Intermittent fasting (IF), or time restricted eating (TRE) has become one of the more prominent trends in the diet and “self health” movement. Although not a new phenomenon (my first blog piece on IF was penned 15 years ago now), time restricted protocols have had substantial staying power over the past decade.
Books on fasting occupy a number of positions on the bestsellers list along with keto/carnivore (fasting is often recommended as complementary here), plant-based and gut health “fixes”.
Why are so many jumping on the fasting bandwagon? Is there something specifically magical about it? Does it help you lose fat more easily? Are there health or therapeutic benefits to it that traditional eating intervals can’t give us? Should this be the default way of eating for most?
Let’s take a deeper look.
When I first published a blog post about intermittent fasting in around 2006 (the now defunct Diet Blog), there were very few human studies available on fasting vs linear restriction. The main players of IF at the time were Martin Berkhan (Leangains) and Brad Pilon (Eat, Stop, Eat). Berkhans protocol is the 8:16 style of fasting – whereby you have an 8 hour eating window and a 16 hour fasting window. Pilon’s method involves not eating for 24 hour stretches 2 times per week while eating “reasonably” the other days.
There are a couple of other popular fasting methods such as Michael Mosley’s 5:2 method, where you would consume 500-600 calories on 2 non-consecutive days throughout the week and eat “normally” on the other 5 days. Below is a chart from Examine.com that gives a rundown of the common types of fasting protocols.
With feverish pop culture and commercial interest, the scientific community is responding by conducting trials on fasting. While there are more data points now, it remains challenging to parse out causal links between fasting and health outcomes due to confounders such as; populations used, fasting protocols employed, study durations, control groups used and so on.
Is Fasting Effective For Fat Loss?
Short answer: Perhaps/It depends/maybe/IDK??
Longer answer: Here’s where we stand right now with the weight of evidence.
Let’s look at some longer term studies first. In these cases, 1 year, using a 5:2 protocol vs continuous energy restriction (here and here), it appears as though IF produces similar weight and fat loss to that of linear restriction (typical meal patterns). Ie. they did not impart a statistically significant advantage.
The results of this systematic review and meta-analysis (9 trials, 782 total participants) also confirms the above conclusions, as do the results of previous meta-analyses conducted in 2018 (here and here).
Further, a year-long randomized control trial Headland et al (2019) on the effect of fasting vs. continuous energy restriction on weight loss and healthy overweight or obese adults showed no significant difference weight loss differences between them.
And the above is icing on a 2014 study comparing IF with linear restriction on outcomes on type II diabetics. Both weight loss strategies yielded comparable reductions in visceral fat mass, fasting insulin, and insulin resistance and no meaningful reductions in fasting glucose concentrations.
Fasting: Helpful For Reducing Calorie Intake
It is worth noting that a good number of the studies within the meta-analyses mentioned above demonstrate that fasting groups fare better with weight/fat loss in studies when there was no control group (ie. they weren’t given a comparison diet).
This is an important finding as it does suggest IF can help people spontaneously eat less, thereby improving fat loss outcomes. Other trials, such as this one support that hypothesis as the fasting participants lost 3kg more than the ad libitum (free feeding) group.
There is definitely a pattern of evidence showing that fasting can help control overall calorie intake in free feeding situations.
Fasting: Is It A Health Hack?
With the weight loss piece out of the way, let’s get to the bigger picture: Do time restricted eating patterns improve health markers?
If you’re learning about IF through an enthusiastic book author or anyone else who makes a living from extolling its virtues, you are likely to be hit with an exhaustive list of ailments that can be “cured” or improved through fasting. Not surprisingly, most of these claims are not reflected in the weight of evidence.
With regard to metabolic health, this study reported that intermittent fasting resulted in larger improvements in total cholesterol and LDL cholesterol compared to continuous caloric restriction, even after adjusting for weight loss.
Other available studies are mixed, however. This study for example, lends credence to the above-and-beyond benefits of fasting – reporting larger improvements in lipid metabolism with intermittent versus continuous energy restriction, and a tendency for improved glucose metabolism. Conversely this one and this one show no difference between treatments with both weight loss, as well as other cardiometabolic health markers such as; blood lipid concentrations, glucose, and insulin.
Human trials of fasting and health benefits are still in nascent stages. I’m curious to see if some of the favorable rodent research on cancer, gut microbiota and circadian rhythm. In the case of cancer, there is some evidence that fasting can lessen the toxicity and improve the efficacy of chemotherapy treatments. There is an interesting theoretical construct of which requires further study.
What is clear is that much of the disease risk reduction has to do with favorable changes in body composition – whether that is achieved through IF or more traditional feeding styles.
