In a recent blog post P. D. Mangan discusses the implications of the metabolic theory of cancer and the ideas of Dr. Laurent Schwartz, French physician and oncologist, who has been using this theory to treat cancer patients. Whether this theory is correct, or not, time (and the scientific method) will tell. But there is one non-controversial observation that has huge implications for treatment–and prevention–of cancer.

As I explained in a previous post Food for Thought, our cells can utilize two main sources of energy: glucose, which is derived from the carbohydrates, and ketones, which are derived from fats. In many types of cancer, cancerous cells lose the ability to metabolize ketones, and are forced to rely solely on glucose. Thus, a simple lifestyle-based strategy to inhibit tumor growth is to switch to a ketogenic diet. In other words, drastically reducing carbohydrates and substituting them with saturated fats will not affect the function of muscle and brain cells, but will starve cancerous cells. In fact, a fat-based diet may also prevent cancer, because most of us have many tiny incipient tumors that are being constantly clobbered down by our immune systems. It’s only when they escape this constant policing action that they become a problem. Starving them of energy gives the immune system additional time to find and destroy them.

And this reminds me of the entry I submitted in response to the Edge question of 2016 a year ago:

WHAT DO YOU CONSIDER THE MOST INTERESTING RECENT [SCIENTIFIC] NEWS? WHAT MAKES IT IMPORTANT?

The responses to this question, including mine, were recently published in a book form, and I think it’s worthwhile to revisit it again, in light of the new information on cancer treatment. It appears that the anti-fat craze not only made us fatter and more stupid, as I point out in this essay, but also more susceptible to dying from cancer.

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Amid all the confusing fluctuations in dietary fashion to which Americans have been exposed since the 1960s, one recommendation has remained unchallenged. Beginning in the 1960s and until 2015 the Americans have been getting consistent dietary advice: fat, especially saturated fat, is bad for your health. By the 1980s, the belief equating a low-fat diet with better health had become enshrined in the national dietary advice from the US Department of Agriculture and was endorsed by the surgeon general. Meanwhile, as Americans ate less fat, they steadily became more obese.

Of course, the obesity epidemic probably has many causes, not all well understood. But it is becoming clear that the misguided dietary advice with which we have been bombarded over the past five decades, is an important contributing factor.

In fact, there has never been any scientific evidence that cutting down total fat consumption has any positive effect on health; specifically, reduced risks of heart disease and diabetes. For years those who pointed this out were marginalized, but recently evidence debunking the supposed benefits of low-fat diets has reached a critical mass, so that a mainstream magazine such as Time could write in 2014: “Scientists labeled fat the enemy. Why they were wrong.” And now the official Scientific Report of the 2015 Dietary Guidelines Advisory Committee admits that much.

There are several reasons why eating a low-fat diet is actually bad for your health. One is that if you lower the proportion of fat in your diet, you must replace it with something else. Eating more carbohydrates (whether refined or “complex”) increases your chances of becoming diabetic. Eating more proteins increases your chances of getting gout.

But perhaps a more important reason is that many Americans stopped eating food, and switched to highly-processed food substitutes: margarine, processed meats (such as the original Spam—not to be confused with email spam), low-fat cookies, and so on. In each case, we now have abundant evidence that these are “anti-health foods”, because they contain artificial trans fats, preservatives, or highly-processed carbohydrates.

While controlled diet studies are important and necessary for making informed decisions about our diets, an exciting recent scientific breakthrough has resulted from the infusion of evolutionary science into nutrition science. After all, you need first to figure out what hypotheses you want to test with controlled trials, and evolution turned out to be a fertile generator of theoretical ideas for such tests.

One of the sources of ideas to test clinically is the growing knowledge of the characteristic diets of early human beings. Consider this simple idea (although it clearly was too much for traditional nutritionists): we will be better adapted to something eaten by our ancestors over millions of years than to, say, margarine, which we first encountered only 100 years ago. Or take a food like wheat, to which some populations (those in the Fertile Crescent) have been exposed for 10,000 years, and others (Pacific Islanders) for only 200 years. Is it surprising that Pacific Islanders have the greatest prevalence of obesity in the world (higher even than in the United States)? And should we really tell them to switch to a Mediterranean diet, heavy on grains, pulses, and dairy, to which they’ve had no evolutionary exposure whatsoever?

Our knowledge of ancestral diets, of course, is itself evolving very rapidly. But it seems clear that we are adapted to eating a variety of fatty foods, including grass-fed ruminants (beef and lamb) and seafood (oily fish), both good sources of Omega-3 fatty acids. Of particular importance could be bone marrow—it is quite likely that first members of the genus Homo (e.g., habilis) were not hunters, but scavengers who competed with hyenas for large marrow bones. It’s very probable that nutrients from bone marrow (and brains!) of scavenged savannah ungulates were the key resource for the evolution of our own oversized brains.

In light of the new knowledge it is clear why Americans are getting fatter by eating low-fat diets. When you eliminate fatty foods that your body and—especially—brain need, your body will start sending you persistent signals that you are malnourished. So you will overeat on foods other than fatty ones. The extra, unnecessary calories that you consume (probably from carbohydrates) will be stored as fat. As a result, you will be unhappy, unhealthy, and overweight. You can avoid those extra pounds, of course, if you have a steely will (which few people have)—then you will not be overweight, merely unhappy and unhealthy.

So to lose fat you need to eat—not fat—but fatty foods. Paradoxically, eating enough fatty food of the right sorts will help to make you lean, as well as happy and—Edge readers, take note—smart!