The ketogenic diet is one of the most popular diets right now—and one of the most misunderstood. While the first ketogenic diet was first developed as a tool for managing childhood epilepsy in the 1920s, other forms of the diet have exploded in popularity as a weight loss strategy over the last 15 years, thanks in part to Dr. Atkins and other weight loss gurus.
The point of the diet is to put the body into a state of nutritional ketosis, where more ketone bodies are being produced than burned, leading to ketones becoming measurable in the breath and urine (and causing test strips like ketostix to turn purple when you pee on them).
Ketones in general are a larger class of chemical, but the kind of ketones we’re talking about here are a biological fuel that our bodies make when they break down fat for energy. By restricting food that isn’t fat to 20% of total calorie intake, the original ketogenic diet (the one for kids with epilepsy) forces the body to get most of most of its energy by burning dietary fat, leading to high ketone levels in the blood. Since high blood ketone levels protect the brain from seizure, this diet makes perfect sense for those kids.
Low calorie diets also leads to ketosis. After about 18 hours of fasting or 3–4 days of moderate caloric restriction, we also see high levels of ketones in the blood as the body begins to turn to its fat stores for energy. Different mechanisms, same result: high blood ketones.
This is where the confusion starts.
If your body is burning fat stores for energy because of caloric restriction, you will go into a state of nutritional ketosis. If your body is burning dietary fat because that’s the only fuel you’re giving it, it will also go into a state of nutritional ketosis. Ketones in the blood only mean you’re burning fat, not that you’re burning body fat.
Online, you’ll find advice like ‘if you’re not seeing success losing weight on keto, eat more fat to kickstart your fat metabolism and get back into ketosis!’. You can also find exogenous ketone supplements for sale. These approaches do increase blood ketones, and can be helpful for athletes, epileptics and for people transitioning to a low-calorie ketogenic diet, but they won’t help anyone lose weight.
Just like every other diet, keto still follows the laws of thermodynamics. Weight loss only happens on a high-fat ketogenic diet if you’re also eating fewer calories than you’re burning.
Ketosis can be a sign of weight loss, but it doesn’t cause it. In fact, extremely high levels of ketones decrease fat metabolism, because your body views extremely high ketone levels as a signal that it needs to preserve fat stores.
To be clear, I’m not trying to knock ketosis. I’ve personally played with ketogenic diets for years, and think they’re great for a lot of things. If you find (like a lot of people) that it’s easier to restrict calories while eating a ketogenic diet, go for it. If you want to go into ketosis for the (very real) cognitive and metabolic benefits, also great.
There are a lot of real, well-researched benefits to ketosis, but magical, no-effort weight loss is not one of them.
P.S. If you’re curious about exactly what’s going on when you enter ketosis, including the fascinating interplay of insulin, glucagon, free fatty acids, fat storage, adrenergic receptor systems, and everything else that’s involved, I highly recommend Lyle McDonald’s book The Ketogenic Diet.
