Intermittent fasting (IF) is the newest diet making waves in the fitness industry. IF occurs in multiple forms with each “expert” putting their own twist on the diet. Some proponents suggest alternating between “feed days” and “fast days” while other diets have periodized hours in which you need to feed or fast. Regardless of which form of IF you follow the proposed benefits are the same. According to proponents IF can help you lose fat, gain muscle, and improve many other markers of health.
Unfortunately, the facts about IF are hard to find because science once again has taken a backseat to marketing and sales. Risks of the diet have been downplayed and benefits of the diet have been exaggerated. Prior to embarking on an IF program here are a few things you should know!
1) If your goal is to maximize lean mass, IF may not be the optimal choice. Recent research has shown that ingesting 20+ grams of whey protein every 3 hours is optimal for stimulating protein synthesis (muscle growth/repair). You cannot make up this protein synthesis effect with one huge meal. Research has shown that approximately 40-50 grams of protein in one sitting maximizes this response (depending on age and protein quality). Therefore if gaining lean mass is one of your goals, I would not recommend an IF diet. It would be more beneficial to eat every 3-5 hours and get a minimum of 20-30 grams of high quality protein per meal. http://www.ncbi.nlm.nih.gov/pubmed/23459753
2) Low energy levels may reduce the amount of work you are able to perform during exercise, thus reducing the magnitude of adaptation. Intense exercise uses carbohydrate as its primary fuel source; once you deplete your stored carbohydrate, your workout intensity will decrease significantly. High intensity exercise is the best form of exercise to maximize calorie expenditure. Per unit of time you burn significantly more calories and trigger an optimal hormonal response for fat loss. Consumption of a balanced meal every 3-5 hours and nutrient timing around workouts is recommended to maximize workout intensity and fat loss (assuming you are in a calorie deficit). Multiple studies conducted during Ramadan have shown reduced athletic performance and increased frequency of hypoglycemia when exercise is performed in a fasted state. Hypoglycemia (low blood sugar), in most cases, will end your workout due to severe nausea, dizziness, and headache. Studies during Ramadan also have shown increased feelings of tiredness and feelings of being unwilling to work. http://www.ncbi.nlm.nih.gov/pubmed/17632999
In addition to reductions in workout intensity, general fatigue throughout the day may lower NEAT (non-exercise activity thermogenesis) which is the sum of all activity throughout the day that is not related to exercise.
3) The way you eat should be sustainable for a lifetime. Before starting an IF diet ask yourself if it is something you can do forever. Most people find it very difficult, which is evident by the EXTREMELY high dropout rate in the few attempted studies (approximately 30%). Hunger levels tend to be very high and uncomfortable for most individuals despite claims of appetite suppression by proponents of the diet. Appetite has been shown throughout research to be best controlled by frequent meals. The only other diets that have been shown to reduce hunger levels are starvation diets and diets prescribing a chronic intake of less than 800 calories. In addition, if you are prone to disordered or binge eating, it may reinforce or increase deleterious behaviors.
Intermittent fasting is simply another way of creating a calorie deficit. For most people it is even harder to adhere to than conventional calorie restriction with no additional benefits. However, if you find you are able to adhere to the IF protocol long term, it is unlikely severe complications will arise. Calorie restriction without malnutrition has been proven to prolong survival; how you choose to accomplish this is a matter of individual preference and tolerance.