The benefits of intermittent fasting are undeniably sexy.
If someone told you that a simple tweak in the timing and duration of your meals (aka intermittent fasting) could lead to faster weight loss, decreased risk of cardiovascular disease and diabetes, and a longer lifespan … what would you do?
Would you jump right onto it? Or does it sound too good to be true?
Well, if the media and mass public are of any indication, you’re likely to give it a try. They did crown it a ‘miracle weight loss solution,’ after all (1).
But what is this ‘it’ in question?
Well, you might have guessed it already: intermittent fasting. So, does it live up to the hype?
Let’s find out what research says about intermittent fasting and whether it’s really a helpful tip for weight loss.
What is intermittent fasting?
You may be surprised to hear this, but intermittent fasting is not a diet.
Traditional diets (like the Atkins, paleo, and ketogenic diet) tell you what you should and should not eat. Intermittent fasting doesn’t.
Types of intermittent fasting
Accordingly, there are different types of intermittent fasting. The following three are the most well-studied and popular:
- Time-restricted eating (TRE): TRE involves complete fasting (absolutely no food allowed) for long hours – anywhere between 16 to 20 hours – and consuming all calories within restricted hours, often referred to as ‘feeding windows.’ The most well-received version of TRE uses a ratio between fasting and eating of 16:8 (16 hours of fasting and eight hours where food is permitted).
- The 5:2 plan: A popular version of intermittent fasting, the 5:2 plan is where you eat a very low-calorie diet (about 500 calories) for two days each week. For reference, a normal-sized plain bagel topped with 3 ounces of cream cheese is 500-550 calories. Thankfully, you don’t have to ‘fast’ on consecutive days. You have your choice of any two days within a week. And on the other five days, you eat as usual.
- Alternate day fasting (ADF): As its name implies, ADF is also the ‘every other day fasting,’ where you need to alternate daily between unrestricted eating and consuming a very low-calorie diet (500 calories). ADF is likely to lead to the most rapid success in terms of weight loss. But it is also the version of intermittent fasting that is least sustainable in the long-run (5).
- Intermittent fasting is a pattern of eating; it doesn’t tell you what to eat, so it’s not a diet.
- There are three popular variants of intermittent fasting: time-restricted eating (TRE), 5:2 plan, and alternate day fasting (ADF).
How does intermittent fasting work?
To understand the science behind intermittent fasting, you must first understand the basics of human metabolism.
Now, don’t worry, this is not about to turn into an advanced biology class.
Throughout these four stages, glucose and fatty acids compete for utilization as fuel by your body. For brevity’s sake, I’ll focus on the first three stages. Most people would pass on the starvation state since you’d need to go without food for more than two days.
Fed state: Point of eating to Hour 3
Whenever you’re digesting and absorbing food after a meal, your body enters the ‘fed state.’ When you’re in the fed state, your insulin levels rise (11). That’s because your body is trying to regulate the increase in blood glucose levels from the food you’ve eaten.
Insulin helps your body use glucose for energy and/or move glucose into your cells for storage as glycogen (12).
Here, there’s a slow down in the breakdown of fats (inhibition of lipolysis). Your body doesn’t see the need to burn fats for fuel since it already has readily-available glucose.
Post-absorptive (early fasting) state: Hour 3 to Hour 18
After Hour 3 (given that you don’t snack), your body goes into the postabsorptive state. The term may sound fancy, but really, all it means is that your body isn’t processing a meal.
As readily-available glucose runs out, your body is forced to dip into and subsequently deplete its glycogen stores for energy.
This state is also where gluconeogenesis occurs. To maintain the energy it needs for healthy bodily functions, your body now synthesizes glucose from protein (amino acids) and fat (glycerol) stores (16, 17).
Fasting state: Hour 18 to Hour 48
If you’ve ignored the growling of your stomach and went without food till Hour 18, your body would likely be depleted of its glycogen stores.
In addition to relying on gluconeogenesis to keep the lights on, your body will also start to use fatty acids and fatty acid-derived ketones for energy.
Thus, during the fasting state, the primary energy source for the body shifts from glucose to free fatty acids. This process is known as the ‘metabolic switch’ (18, 19). You’re now burning fat for fuel.
This is the state that brings about the intermittent fasting benefits that people are after.
Past Hour 48, and you’d be in starvation state. But again, for brevity’s sake, I won’t be covering this state.
- The ‘feed-fast cycle’ is the underlying principle behind intermittent fasting and it has four states: fed state, post-absorptive state, fasting state, and starvation state.
- During the fed state (point of eating – Hour 3), there’s an abundance of blood glucose for energy. Your body doesn’t burn fat.
- During the post-absorptive state (Hour 3 to Hour 18), you’re running low on blood glucose and glycogen stores. You start synthesizing glucose from protein and fat.
- At the fasting state (Hour 18 to Hour 48), the ‘metabolic switch’ occurs; your body’s primary energy source is now free fatty acids (from fats), instead of glucose.
Is intermittent fasting effective for weight loss?
So, here’s the question you’re probably wondering about. Given that intermittent fasting forces your body to burn your fat storage for fuel, it has to be useful for weight loss, right?
But perhaps the better question to ask is: ‘Is intermittent fasting better for weight loss in comparison to other dieting methods?’
Unfortunately, evidence suggests that people who practice intermittent fasting don’t see better weight loss results in comparison to those following the conventional approach of cutting calories (25, 26, 27).
It seems that in the end, fat loss is still ultimately dictated by how many calories you eat versus how many calories you burn (28, 29).
Other benefits of intermittent fasting
Okay, now you’re scratching your head. If intermittent fasting doesn’t yield better results for weight loss, why is everyone so obsessed with it?
