It usually takes three to four days for your body to go into ketosis because you have to use up your body's stores of glucose, i.e., sugar first, Keatley says. Any major diet change can give you some, uh, issues, and Keatley says he often sees patients who complain of IBS-like symptoms and feeling wiped out at the beginning of the diet. (The tiredness happens because you have less access to carbs, which give you quick energy, he explains.)
This seminal randomized trial, published in JAMA in 2007, involved 300 women and measured their weight loss on the Atkins diet compared to the Zone, Learn, and Ornish diets. The researchers found that while women on Atkins shed a few more pounds, the weight loss on low-carb diets was “likely to be at least as large as for any other dietary pattern” and “the magnitude of weight loss [on Atkins] was modest, with a mean 12-month weight loss of only 4.7 kg.” In other words, long-term weight loss on Atkins wasn’t meaningfully different from the other diets.
Long-term use of the ketogenic diet in children increases the risk of slowed or stunted growth, bone fractures, and kidney stones. The diet reduces levels of insulin-like growth factor 1, which is important for childhood growth. Like many anticonvulsant drugs, the ketogenic diet has an adverse effect on bone health. Many factors may be involved such as acidosis and suppressed growth hormone. About one in 20 children on the ketogenic diet develop kidney stones (compared with one in several thousand for the general population). A class of anticonvulsants known as carbonic anhydrase inhibitors (topiramate, zonisamide) are known to increase the risk of kidney stones, but the combination of these anticonvulsants and the ketogenic diet does not appear to elevate the risk above that of the diet alone. The stones are treatable and do not justify discontinuation of the diet. Johns Hopkins Hospital now gives oral potassium citrate supplements to all ketogenic diet patients, resulting in one-seventh of the incidence of kidney stones. However, this empiric usage has not been tested in a prospective controlled trial. Kidney stone formation (nephrolithiasis) is associated with the diet for four reasons:
Eating carbs drives up insulin production, the hypothesis suggests, stirring hunger and causing the body to hold on to fat and suppress calorie burn. But when you replace carbs with fat, you subdue hunger, boost calorie burn, and melt away fat. With fewer carbs, your body also doesn’t produce as much insulin — and that increases the rate of ketogenesis and decreases the body’s need for glucose.