Your body is an amazing machine that is able to extract energy from just about anything you eat. While glucose is your body's preferred energy source, you can't convert fat into glucose for energy; instead, fatty acids or ketones are used to supply your body with energy from fat.
Fat for Energy
Fat is a concentrated source of energy, and it generally supplies about half the energy you burn daily. During digestion and metabolism, the fat in the food you eat is broken down into fatty acids and glycerol, which are emulsified and absorbed into your blood stream. While some tissues -- including your muscles -- can use fatty acids for energy, your brain can't convert fatty acids to fuel.
If you eat more fat than your body needs, the extra is stored in fat cells for later use. Fat has more than twice as many calories per gram as carbs and protein, which makes it an efficient form of stored energy. It would take more than 20 pounds of glycogen -- a type of carbohydrate used for fuel -- to store the same amount of energy in just 10 pounds of fat.
Your Body Makes Glucose From Carbs
Almost all the glucose in your body originated from carbohydrates, which come from the fruit, vegetables, grains and milk in your diet. When you eat these carb-containing foods, your digestive system breaks them down into glucose, which is then used for energy by your cells. Any excess glucose is converted into glycogen, then stored in your muscles and liver for later use. Once you can't store any more glucose or glycogen, your body stores any leftover carbs as fat.
Fat Converted to Ketones
Glucose is your brain's preferred source of energy. However, when glucose is in short supply, your brain can use ketones -- which are derived from fat -- for fuel. Since your brain accounts for approximately one-fifth of your daily calorie burn, your body will typically contain some ketones as a backup source of energy for your brain. When you go on a very low-carb diet, your ketone levels increase so that your brain still has access to the fuel it needs.
Glucose Versus Ketones
While it's normal for your body to use fat as energy, the benefits of ketones versus glucose is not well-known. Ketones help suppress appetite, which is beneficial if you're following a low-carb diet to lose weight, according to a 2015 review article published in the Frontiers of Psychology. When you're following a low-carb diet to get into ketosis -- a state where you're primarily using ketones for energy -- you need to drink plenty of water to avoid dehydration.
High levels of ketones increase the acidity of the blood, which can make some people sick very quickly. In addition, people with diabetes need to avoid high ketone levels to prevent damage to the kidneys and liver.