Just like how increasing protein intake can help with weight loss, there must plenty of health benefits associated with going hungry for the most part of the day, right?
Just so everyone is clear, insulin resistance occurs when cells in your muscles, fat, and liver don’t respond well to insulin and can’t use glucose from your blood for energy.
To make up for this shortfall, your pancreas doubles down on producing more insulin.
Despite the increase in insulin, your blood sugar levels continue to rise. The development of insulin resistance often marks the beginning of diabetes.
So, does intermittent fasting help with insulin resistance?
In line with this, many – but not all – large-scale observational studies have also shown a reduced risk of diabetes in participants following an intermittent fasting eating pattern (37).
That settles it, then. Plus one to the benefits of intermittent fasting?
Not so fast. Take a closer look at the existing scientific literature, and you’ll find that few are focused on healthy, non-obese individuals.
How can we generalize research findings from obese participants to the broader population?
We can’t – even the evidence agrees. So far, the majority of human trials involving normal-weight participants have failed to demonstrate beneficial effects of fasting on insulin cardiometabolic health indicators, such as insulin resistance (38, 39).
So, could it be possible that weight loss resulting from intermittent fasting helped with insulin resistance, rather than intermittent fasting itself?
Cardiovascular diseases (CVD) are a severe problem in the modern world. How serious?
According to the World Health Organization (WHO) data, 17.9 million people die every year due to CVD (43) – about one-third of all deaths!
Now, you can understand why so many people are hopeful that intermittent fasting would help reduce traditional cardiovascular risk factors.
Fortunately, it does not disappoint – there are benefits of intermittent fasting here.
However, once again, it’s always better to look beyond the surface.
Intermittent fasting does reduce CVD risk factors, but really, what’s the mechanism at work?
While researchers have yet to find any definitive answers, we could take a closer look at how intermittent fasting affects blood pressure.
Even though intermittent fasting demonstrates beneficial effects on blood pressure, these changes appear to be driven by weight loss.
Google ‘Intermittent fasting and cancer,’ and you’ll find a slew of articles that tell you that it can help prevent the dreadful, big C. And they have their sources, too.
By the end of the pieces, it wouldn’t be surprising if you thought you’d found the cure for cancer.
After all, they do make it sound like intermittent fasting can cure all types of cancer.
And that’s important. You see, while physiologically similar to mice, we are also wildly different in many ways.
We can’t generalize findings from animal models to humans.
So, until there’s further research performed on humans, we can’t say for sure if intermittent fasting is useful for cancer prevention. Only time will tell.
- While intermittent fasting is effective in weight loss, it doesn’t prove to be better than ‘traditional’ diets focused on calorie restriction.
- Intermittent fasting helps with insulin resistance in obese individuals. But can this finding be generalized to the broader population? Also, could the beneficial results be due to weight loss, rather than intermittent fasting itself? The results are inconclusive.
- Intermittent fasting reduces cardiovascular disease risk factors, but again, could this effect be simply due to weight loss?
- As of now, no current studies have established links between intermittent fasting and cancer in humans; research is still sorely lacking in this field.
Is intermittent fasting dangerous?
While intermittent fasting is unlikely to pose any serious health issues, it comes with some potential problems.
Intermittent fasting is not sustainable
And the main issue is that of sustainability. Can you maintain this restrictive way of eating for extended periods and – more crucially – should you?
Figuring out how to fit those feeding and fasting windows into your work and social life, and then fuel and refuel appropriately around your workouts, can be both a logistical nightmare and health concern.
This is especially true if you wake up very early, work very long hours, or go to bed very late.
Intermittent fasting can lead to eating disorders
Given that it’s human nature for people to want to reward themselves after doing arduous work (like fasting for 16 hours), there’s a danger of over-indulging in unhealthy dietary habits on non-fasting hours or days (57).
You can think of it as holding your breath underwater for a long time. The moment you surface, your first breath isn’t going to be gentle; you’re going to be gasping for air.
That’s how you’re (most likely) going to eat when you come out of a fasting window. Even if you’re willing to try out meal prep for better control over your nutrition, it can be difficult to overcome these natural hunger cues.
Many nutrition experts believe that intermittent fasting looks like an old story in new clothes: promoting a supposedly ‘healthy’ ideal when really, it’s encouraging a socially acceptable way to engage in restrictive and disordered eating. And this problem is masked by the widely touted benefits of intermittent fasting.
So, if you have a history of eating disorders, it’s probably best to avoid engaging in intermittent fasting at all. It can turn out to be a very slippery slope, indeed.
Instead, focus on how you can build a healthy relationship with food that’s sustainable and beneficial for you well-being.
- Intermittent fasting is safe for most people; it doesn’t bring about severe health concerns.
- However, a potential issue relating to intermittent fasting is that of sustainability – it can be difficult to adhere to a ‘schedule’ for eating in social and work settings.
- Research also shows that intermittent fasting can lead to eating disorders. As such, intermittent fasting is generally not encouraged for anyone with a history of disordered eating.
Want to try intermittent fasting?
If we were to take a balanced, objective look at intermittent fasting, it’s apparent that other than being an effective weight loss strategy (that isn’t any more superior than traditional calorie-restriction), it doesn’t offer any additional benefits.
So, if you’re planning to try intermittent fasting, be sure to ask yourself why.
What are you hoping to achieve with intermittent fasting? If it’s anything other than weight loss, don’t bother (for now).
If you are intermittent fasting for weight loss, be brutally honest with yourself: can you keep up with this pattern of eating in the long run?
Also, are you in a suitable mental state to stay on such a restrictive diet?
If you don’t think you could sustain the diet, or that you’re not in a healthy headspace, you’d be better off if you steered clear of intermittent fasting.