What Are the Disadvantages of a Keto Diet?

Updated July 2026

If you've ever tried to lose weight, you've probably at least been tempted by keto. The ketogenic diet promises weight loss, and especially body-fat loss, based on the insulin-blunting effect of eating very few carbohydrates. But the logic is not as clean as it sounds, and the trade-offs are real.

Where did the ketogenic diet come from?

It began as a medical treatment, not a weight-loss plan. The ketogenic diet was originally developed to reduce seizures in children, because powering the brain with ketones instead of glucose lowered seizure frequency. Normally the carbohydrates you eat become glucose, the body and brain's preferred fuel. When you don't eat enough carbs, the body switches to a backup system: the liver turns stored and dietary fat into fatty acids and ketone bodies, and those ketones can fuel the brain. It's an adaptive survival mechanism, the same one that kicks in during any near-starvation, ensuring your brain keeps running when food is scarce.

Does eating fat instead of carbs really help you lose fat?

This is where keto gets suspect. Advocates reason that since insulin is a storage hormone, driving insulin as low as possible should improve blood sugar, improve cholesterol, and shrink fat stores, and the way to do that is to eat mostly fat instead of carbs. A typical keto plan is about 5% of calories from carbs, 15% from protein, and a striking 80% from fat. For contrast, people in the world's Blue Zones, the communities with exceptional healthy longevity, get about 65% of their calories from carbohydrates. Keto advocates would, in effect, tell you all those centenarians have it backwards.

But how do you lose net body fat while deliberately eating large amounts of it? The short answer is you mostly don't. Keto can make the scale drop, but objective studies show fat stores don't change much; most of the early loss is water and protein. That's why the leg muscles of CrossFit participants put on a ketogenic diet shrank by as much as 8%. The benefits usually credited to keto, such as lower insulin, lower cholesterol, lower blood pressure, and weight loss, come largely from short-term studies in people with type 2 diabetes, and they're the same changes you'd see with plain starvation, or frankly with any illness that causes weight loss. Improving a few markers while losing weight doesn't automatically mean you're healthier. (More on keto and diabetes here.)

What does keto do to your cholesterol?

It tends to push it the wrong way. Blood sugar may fall, since you're severely restricting the main source of circulating glucose. But cholesterol is another story: studies have shown LDL increases of about 35% on average in people on keto plans. This is because saturated fats can negatively impact LDL receptor function, reducing our cells' ability to utilize the cholesterol that's floating around in our bloodstreams. In my own practice, I've watched a wave of patients develop high cholesterol after adopting this way of eating. High cholesterol, preserved body fat, and lost muscle is not a trade most people would knowingly accept for a better number on their scale.

So what's the real lesson from keto?

That the people who feel better on low-carb are usually just people who stopped eating garbage. Human experience points the opposite direction from keto: the communities with exceptional healthy longevity eat whole-food, plant-based, carbohydrate-rich diets, and even the "hunter-gatherer" groups famous for near-absent heart disease turn out to be far more gatherer than hunter. The only reason a low-carb diet makes some people feel better is that they've stopped eating the wrong carbs. A diet rich in beans, greens, nuts, seeds, fruits, vegetables, and whole grains in their least-processed form is the real key, and people who eat that way don't just live longer, they feel great doing it. That's the whole idea behind eating real food, not too much, mostly plants, and it's why Step One Foods are built from oats and cranberries and chia, not beef and butter.

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