Below is an approximation of this video’s audio content. To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video.

Blood sugar, also known as blood glucose, is the universal go-to fuel for the cells throughout our bodies. Our brain burns through a quarter-pound of sugar a day, its “preferred metabolic fuel.” Our body can break down proteins and make glucose from scratch, but most comes from our diet in the form of sugars and starches. If we stop eating carbohydrates, or stop eating altogether, most of our cells switch over to burning fat. But fat has difficulty getting through the blood-brain barrier. But our brain has a constant massive need for fuel, one organ accounting for up to half of our energy needs. Without it, the lights go out…permanently.

To make that much sugar from scratch, our body would need to break down about a half-pound of protein a day. That means we’d cannibalize ourselves to death within two weeks. But people can fast for months. The answer to the puzzle was discovered in 1967. Harvard researchers famously stuck catheters into the brains of obese subjects who had been fasting for over a month, and discovered that ketones had replaced glucose as the preferred fuel for the brain. Your liver can turn fat into ketones, which can then breach the blood-brain barrier and sustain your brain if you’re not getting enough carbohydrates. Switching fuels has such an effect on brain activity that it has been used to treat epilepsy since antiquity.

The prescription of fasting for the treatment of epileptic seizures dates back to Hippocrates. In the Bible, Jesus seems to have concurred. To this day, it’s unclear why switching from blood sugar to ketones as a primary fuel source has such a dampening effect on brain overactivity. How long can you fast, though? To prolong the fasting therapy, in 1921 a distinguished physician scientist at the Mayo Clinic suggested trying what he called a “ketogenic diet,” a high-fat diet designed to be so deficient in carbohydrates it could effectively mimic the fasting state. “[R]emarkable improvement” was noted the first time it was put to the test—efficacy that was later confirmed in randomized, controlled trials. Ketogenic diets started to fall out of favor in 1938 with the discovery of the anti-seizure drug that would become known as Dilantin, but ketogenic diets are still in use today as a third- or fourth-line treatment for drug-refractory epilepsy in children.

Oddly, the success of ketogenic diets against pediatric epilepsy seems to get conflated by “keto diet” proponents into suggesting a ketogenic diet is beneficial for everyone. But you know what else sometimes works for intractable epilepsy? Brain surgery. But I don’t hear people at the gym clamoring to get their skulls sawed open.

Since when do medical therapies translate into healthy lifestyle choices? Scrambling brain activity with electroshock therapy can be helpful in some cases of major depression, so what…pass the electrodes? Ketogenic diets are also being tested to see if they can slow the growth of certain brain tumors. Even if it works, you know what else can help slow cancer growth? Chemotherapy. So why go keto when you can just go chemo?

Promoters of ketogenic diets for cancer, paid for by so-called “ketone technology” companies that will send you salted caramel bone broth powder for a hundred bucks a pound. Or companies that market ketogenic meals report “extraordinary [anecdotal] responses” in some cancer patients, but more concrete evidence is simply lacking. Even the theoretical underpinnings may be questionable. A common refrain is that “cancer feeds on sugar.” But all cells feed on sugar. Advocating ketogenic diets for cancer is like saying Hitler breathed air, so let’s boycott oxygen.

Cancer can feed on ketones, too. Ketones have been found to fuel human breast cancer growth, and drive metastases in an experimental model—more than doubling tumor growth. Some have even speculated that may be why breast cancer often metastasizes to the liver, the main site of ketone production. If you drip ketones on breast cancer cells in a petri dish directly, the genes that get turned on and off make for a much more aggressive cancer, associated with a significantly lower five-year survival in breast cancer patients. Researchers are even considering designing ketone-blocking drugs to prevent further cancer growth by halting ketone production.

And think about what eating a ketogenic diet might entail. High animal fat intake may increase the mortality risk among breast cancer survivors, and potentially play a role in its development in the first place through oxidative stress, hormone disruption, or inflammation.

Men, too: “a strong association [has also been found] between saturated fat intake and prostate cancer progression.” Those in the top third of consumption of these kinds of fat-rich animal foods appeared to triple their risk of dying from prostate cancer. Not necessarily fat in general—no difference in breast cancer death rates based on total fat intake—but saturated fat intake may negatively impact breast cancer survival, a 50 percent increased risk of dying from breast cancer. There’s a reason the official American Cancer Society and American Society of Clinical Oncology Breast Cancer Survivorship Care Guidelines recommend a dietary pattern for breast cancer patients that’s essentially the opposite of a ketogenic diet: “high in vegetables, fruits, whole grains, and legumes, meaning beans, split peas, chickpeas and lentils, and low in saturated fats.”

So far, not a single clinical study has shown “a measurable benefit from a ketogenic diet for any human cancer.” There are currently at least a dozen trials underway, however, and the hope is that at least some cancer types will respond. Still, even then that wouldn’t serve as a basis for recommending ketogenic diets for the general population any more than recommending everyone go out and get radiation, surgery, and chemo for kicks.

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