubes

X. Twitter Discussions

XI. On My Interest in Diet and Health

See the last section of "Five Books That Have Changed My Life" and the podcast "Miles Kimball Explains to Tracy Alloway and Joe Weisenthal Why Losing Weight Is Like Defeating Inflation." If you want to know how I got interested in diet and health and fighting obesity and a little more about my own experience with weight gain and weight loss, see “Diana Kimball: Listening Creates Possibilities” and my post "A Barycentric Autobiography. I defend the ability of economists like me to make a contribution to understanding diet and health in “On the Epistemology of Diet and Health: Miles Refuses to `Stay in His Lane’.”