Intermittent fasting is quickly gaining popularity in the dieting world. A quick online search about this diet will show hundreds of success stories, with claims like “naturally lead to fat loss” and “eat, fast, and live longer,” (1). Based on the ancestral paleolithic diet, it is believed that brief periods of fasting protect lean body mass, protein, and glycogen stores (2). In a study published by the American Society for Nutrition, researchers determined that there was no significant difference in protein or fat metabolism or weight change when compared to a standard diet (2).
In fact, this study found that an intermittent fasting (IF) diet decreases resting energy expenditure (REE), how many calories your body burns at rest in a day, and may lead to weight gain if the diet is adopted for a long period of time. Increased glucose sensitivity was anticipated with this diet, but the results proved no significant increase in sensitivity (2). Given these results, this study disproves the hypothesis that an intermittent fasting diet will improve fat and muscle composition.
This study compared the effects of an intermittent fasting diet to a standard diet (SD) in eight healthy men. These men were put on the IF diet for two weeks followed by two weeks of the SD, with a four week break in between to eliminate the possibility of one diet affecting another. It is important to note that the study participants received the same amount of total calories each day in the SD and IF diet. Testing included: blood samples to measure glucose and its regulatory hormones, lipid profiles, and muscle composition. Weight was also recorded regularly (twice a week) to monitor any weight loss for either diet (2). Results from this study indicated that the average REE of its participants was 1,716 calories per day with the standard diet and decreased down to 1,698 calories with the IF diet (p<0.05), showing that the IF diet may decrease REE. Lipolysis, or breakdown of fat, was 0.4 µmol for both the SD and IF diets (not significant), meaning there was no significant difference in fat breakdown between the two diets. KIV, an indicator of protein synthesis, was 1.44 and 1.41 for the SD and IF diets, respectively (not significant). This concludes, again, that there is no significant difference in protein synthesis between these two diets (2).
Though this study seems to debunk many of the claims that are associated with an intermittent fasting diet, a close look at the study’s strengths and limitations is necessary before determining the validity of this diet. Using the same men for both the IF and standard diets proves to be a strength for this study because it eliminates the variability of body composition and metabolism between people that would occur with different treatment (IF diet) and control (SD) groups. This study also provided the same amount of calories each day with both diets. In doing so, the researchers were able to determine whether intermittent fasting caused weight loss and improved body composition or if these results were merely due to a reduction in caloric intake (commonly seen with the IF diet).
The participants of this study were all healthy males without any chronic diseases or other medical complications. Having such a specific test population limits the study’s ability to draw conclusions for an entire population. This demographic accounts for only a small percentage of the national population, so making sweeping generalizations from these results may be inaccurate. This study was also comprised of only eight participants, making the chance of error much more likely. Similar studies are needed to verify the effects of an intermittent fasting diet.
Luckily, multiple studies have been done in this area to determine the effectiveness of intermittent fasting. Another study also used healthy men as test subjects, but unlike the study previously mentioned, showed an increase in insulin sensitivity with intermittent fasting (3). With an increase in insulin sensitivity, intermittent fasting may provide benefits for those who are at risk for Type 2 Diabetes. A different study researched the effect of an alternate-day fasting diet in obese adults. This study found that this type of fasting diet increased weight loss and improved lipid profiles, such as cholesterol, LDL, and triglyceride levels (4). These positive effects may lead to decreased risk for cardiovascular disease (CVD), and in turn, an increase in life expectancy (5).
With such conflicting results between studies, how can you decide if an intermittent fasting diet will work for you? These studies show that those who are at risk for Type 2 Diabetes may benefit from the IF diet because of the diet’s potential to increase insulin sensitivity. This diet may also benefit those who are overweight, obese, or have an increased risk for CVD and other chronic diseases by improving lipid profiles. For individuals who are at a healthy weight and have a low risk for heart disease, this diet may be less effective. Research shows that the IF diet has little effect on fat loss and lean body mass and may even decrease energy expenditure (REE), potentially leading to weight gain if this diet is implemented for an extended period of time. Weight loss and improved body composition in healthy individuals on the IF diet may be attributed to a decrease in overall caloric intake, commonly seen in the IF diet, rather than the act of fasting itself.
- Fit Body HQ. Intermittent Fasting for Fat Loss: What It Is, How You Do It, & Does It Work? Available at: http://www.fitbodyhq.com/nutrition/diets/intermittent-fasting-for-fat-loss-what-it-is-how-you-do-it-does-it-work/.
- Soeters M, Lammers N, Dubbelhuis P, et al. Intermittent Fasting Does Not Affect Whole-Body Glucose, Lipid, or Protein Metabolism. Am J Clin Nutr. 2009; 90:1244-51.
- Halberg N, Henriksen M, Soderhamn N, et al. Effect of Intermittent Fasting and Refeeding on Insulin Action in Healthy Men. J Appl Physiol. 2005; 99: 2128-36.
- Varady K, Bhutani S, Church E, Klempel M. Short-term Modified Alternate-day Fasting: A Novel Dietary Strategy for Weight Loss and Cardioprotection in Obese Adults. Am J Clin Nutr. 2009; 90:1138-43.
- Johnson J, Laub D, John S. The Effect on Heath of Alternate Day Calorie Restriction: Eating Less and More Than Needed on Alternate Days Prolongs Life. Medical Hypothesis. 2006; 67:209-211.
Original article found at: http://ajcnnutrition.bbclovefood.com/content/90/5/1244.full
*Image found at http://www.fitbodyhq.com/category/nutrition/
Reviewed by Viktoriya Wolff