All diet plans have positives and negatives, and extreme diet plans such as one-day fasting are no exception. While proponents of one-day fasts suggest that such plans can be more effective than other weight loss plans, research suggests that fasting can be detrimental not only to your weight loss efforts but your overall health. Consult a doctor prior to starting any diet plan.
If you're used to eating every day, starting a plan that includes one-day fasts can be difficult to adjust to. Even diets that allow you to eat every day are difficult due to the reduced calorie intake, so you may find it very difficult to deal with hunger when that reduced calorie intake has to sustain you for two days. According to a study from the January 2011 issue of "The Journal of Nutrition," reducing the frequency of your meals significantly reduces your ability to control your appetite. This may cause you to cheat on your diet or at the very least, make it unpleasant.
Food provides your body with energy, so depriving your body of food may encourage it to burn stored fat for energy. Some research backs up this suggestion; a study published in the January 2005 issue of "The American Journal of Clinical Nutrition" found that fasting every other day promoted an increased rate of fat burning. Subjects participating in the one-day fasts burned 15 g more fat each day, which equates to 135 calories.
If you're active or plan on engaging in exercise to enhance your weight loss, one-day fasts can be detrimental. A lack of food can compromise your exercise intensity; research published in the June 2007 edition of "British Journal of Sports Medicine" found that fasting significantly reduced endurance, speed and agility. Reduced exercise performance can inhibit your strength-gaining results and will also reduce the amount of calories you burn in each session, compromising weight loss. According to the study, reductions in performance remained for two weeks after the fasting period ended.
Your dietary intake affects your hormone levels, and research suggests that some of the hormonal effects can be beneficial. A study published in the December 2005 edition of "Journal of Applied Physiology" found that fasting could promote improvements in insulin sensitivity. Improved insulin sensitivity may help you gain more muscle and keep fat at bay, although the foods you eat also have an effect on body composition.