Our bodies are fueled on a mixture of carbohydrates, protein, and fat. However, when carbohydrates are highly restricted or removed, the body’s small stores are quickly depleted. We are then forced to find an alternative fuel to provide energy to our organs. Most organs automatically choose free fatty acids (FFA). But there is caveat: Your brain and nervous system, which are normally fueled by glucose, cannot use FFA—they rely on ketone bodies for fuel.

The human brain is like a hybrid car; it can feed off of two forms of power—glucose and ketones. When the brain doesn’t have glucose to run on, then it starts using ketone bodies, a by-product of the incomplete breakdown of FFA in the liver, serving up a non-carbohydrate, fat-derived fuel. By eating a diet of primarily fat (sadly, no, not the kind of fat from cheeseburgers and fries, but the fats found in coconut oil, nuts, avocado, and fatty fish like salmon or sardines), you increase the amount of FFA in your blood, producing ketone bodies at accelerated rates.

The accumulation of ketones in the bloodstream induces a metabolic state called ketosis (aka the end-goal of the ketogenic diet). During ketosis, there is a decrease in the production and utilization of glucose. There is also a decrease in the breakdown of protein (found in your muscles) to be used for energy. So, your body burns fat, without sacrificing any muscle—your basic low-fat diet can’t say the same. This is the only known “protein-sparing” weight-loss technique; therefore, many individuals use ketogenic diets to lose body fat while maintaining muscle mass and keeping their metabolism running (try doing that on your average low-cal diet).

Ketogenic diets cause our bodies to burn stored fat as fuel by affecting two hormones: insulin and glucagon. Restricting carbohydrates from the diet decreases insulin and increases glucagon. As a result, our bodies quickly move from storage mode to burn mode—we release triglycerides from fat cells and burn FFA, producing ketones as brain fuel. In addition to insulin and glucagon, a number of other hormones are also affected, all of which help to shift fuel use away from carbohydrates and towards fat.

When given the choice, your body will always choose storage mode and carbohydrates in the form of glucose. Without carbohydrates, which are needed for energy, it can be very hard to sustain your hot yoga and HIIT class. For this reason, individuals who wish to use a ketogenic diet to burn body fat and still go full-force with their workouts must integrate carbohydrates. There are two ways to do this: You can consume carbohydrates immediately before and after exercise to sustain performance without affecting ketosis; or you can alternate periods of ketogenic dieting with periods of moderate carbohydrate consumption. The period of carbohydrate eating is a must to sustain explosive or demanding exercise. If you’re planning to keep up with your intense fitness routine and want to try a ketogenic diet, talk to your doctor or nutritionist to make sure you’re going about it the right way.

Everyone will have a different carbohydrate ceiling that pushes the needle from storage mode into fat-burning mode. Some people may need to restrict carbohydrate consumption to 20g a day, while others may get away with eating close to 30g. Urine test strips are the easiest way to test for the presence of ketones.

Attaining ketosis happens most efficiently when carbohydrates are limited, fat consumption (but only the healthy fats!) is high, and protein intake is moderate. In fact, overeating protein is one of the most common mistakes, because too-much protein actually causes glucose production.

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