A ketogenic (or "keto") diet is an eating plan that's designed to seriously minimize carbohydrates, your body's favorite fuel source, and dramatically increase fats. The idea is that as carbohydrate levels drop, the body becomes forced to burn stored fat as its primary source of fuel, which can result in often dramatic weight loss. The diet represents a total turnaround from how most people eat: while the suggested American diet is about 50 percent carbohydrate, 15 percent protein, and 35 percent fat, the breakdown on most typical keto diets is 5 to 10 percent carbs, 70 to 75 percent fat, and the rest from protein.

The "keto" part refers to , which are water-soluble molecules that the liver makes when metabolizing fats, particularly when carbohydrate intake is low. Ketones can be used for energy by most tissues in your body, including the brain, which can't use unrefined fats as fuel.

Your body is always using a mix of fat and glucose for energy, but in a non-keto-adapted state, it reaches for glucose first, since only are normally generated during fat metabolism, and some tissues of the body—for example, the heart—prefer using ketones when they're available. The brain can't use fat, so it depends upon glucose when you're in a non-keto-adapted state.

If glucose is the body's normal go-to source of energy, you may be wondering what happens when it suddenly doesn't have enough to use as its main fuel.

Once stores of glycogen (the way the body warehouses glucose) become depleted, your brain and other organs begin the process of adapting to using fats and ketones instead of glucose as its main fuel. But reaching ketosis, the state in which fat provides most of the fuel for your body, isn't usually a pleasant experience. 

While the length of time it takes to adapt to a keto diet varies, the process begins after the first few days. Then, after about a week to 10 days, many low-carbers suddenly start to feel the positive effects of keto-adaptation.

They report improved mental concentration and focus and more physical energy as well.

By the end of the second week (sometimes up to three weeks), the body has usually accomplished the majority of its work in adapting to using fat for energy. By this point, hunger and food cravings are diminished and stamina and vitality increase.

After this, the body continues to make more subtle changes. For example, it gradually becomes more conserving of protein, so people often crave less protein. Another change that athletes often notice is less lactic acid buildup in their muscles with long training sessions, which translates into less fatigue and soreness.

 It can take up to 12 weeks for these changes to occur and for you to fully reach ketosis.

There are a couple of ways you can get over the hurdle of the first week of carbohydrate withdrawal:

Research so far shows that ketogenic diets (and low-carb diets in general) can lessen the symptoms of metabolic syndrome, , and polycystic ovarian syndrome (PCOS). Keto diets are also successfully used to . Studies indicate they may as well, such as Parkinson's disease, though more research is needed. 

The more scientists look at the keto diet, the more positive benefits they seem to find. For example, people on these diets have less of the saturated fat in their blood that's linked to insulin resistance, metabolic syndrome, and heart disease.

 Emerging research also shows that using ketones for energy may decrease oxidative stress and inflammation in the body, and may even be involved in turning on some genes that may be beneficial to health. 

Some people find that their ketosis is pretty stable as long as they eat a low-carb diet under about 50 grams of carbs a day, while others find they need to eat fewer carbs to stay in ketosis. Athletes and heavy exercisers often can eat more than 50 grams of carbs and still stay in ketosis. Other influences, such as hormonal fluctuations and stress, have been known to throw people out of ketosis.

Some people find value in measuring their blood ketones, which can be done at home using a special meter and test strips. But most low-carb diet authors don't recommend bothering with it. If you're getting the benefits you hoped for on a keto diet, worrying about how high your ketones are may just add a level of complication you don't need.

Keep in mind that, when adhered to, the keto diet does its job—but the margin of error is extremely thin. People on the diet who eat foods with sugar or high carbs (even only once), cause insulin to be secreted; the body goes from ketosis to high fat storage mode (because is being fed high amounts of fat and carbs/sugar), which can lead to significant weight gain.