Intermittent fasting has taken the diet world by storm, with many touting its benefits for weight loss, metabolic health, inflammation, longevity and more. But is it an effective, long-term strategy for weight loss, or just another diet?

Intermittent fasting has generated considerable interest among nutrition researchers, but much of this research is still in its infancy. Some small studies in humans have started to move beyond the hypothesis-generating studies (usually done in animals), so we are building up a base of data to help us understand the potential benefits of intermittent fasting. Read on to learn what the current evidence says about what intermittent fasting can (or can’t!) do for you. 

What Is Intermittent Fasting?

Intermittent fasting, also known as time-restricted eating, is a way of eating that restricts food intake to a certain number of hours during the day. The most common style of intermittent fasting is 16:8, meaning fasting for 16 hours and eating for 8 hours, but there are several other intermittent fasting methods:

• The 5:2 Method: Eat normally for 5 days per week. Restrict your calories to 500 per day on two days per week.
• Alternative Day Fasting: Fast every other day, limiting calories to 25% of your usual intake.
• Eat Stop Eat Method: Fast for 24 hours every 1-2 weeks.
• One Meal A Day: Eat a single meal with all of your daily caloric requirements.

But let’s not lose sight of one thing: Most of us already practice intermittent fasting when we sleep. Let’s say you sleep for 8 hours and stop eating 2 hours before bed – you’re already fasting every day for 10 hours (keep that in mind as you continue reading!).

What’s The Pathway Between Intermittent Fasting and Weight Loss?

One of the most commonly talked about benefits of intermittent fasting is that it helps with weight loss. You hear stories about people who struggled to lose weight for years until they finally met their goals with intermittent fasting. So, how does it work?

Well, there may be nothing special going on here except calorie restriction. You’ve restricted the amount of time you can eat; therefore, you also decrease your total calorie intake. This creates a calorie deficit, which helps with weight loss.

While animal studies have shown considerable benefits on weight loss and other markers of metabolic health, such as fasting glucose, insulin and inflammation, human studies are largely disappointing.

In a recent randomized trial on intermittent fasting, 138 participants practiced intermittent fasting (with an 8-hour eating window) or daily calorie restriction. After one year, the two groups had no differences in weight loss, waist circumference, fat mass, BMI, or metabolic health markers. These findings suggest that intermittent fasting achieves similar results to calorie restriction. Several meta-analyses of intermittent fasting trials support these findings. 

But, we know that calorie restriction often isn’t maintained over time, and many participants in weight loss trials regain lost weight within 2-5 years. So, does intermittent fasting provide a better alternative for long-term weight loss?

The truth is, we don’t really know yet. A 2016 meta-analysis of “long-term” studies lasting more than six months on intermittent fasting showed no difference between intermittent fasting and continuous energy restriction for weight loss. Six months isn’t exactly long-term though, is it?

Interestingly, the trials included in this analysis that had the longest follow-up periods (12-18 months) were mostly conducted in the late 1990s and early 2000s, using different parameters for intermittent fasting than most trials use today. More up-to-date, long-term studies are needed before we can understand whether or not intermittent fasting is superior to calorie restriction in terms of maintaining weight loss.

What About Blood Sugar and Insulin Levels?

Another potential benefit of intermittent fasting that gets significant attention is stabilizing blood sugar and insulin levels. Large spikes and dips in blood sugar, and therefore insulin, are associated with several chronic diseases.

The idea is this: By giving the body a prolonged period of “rest” from eating, you allow insulin levels to drop. Since insulin is a hormone also involved in fat storage, this can have metabolic and weight loss benefits.

But does this idea hold true in the research? Unfortunately, not very well. A meta-analysis from 2021 showed that intermittent fasting could significantly decrease fasting blood sugar and insulin resistance. The words “significantly decrease” sound great, but that’s simply referring to statistics and probabilities. What you really want to know is: Will that difference affect your health? The answer: Probably not that much. In the case of this study, the change in parameters is unlikely to have an actual benefit on health.

Let’s Talk About Circadian Rhythms & Hormones

As humans, we are governed by a circadian rhythm, which is the sleep-wake, or light-dark, cycle. We have an internally-driven clock that functions on a 24-hour time period. Circadian rhythms are intricately linked with the endocrine system, which is the system of your body that controls your hormones.

If you think about weight loss, you probably understand that it’s complicated for most people, and not necessarily as simple as energy in, energy out. Dysregulated circadian rhythms are a likely contributing factor to this complexity. Here are some examples:

• Melatonin is a hormone responsible for maintaining your circadian rhythm. Production of melatonin is stimulated in the dark and suppressed in the light, and has a critical role in regulating sleep.
• Leptin is one of the hormones that regulates your food intake. Leptin secretion matches a circadian cycle, peaking at nighttime. This makes physiological sense because as we prepare for sleep, we want to turn off appetite. However, leptin levels tend to become chronically elevated with excess fat stores, and the body may start to lose its ability to innately respond to fullness cues.
• Cortisol is one of your stress hormones that also contributes to how your wakefulness changes throughout the day. Cortisol secretion peaks just before waking in the morning and then tapers off throughout the day until reaching its lowest levels at nighttime.

Can Intermittent Fasting Affect Circadian Rhythms?

If circadian rhythms and food intake are connected (which they are, based on the hormones we discussed above!), then this is one possible pathway for the potential health benefits of intermittent fasting.

A 2021 systematic review showed that intermittent fasting can directly impact the gut microbiome. The microbiome is thought to be involved in the regulation of circadian rhythms through cyclical changes in diversity, composition and metabolite production that give the body specific signals at specific times. The authors hypothesized that intermittent fasting could be a time indicator to help synchronize the circadian system.

However, it’s important to recognize that this paper included 18 animal studies, but only 4 human studies. Interestingly, humans and rodents have opposite circadian patterns, meaning that data from animal studies need to be interpreted with caution.

Another meta-analysis from 2021 looked at leptin concentrations of mostly overweight people practicing intermittent fasting. They found that intermittent fasters had significantly lower leptin levels than people not practicing intermittent fasting. This suggests that intermittent fasting could potentially counter those chronically elevated leptin levels.

So, there is a plausible relationship between intermittent fasting and circadian rhythms. But, is this relationship a result of intermittent fasting itself, or simply because people who intermittent fast are much less likely to eat late at night? Perhaps, we would see the same results if we compared people who generally stop eating for the night around 7-8 pm versus those that eat until 10 pm-12 am. We need more well-designed, long-term studies to fully understand what’s going on here.

Is It Safe?

The safety of intermittent fasting depends largely on the time-restricted eating patterns you adopt. For most people, a 16:8 fasting window is likely safe because it doesn’t differ that much from a typical eating pattern. However, you should exercise caution with more restricted feeding windows, especially those like one-meal-a-day.

Intermittent fasting could increase the risk of disordered eating behaviours, especially if done in a very regimented way or amongst people with a history of disordered eating. If you are pregnant or breastfeeding, clinically underweight or have any other condition that increases your nutrient requirements, you should probably also skip intermittent fasting.

Takeaway

Intermittent fasting is one method to achieve calorie restriction. Some people argue that it’s less restrictive because instead of restricting specific foods, it limits the amount of time you can eat. But, there’s still restriction. If intermittent fasting is something you want to explore, it’s likely safe, but it’s not a magic bullet. Many of the proposed benefits of intermittent fasting may not be unique to the method of eating but could be achieved through other means and without a potentially restrictive label.