And that's what concerns physicians. Detoxification benefits remain unproven while, in fact, the opposite can happen, says Dr. C. Wayne Callaway, a specialist in endocrinology, metabolism and clinical nutrition at George Washington University Medical School. Ketosis sets in when you fast for more than 18 hours, says Dr. William Daughaday, clinical professor specializing in endocrinology and metabolism at the University of California, Irvine. You're burning so much fat that your body can't handle effects of byproducts of fat oxidation. Fasting can exacerbate conditions, including Refsum's disease and Reye's syndrome, he said.
Neurologic problems can result.
The risk goes up to about 67 percent. And if you've lost at least 22 pounds, the risk doubles. When you fast, your blood pressure, blood sugar and blood fats decrease, but these are transient effects, Callaway says. They go up as soon as you eat. Worse, you also lose bone and muscle.