Intermittent Fasting: What’s all of the hype about?

Intermittent fasting is a hot topic in the diet and nutrition world. There are many books, blogs, celebrities and even apps touting the many health benefits of this pattern of energy consumption.  The question is whether there is sufficient clinical research to supports these claims.

Intermittent fasting has been a part of religious practices for centuries. Christian, Muslim, Buddhist, Hindu and Jewish populations all perform intermittent fasting at different times throughout the year.  Clinical studies on intermittent fasting are still quite limited and what we do know comes mostly from: animal studies, a handful of human trials with small sample sizes, and observational studies amongst religious groups. Though limited, evidence from some of these studies suggests that intermittent fasting might be beneficial in the treatment of some health conditions like: obesity, diabetes, high cholesterol, inflammation and cancer.

In a 2012 meta-analysis of individuals who participated in Ramadan, where fasting varied from 11-22 hours per day, 21 of the 35 studies included found a significant reduction in body weight.  Another 2013 meta-analysis of those who participate in Ramadan, found that the fasting period associated with Ramadan lead to a decrease in LDL and fasting blood glucose.  In women, HDL, “the good cholesterol,” was also increased.  Among men, there were reductions in total cholesterol and triglycerides.

Intermittent fasting is done in different ways and studies that have sought to further investigate the observational findings associated with religious fasting, have employed different fasting techniques.  The 5:2 diet is popular. This involves five days of habitual eating and two non- consecutive days of severe caloric restriction (20-25% of energy needs).  A pilot study of patients with type 2 diabetes examined the 5:2 technique versus continuous or daily energy restriction, what we think of when people say they are on a “diet,” and found both techniques resulted in similar amounts of weight loss and a reduction in HbA1C (a marker of blood glucose levels).

Not all studies on intermittent fasting have shown to have completely positive outcomes.  Some studies where complete calorie restriction is required for greater than 24 hours, have resulted in a negative impact on certain biomarkers.  For example, one study examined the difference in weight loss and glucose metabolism between three groups.  One group had 0% energy restriction for 36 hours, the second 100% energy restriction for 36 hours, and the third 75% energy restriction for 36 hours.  The group that was required to fast continuously for 36 hours was found to have impaired glucose metabolism and reduced insulin sensitivity upon breaking the fast.  These findings are in accordance with other research on prolonged fasting.  The body responds as it would to short-term starvation where glucose conservation and glycogen repletion are favored.  The 75% energy restriction group did not have the same impairment of glucose metabolism as the 100% calorie restricted group.  Weight loss was found in both fasting groups.

Time restricted fasting (TRF) appears to have many positive outcomes and few negative ones, assuming the fasting period is in line with circadian rhythms.  This type of intermittent fasting involves daily fasting periods from 12-21 hours.  In rodents, studies of TRF resulted in a reduction of weight, total cholesterol, triglycerides, glucose, insulin and inflammatory markers like IL-6, TNF-a and CRP.  Further, insulin sensitivity was improved.

One TRF human clinical trial involved 34 resistance trained male athletes.  In this study, the experimental group was required to consume all calories between 1 p.m. and 9 p.m., at set meal times of 1-2 p.m., 4-5 p.m. and 8-9 p.m.  The control groups meals were set at 8-9 a.m., 1-2 p.m. and 8-9 p.m.  Snacks were not permitted between meals.  The findings of this study after eight weeks were a significant reduction in fat mass, blood glucose, insulin levels, TNF-a and IL-1B (also pro-inflammatory) only in the TRF group.  Muscle mass was also maintained and strength was increased as all athletes participated in regular resistance training throughout the study.  A similar 2017 study involving athletes performing resistance training and TRF, found greater improvements in muscle size and strength in the TRF group compared to control.  Another human study was performed where a single meal with no calorie restriction was consumed in the afternoon each day for eight weeks and was compared to an isocaloric diet that was divided into three meals per day.  This study found that the one meal a day group experienced significant weight loss, decrease in fasting glucose and improvements in LDL and HDL compared to the three meal a day group.

As mentioned previously, though TRF seems to have a positive influence on a number of biomarkers, it is important to ensure meals adhere to circadian rhythms.  In a TRF study where rodents ate in opposition to circadian rhythms, these rodents became obese and were at an increased risk for developing type 2 diabetes.  A second group of rodents who consumed the same number of calories, but ate according to circadian rhythms, were protected from: obesity, fatty liver, hyperinsulinemia and inflammation.  It is also well known that night shift workers are at an increased risk for obesity and that night time eating can lead to poor sleep.  Poor sleep is also associated with insulin resistance, increased risk of obesity, diabetes, cardiovascular disease and cancer.  Interestingly, an analysis of 2337 breast cancer survivors who fasted less than 13 hours per night during seven years of follow up, were found to be at an increased risk for recurrence.  This analysis also showed that short nightly fasting was associated with an increase in HbA1C.

The research seems to suggest that intermittent fasting can be helpful for weight loss and impacting other biomarkers in a positive way.  However, it also seems to suggest that some forms of intermittent fasting may be better than others, as some methods may have some negative effects.  It is also safe to say that more research is required in this area, but the research does provide some hints that “breakfast” may not be the most important meal of the day.


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Carter, S., Clifton, P.M., Keogh, J.B. (2016). The effects of intermittent compared to continuous energy restriction on glycaemic control in type 2 diabetes; a pragmatic pilot trial. Diabetes Research and Clinical Practice. 2016;122:106-112: DOI: 10.1016/j.diabres.2016.10.010.

Moro, T., Tinsley, G., Bianco, A., Marcolin, G., Pacelli, Q.F., Battaglia, G., Palma, A., Gentil, P., Neri, M., Paoli, A. (2016). Effects of eight weeks of time-restricted (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males.  Journal of Translational Medicine.  2016;14:290: DOI: 10.1186/s12967—016-1044-0.

Patterson, R.E., Sears, D.D., (2017). Metabolic Effects of Intermittent Fasting. Annual Review of Nutrition. 2017;37:371-393: DOI: 10.1146/annurev-nutr-071816-064634.

Tinsley, G.M., Forsse, J.S. Butler, N.K., Paoli, A., Bane, A.A., La Bounty, P.M. (2017). Time-restricted feeding in young men performing resistance training: A randomized controlled trial. European Journal of Sports Science. 2017;17(2):200-207: DOI: 10.1080/17461391.2016.1223173.

Photo by Sonja Langford on Unsplash

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