Low-carb dieters who consume very few carbohydrates often go into ketosis. Ketosis develops when you use up your glycogen stores and need an alternate source of energy. Your body forms ketone bodies as it breaks down fat to use for energy, and you excrete ketones in the urine. Low-carb dieter use ketone test strips to ensure that they're following the diet correctly and burning fat. Caffeine might disrupt glucose metabolism, which could affect ketosis, although only anecdotal evidence of this exists.
Ketosis and Insulin Resistance
Insulin resistance, the inability of cells to respond to and absorb glucose, can raise glucose levels and cause weight gain. Ketosis decreases insulin resistance by improving insulin sensitivity, meaning the ability of cells to absorb glucose. Insulin helps cells take glucose from the bloodstream to use for energy. Improving insulin sensitivity helps you lose weight. Caffeine might increase insulin resistance.
Caffeine and Insulin Resistance
Caffeine might increase insulin resistance, which might make losing weight more difficult and also increase your chances of developing type 2 diabetes, although this has not been clinically proven. A study conducted by researchers from Duke University Medical Center in the August 2004 issue of "Diabetes Care," published by the American Diabetes Association, discussed the effects of caffeine on blood glucose levels and insulin sensitivity. This study showed that 250 mg of caffeine did not change fasting glucose levels but did raise glucose levels after consumption of 75 g of glucose compared with placebo.
Although caffeine might raise glucose levels after eating a meal high in carbohydrates, it's unclear that this effect occurs after a low-carbohydrate meal such as those eaten by low-carb dieters. It's also unclear whether blood glucose would rise high enough to keep a low-carb dieter out of ketosis. Lyle McDonald, author of "The Ketogenic Diet: A Complete Guide for the Dieter and Practitioner," refutes the idea that caffeine can interfere with ketosis, stating that this theory is contradictory to the known effects of caffeine.
If you follow a low-carb diet and have trouble achieving ketosis, cutting out caffeine altogether or cutting down on caffeine and then rechecking your ketone levels would help you determine whether caffeine affects your blood glucose levels.