Intermittent fasting has been proposed as a method of weight loss in recent years. However, questions remain as to whether it is an effective and safe strategy to manage body weight. A recently published investigation attempts to answer this question. Eighteen men and women completed two three day trials in a crossover design. On the first day, subjects consumed 100% of their energy needs. On the second day, a standardized breakfast was consumed, hormones and appetite were measured and an all you can eat breakfast and lunch was served to all subjects. On day three, an all you can eat breakfast was offered. The second trial started with subjects consuming only 25% of energy needs. Days 2 and 3 in the second trial were identical to the first trial.
There were several important results reported:
1) When subjects restricted their energy intake on day 1, they consumed 7% more energy on day 2. By day 3, they consumed no more calories than they did when they did not fast.
2) Appetite was higher on day 2 when subjects fasted, but not different on day 3.
3) Blood glucose was significantly higher after energy restriction.
4) Energy expenditure was significantly lower on the morning of day 2 after fasting. American Journal of Clinical Nutrition 2016; 104:1545-53.
Take Home Message
This is a fascinating study to me. I have never recommended fasting as a method of weight loss. Is it effective? In the short term, probably. I would argue that it is not in the long term. I think you win the battle but lose the war by fasting. While you will certainly lose some weight, it is my opinion that starving yourself recruits some of our body’s most effective defense mechanisms. Specifically, metabolic rate is slowed and we start to burn muscle in an effort to spare body fat. This combination makes long term weight loss virtually impossible.
A lot of my concerns about fasting were validated by this study:
1) Subjects were very hungry the day of the fast and even the second day as well. This makes compliance tough because most people are pretty miserable when they are hungry and won’t let themselves stay hungry for long.
2) Subjects consumed more calories the next day than if they had eaten normally.
3) Their energy expenditure was lower the next day. In other words, the fasting seemed to slow down their metabolism. Not good.
4) Most concerning was that their blood sugar and insulin levels were significantly greater when they fasted. This indicates impaired glycemic control, which is not a good thing at all.
5) On a side note, if you are cutting your calories by 75% up to 4 days per week, it is going to be impossible to get enough vitamins, minerals, antioxidants, fiber, etc on those days. These nutrients are hugely important for our health and for chronic disease risk reduction.
In conclusion, there are no shortcuts to losing weight. It takes a lot of work, but the path to a healthy weight and a healthy body is a combination of a balanced diet, a good cardiovascular exercise program and consistent resistance training.