Many people consume far more carbohydrates in their diets than they actually need. Once, a patient asked me how much carbohydrates she should be eating every day, because her dietitian told her she needs at least 250 grams per day or she could get brain injury from “hypoglycemia.” The Dietary Guidelines published by the USDA, The Institute of Medicine, and The American Heart Association recommend that carbohydrates make up 45 to 65 percent of total daily calories. Based upon 2,000 calories/day, between 900 and 1,300 calories should be from carbohydrates (225 and 325 grams of carbohydrates a day). Historically, the brain has been considered the only organ that requires glucose as a fuel source, utilizing approximately 100-140 g glucose per day. However, with ketoadaptation, the CNS reduces the obligatory glucose requirement by approximately 80%, resulting in a true utilization of 20-28 g glucose/d.
Endogenous glucose production through gluconeogenesis is approximately 2.8-3.6 g/kg/d, or approximately 210-270 g/d in a 70kg human, far greater than the obligatory requirement of 20-28g glucose/d of the ketoadapted human. According to the Food and Nutrition Board of the Institute of Medicine of the US National Academies of Sciences, “The lower limit of dietary carbohydrate compatible with life apparently is zero, provided that adequate amounts of protein and fat are consumed.” This does not mean that patients should strive to eat zero carbohydrates; what it means is that half your calories don’t need to come from carbohydrates.