Intermittent fasting (IF) – typically defined as anywhere from 12 to 18 hours without food in each 24 hour period OR full days of fasting interspersed throughout the week – is a dietary pattern that focuses on when to eat, not what to eat. Research has linked IF to a myriad of health benefits, including weight loss, improved brain function and inflammatory markers, reduced insulin resistance and heart disease risk, cancer prevention, and anti-aging. While this list is impressive, studies on IF are in their early stages and are limited due to the lack of human, randomized controlled studies, small sample sizes, short study periods, and minimal follow-up with subjects. Despite these limitations, IF exemplifies many positives and could be a viable and healthy weight loss option for some individuals.
There are three main methods of practicing IF (on non-fasting days/times, there are no restrictions, caloric or otherwise):
1). Time restricted feeding– Food is consumed within a restricted window each day, for example 9am to 5pm; complete fasting (water only) otherwise
2). Whole-day fasting– 1 or 2 days per week consists of complete fasting or no more than 500 calories (or 25% of your daily caloric intake); no restrictions on non-fasting days
3). Alternate-day fasting– Every other day is a fasting day, which consists of complete fasting or no more than 500 calories (or 25% of your daily caloric intake); no restrictions on non-fasting days
IF For Weight Loss
While fasting can lead to a lower overall intake of calories and is one of the main reasons IF leads to weight loss, this is not the only basis for IF’s weight loss success. IF actually triggers a series of changes on a cellular level that promotes fat loss and increased calorie expenditure. When practiced regularly, IF increases human growth hormone and the release of norepinephrine (the fat burning hormone) and decreases insulin levels – all key factors in weight loss. In fact, studies show that these hormonal changes can result in a 3.6 to 14% increase in metabolic rate. Fasting may also increase the amount of brown fat in our bodies – possibly mediated through changes in the gut microbiome – brown fat burns energy (or calories), while white fat stores energy – which could also explain fasting’s effects on weight loss.
A review of 40 fasting studies for weight loss showed that IF is an effective dieting tool, resulting in an average of 9 pounds lost over 10 weeks. But how does IF compare to daily calorie restriction, which is the most common diet for weight loss? Based on the study findings, researchers concluded that with a dropout rate as high as 60% for some studies, IF is not necessarily a diet with a higher compliance rate than calorie restriction. A smaller, less extensive review on IF for weight management included 6 studies and found a drop out rate of 20%, which is lower than most other weight management programs. This difference in attrition rates tells us that attrition is most likely very dependent on the population and type of IF intervention utilized.
The 40-study review also found that overall, IF did not result in more weight loss or body composition changes when compared to calorie restriction. There is some discrepancy here as well; a small sample of studies show that IF results in greater fat loss or insulin resistance when compared to cutting calories, but based on the research as a whole, IF seems comparable to calorie restriction in these areas.
The downfall of this large 40-study review is that the studies analyzed varied widely in sample size, timeframe, and the type of IF intervention used (time restricted vs. whole-day vs. alternate-day). So while these findings offer some insight, there’s still so much we don’t know about IF, including which IF method, frequency, and time length is most effective for therapeutic and weight loss effects, its safety in specific populations (the elderly and sick), and the psychological and dietary behavior impacts fasting has on those who practice it as well as surrounding friends and family who witness it (children, for example).
While it seems probable that IF could trigger overeating during non-fasting days/windows, research does not show this. In fact, some studies show that fewer calories are consumed during non-fasting times. Compliance could be an issue though. For those accustomed to eating every 2 to 3 hours, IF can be a difficult dietary pattern to adopt. If you decide to adopt IF, finding the right fasting pattern to fit your lifestyle is important. And if you’re someone who has a difficult time restricting calories daily and following “food rules”, IF could be a good option for you.
Other IF Benefits
While some of us are focused solely on inches/pounds lost when trying to lose weight, it’s important to choose a diet that not only promotes weight loss, but also promotes health. Fasting accomplishes both. Fasting alters the expression and function of genes playing a role in longevity and disease prevention and increases the initiation of cellular repair, explaining why IF may be an important tool in anti-aging and disease prevention. Human studies show convincing results that IF is protective against cancer, diabetes, cognitive aging, and heart disease. IF has also proven to have profound effects on increasing lifespan in animals (rats, mice, and fruit flies) and in vitro, but these findings are much less evident in humans.
However, positive alterations in gut health have been shown to be a universal outcome of IF in both animal and human studies. These changes include an increase in bacterial diversity and richness, as well as higher levels of beneficial bacteria. Because of IF’s significant impact on improving the gut microbiome, scientific exploration is currently underway using IF as a treatment for diabetes-associated retinopathy and for the management of multiple sclerosis.
Want To Try IF? Here’s Where To Begin
For those of you who are interested in IF, you may be wondering where to begin. Here’s your answer: The most recent scientific evidence on IF suggests that a circadian rhythm approach, one that restricts eating to an 8 to 10 hour period during the daylight hours may be the most effective. And when combined with a healthy lifestyle (especially a healthy diet and exercise), benefits are exponential. Based on a small study, an 8-hour feeding window early in the day (7am to 3pm) resulted in drastically lower insulin levels, improved insulin sensitivity, lower blood pressure, and decreased appetite when compared to a more extended feeding window of 12 hours (7am to 7pm).
While more studies are needed to confirm these findings, time restricted feeding can be a simple way to introduce fasting into your daily life without significantly altering your lifestyle. If 7am to 3pm seems challenging, try 10am to 6pm, or even 8am to 6pm. Remember, the earlier in the day the better, and avoid snacking in between meals.
When Not To Fast
While fasting may seem like the answer to your weight loss (or gut) woes, some individuals should avoid fasting. These populations include those with a history of an eating disorder, women who are pregnant or nursing, and children. Remember to consult with your healthcare provider before starting any new dietary plans.