Fasting And Autophagy: Key Marker or Red Herring?
Autophagy is the process of breaking down parts of or entire cells into amino acids and fatty acids. Basically it’s your cells cleaning up damaged/unnecessary components – an automated trash removal system. Autophagy plays an important role in aging and protecting against disease and dysfunction.
There are reasons to be both optimistic and skeptical when it comes to autophagy. It should come as no surprise that the fasting cultists have jumped on the autophagy train – given there is some abstract and yet-to-be-proven connection between the 2. As usual the more fervent in the fasting community have put the cart way before the research horse.
As of this writing, there’s very little evidence that fasting is an effective way to increase autophagy. Human trials are sparse and at least for now, it seems you can achieve enhanced autophagy through calorie restriction of all kinds. While autophagy is happening all the time in your body naturally, there is evidence that exercise can enhance these effects while in a deficit (exercise is good for just about everything).
We are still very much in the preliminary stages of understanding the effects of autophagy on disease progression. We do know, however that it is an incredibly complex process and thus proceed with healthy doses of skepticism when someone tells you to “fast cuz autophagy”
Fasting and Exercise: Should I Do Fasted Cardio? How Does It Impact My Gainz?
Let’s start with the muscle. Will fasting impact your strength performance or your ability to put on/save muscle? Probably not.. With a caveat. Tinsley et al conducted 2 separate studies on the topic. The first “leangains” study in 2016, fat-free mass, muscle and maximal strength were maintained in both groups – with the time restricted eating group even losing a bit more fat. This study was on men. 3 years later, Tinsley repeated the study in women. Again, the results showed similar increases in muscle and strength gains, with a slight decrease in fat for the time restricted eating group.
The aforementioned caveat being:Training was more effective in the fasting groups when coinciding with fed states. In other words, the timing of training sessions would something to consider if fasting to ensure proper fuel/maximize results.
Does Fasted Cardio = More Fat Loss?
While fasted cardio increases fat burning during exercise, this is compensated for by reduced fat burning during the rest of the day. Conversely, fed cardio burns less fat during exercise but this is compensated by increased fat burning the rest of the day. Keep in mind that fat “burning” is not the same as losing body fat. When it comes to actual fat loss, there doesn’t seem to be a difference whether fed or fasted – making this a preference thing.
Summary Of The Fasting Evidence
- Fasting protocols may help manage appetite and control calorie intake compared to regular feeding windows.
- When controlled for calories, there is no significant difference between fasting and linear restriction.
- The biggest advantage of fasting is likely due to providing “guardrails” which facilitates a lower calorie intake over the course of a day.
- There is no advantage to fasted cardio when it comes to fat loss.
- You likely don’t have to worry much about muscle loss with fasting unless you have very specific muscle-building goals.
- Most health benefits from fasting appear to be a result of caloric restriction and the resulting fat loss vs the fasting itself.
- Autophagy is an oversimplified buzzword and is not universally healthful or disease-fighting. Fasting in this context is a non-issue at this point.
- Fasting does not seem to confer greater benefits in cardiometabolic health when compared to control groups.
- If you are learning about fasting from a book author or anyone that has “fasting” in their social bio, you are being sold on exaggerated claims.
What does this mean for you?
- IF *can* be a potentially helpful strategy for fat loss and improved health – provided it is:
- a) is something you can adhere to
- b) Helps you maintain a caloric deficit (if fat loss is your goal)
- c) Something you respond well to.
- IF may provide a good “shake up” to your current routine if you feel you’ve plateaued.
- Fasting can be helpful in providing some restraint and boundaries that induce some self-discipline necessary to spark changes.
- Fasting is not for everyone – if you have conditions related to hypoglycemia, you may not be a good candidate for this way of eating. If you have tried it for a while and are generally not feeling well with it, it’s time to stop.
Every new eating pattern will come with some discomfort and require adaptation – fasting or not. The human organism is highly adaptable and the feelings of low blood sugar are mostly psychological rather than physiological. For example, symptoms of hypoglycemia do not necessarily mimic actual hypoglycemia. Studies have shown that during Ramadan fasting, brain function remains unimpaired. There is pretty robust evidence showing that mental performance stays intact for the better part of 24 hours in the absence of food.
- If you are going to give fasting a shot, I recommend starting with either a 16:8 (fasting for 16 hours, eating in an 8 hour window) or the 5:2 method (2 non consecutive days of eating 500-600 calories) You can try a couple of weeks on each to see how you feel and how it works for your life. Give any given method at least 6 weeks.
If you’re stuck and frustrated with attempts to lose fat and get your health in order, find a reputable, objective and empathic coach who can best advise you on nutritional guidance and a comprehensive fat loss program (hi).