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.

Given the decades of use of ketogenic diets to treat certain cases of pediatric epilepsy, a body of safety data has accumulated. Nutrient deficiencies would seem to be the obvious issue. Inadequate intake of 17 micronutrients—vitamins and minerals—has been documented in those on strict ketogenic diets.

Dieting is a particularly important time to make sure you’re meeting all your essential nutrient requirements, since you may be taking in less food. Ketogenic diets tend to be so nutritionally vacuous that one assessment estimated that to get a sufficient daily intake of all essential vitamins and minerals. you’d have to eat more than 37,000 calories a day.

That’s one of the advantages of more plant-based approaches. As the editor-in-chief of the Journal of the American Dietetic Association put it: “What could be more nutrient dense than a vegetarian diet?” Choosing a healthy diet may be easier than sticking 50 sticks of butter in your coffee.

And we’re not talking about just not reaching your daily allowances. Children have gotten scurvy on ketogenic diets, and some have even died from selenium deficiency (which can cause sudden cardiac death). The vitamin and mineral deficiencies can be solved with supplements, but what about the paucity of prebiotics, the dozens of types of fiber and resistant starches found concentrated in whole grains and beans that you’d be missing out on?

Not surprisingly, constipation is very common on keto diets, but as I reviewed before, starving our microbial self of prebiotics can have a whole array of negative consequences. Ketogenic diets have been shown to reduce the richness and diversity of our gut flora. Microbiome changes can be detected “within 24 hours” of switching to a high-fat, low-fiber diet. The lack of fiber starves our good gut bacteria, but we used to think dietary fat itself was nearly all absorbed in the small intestine. But based on studies using radioactive tracers, we now know that about 7 percent of the saturated fat in a fat-rich meal can make it down to the colon, which may result in detrimental changes in our gut microbiome, weight gain, increased leaky gut, and pro-inflammatory changes. For example, a drop in beneficial bifidobacteria and a decrease in overall short-chain fatty acid production—both of which would be expected to “increase the risk of…gastrointestinal disorders.”

Okay, but striking at the heart of the matter, what might all that saturated fat be doing to our heart? If you look at low-carbohydrate diets and all-cause mortality, those who eat lower-carb diets suffer “a significantly higher risk of all-cause mortality,” meaning they live, on average, significantly shorter lives. From a heart disease perspective, though, it matters if it’s an animal fat or plant fat. Based on the famous Harvard cohorts, eating more of an animal-based low-carb diet was associated with higher death rates from cardiovascular disease—a 50 percent higher risk of dying from a heart attack or stroke—but no such association was found for lower-carb diets based on plant sources.

And it wasn’t just from Harvard. “Low carbohydrate dietary patterns favouring animal…protein and fat, from sources such as [red meat and chicken], were associated with higher mortality; whereas those [favoring] plant protein…and fat…, [from things like] vegetables, nuts, peanut butter, and whole grain[s], were associated with lower mortality…”

Cholesterol production in the body is directly correlated to body weight. Every pound of weight loss by nearly any means is associated with about a one-point drop in cholesterol levels in the blood. But put people on very low-carb ketogenic diets and the beneficial effect on LDL bad cholesterol is blunted or even completely neutralized. Counterbalancing changes in LDL size or HDL (what we used to think of as good cholesterol) are not considered sufficient to offset this risk. You don’t have to wait until cholesterol builds up in your arteries to have adverse effects, though. Within three hours of eating a meal high in saturated fat, you can see a significant impairment of artery function. Even with a dozen pounds of weight loss, artery function worsens on a ketogenic diet instead of getting better—which appears to be the case with low-carb diets in general.

So: bad for the gut, bad for the heart, but is it bad to the bone? We’ll find out next.

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