That doughnut you eat first thing in the morning on the way to work signals the body to produce glucose and insulin. Glucose is the easiest molecule for the body to convert into energy, which is why it is the body’s preferred energy source. Insulin is produced to process the glucose in the bloodstream. When glucose is used as a primary energy source, fats are not needed for energy, so they are stored — and our waistlines expand. For most people’s day-to-day diet, glucose is the main energy source because of its abundance. However, because the body doesn’t store excess glucose, any leftovers are converted into fat, which is more easily stored. Again, this causes weight gain. When the body runs out of glucose, the brain says, “Send more!” and signals the appetite to reach for a quick carb snack rather than burning stored fat. This signaling system is central to the efficiency of the body’s metabolism and is the basis of the typical hunger cycle.
The keto diet trains the body to stop depending on glucose and increase its reliance on fats for energy, based on the premise that when carb intake is lowered, the body begins to look for alternative energy. This drives the shift into a new metabolic state: ketosis. When the body is in this metabolic state, ketones are produced from the breakdown of fat in the liver, which works as a survival mechanism in the body. While the body produces insulin from the carbs we eat, fat cells stay stored in the body and do not enter the bloodstream. Reducing carb intake lowers glucose levels, along with blood sugar levels, which in turn lowers insulin levels. The process allows the fat cells to release their stored water, enter the bloodstream, and head to the liver. Advocates of the keto diet say the goal is to move into this metabolic state of ketosis. They remind the dieter that ketosis does not starve the body, as its detractors have said. Instead, it starves the body of carbohydrates, allowing weight loss.