By Rachel Vincent, RD, LDN
According to Google’s latest “Year in Search” report, intermittent fasting was among the most searched diet terms in 2018 and is still on the rise in 2019. Notably, a recent March article on Cooking Light’s website featured Minneapolis Chef Mike DeCamp of P.S. Steak, who lost 55 pounds shrinking his eating window to a span of just 8 hours, between 5 pm-1 am during his typical work schedule. As with all new dietary approaches, we should first examine the supporting research to understand how effective this strategy might be, and increase awareness of any potential drawbacks, side effects or safety concerns before deciding whether to try it out.
Types of Fasting Regimens
There are four major classes of intermittent fasting:
- Complete alternate-day fasting consists of periodically fasting from all caloric food and drink, followed by periods of liberal eating.
- Modified alternate-day fasting involves periodically decreasing calorie intake to about 20-25 percent of energy needs, followed by periods of liberal eating. A popular example is the 5:2 Fast diet, published by British journalists Michael Mosley and Mimi Spencer, in which individuals eat 500 calories per day twice per week, and eat normally for the other 5 days.
- Time-restricted feeding limits calorie intake to a specific time window to induce fasting periods every day.
- The fourth type, religious fasting, varies in approach. Examples of religious traditions that feature fasting include Islam during the holy months of Ramadan, Latter-Day Saints during the first Sunday of every month and Seventh-Day Adventists when extending their nighttime fast by consuming the last two of their daily meals in the afternoon.
Intermittent Fasting Human Studies and Weight Loss
A 2018 systematic review and meta-analysis of 11 randomized controlled trials comparing modified alternate-day fasting with continuous energy restriction in humans showed no significant different in weight-loss percentage between the two approaches. Participants experienced approximately 5.2-12.9 percent weight loss over a span of 8-24 weeks. Five trials measured waist circumference and did not show any differences between the two diets.
A 2017 narrative review of six randomized comparisons of alternate-day fasting and continuous energy restriction showed comparable reductions in body weight. Participants experienced approximately 3.6-7.8 percent weight loss over a span of 12-26 weeks. Four of the studies showed equivalent body fat loss, while one reported a greater loss of body fat in the intermittent fasting regimen. However, small participant sample sizes in these studies led to difficulties detecting significant differences.
A 2018 study of time-restricted feeding centered on 23 participants who restricted eating between 10 am and 6 pm. There were no limitations on the types or quantities of the foods consumed for either group. Control group participants were instructed to maintain their weight and not change their dietary or activity habits. The results showed the 8-hour time-restricted group decreased energy intake by about 300 calories per day and experienced an average of 3 percent body weight loss in 12 weeks without calorie counting.
Potential Drawbacks, Side Effects and Safety Concerns
The appeal of these approaches, in focusing more on when you eat than what you eat could also be a potential nutritional drawback. Cookies, candy, chips and soda consumed in an 8-hour window are still less healthy than fruits, vegetables, whole grains and legumes consumed in a 14-hour window. Interestingly, in the Cooking Light article, Chef DeCamp noted that he didn’t just limit his eating timeframe, but also shifted his choices toward plant proteins, vegetables, legumes and omega-3 fats, which undoubtedly contributed positively to his weight-loss results.
As far as potential side effects, a 2018 systematic review identified a few mild adverse effects in less than 20 percent of intermittent fasting research participants, including low energy, persistent hunger, headaches, irritability, trouble concentrating, constipation and feeling cold. Dehydration and electrolyte imbalances can also be a potential risk of the more severely restrictive fasting approaches.
Another consideration that remains unclear is determining the long-term effects of continuing to follow an intermittent fasting regimen. To date, there are no long-term human studies with follow up greater than one year confirming the sustainability or effectiveness of intermittent fasting.
Additionally, individuals with diabetes, who use medications that require food intake, children and adolescents, women who are pregnant or breastfeeding, and those with a past or current history of eating disorders such as anorexia or bulimia should not try intermittent fasting.
Thus far, research suggests that short-term intermittent fasting results in comparable weight-loss outcomes in individuals with obesity to those following a traditional daily calorie restricted program. This approach is not for everyone, however with appropriate support from a nutrition expert, fasting can be a useful tool in select cases. More research identifying long-term outcomes would be helpful before deciding to adopt this as a permanent lifestyle change for weight maintenance.
Rachel Vincent, RD, is a clinical nutritionist educator at Mission Weight Management.