Does intermittent fasting for fat loss work?
Intermittent fasting is one of the most confusing eating methods out there.
There are tons of studies examining it’s effects on weight loss, but not a lot of helpful conclusions. In fact, I’ve been meaning to write about intermittent fasting for awhile now, but honestly felt a bit overwhelmed.
Ultimately, I knew I had to dive into this topic and sift through the research myself.
Which I finally did! And WOW is there a lot out there.
This post discusses everything you need to know about intermittent fasting for fat loss: Does it work? Is it safe? What actually happens to your body when you fast? Is it more effective than just restricting your calories every day?
Here is what we know about intermittent fasting so far.
That doesn’t mean it won’t change. But these are the conclusions scientists can give us right now, backed by carefully considered studies.
Let’s burn some fat!
Please note: Check with your physician before starting any exercise routine or starting any particular diet. See this Disclaimer for more details. This post may contain affiliate links. For details, please visit my Disclosure page. Thank you!
6 Truths About Intermittent Fasting for Fat Loss
1. Can intermittent fasting can help you lose fat?
Multiple studies have shown that intermittent fasting can be an effective method for weight loss.
So how does it work?
Well, this is where it gets a bit more tricky.
Here’s the rationale behind the different reasons intermittent fasting can work for weight loss:
- It helps you achieve a calorie deficit
- It helps decrease cravings
- It helps you burn more fat
It helps you achieve a calorie deficit
Because you’re only “allowed” to eat within a certain window of hours, this consequently can make it a bit tougher to overeat.
A lot of people attribute their weight loss with intermittent fasting to this reason alone.
If you’re a big time snacker and have difficulty limiting yourself, this may do the trick for you.
For example, compare these 2 methods of eating:
While it’s definitely not IMPOSSIBLE to overeat, it’s a bit harder when your all meals are smushed close together.
However, a systematic review of intermittent fasting studies suggests that you can lose weight and fat with intermittent fasting alone, even without limiting calories.
It helps decrease cravings
Another reason intermittent fasting can work is because your body adapts to the schedule. Some people find that while they used to struggle with day-long cravings, their bodies now no longer beg for unhealthy snacks, particularly at night.
A fasted state burns more fat
This is a newer reason to me, but there’s some pretty convincing evidence out there to support this.
I’d always felt a bit unsure about intermittent fasting because I’d been taught that your body may use muscle for energy instead. Meaning that if your bloodstream didn’t have enough glucose handy (because you hadn’t eaten recently), then there’s a chance your body may start breaking down muscle for energy INSTEAD of fat. And that this was especially a concern when exercising in a fasted state.
Why isn’t this ideal?
Because we want to be losing fat, not losing muscle. Excess fat is what increases risk of disease. Fat is also the culprit hindering most people’s body aesthetic goals.
But there are now studies that suggest muscle loss isn’t a big concern with intermittent fasting, and that this eating method may even promote muscle retention WHILE burning more fat.
And the most interesting part is our bodies seem to “flip a metabolic switch” when we fast.
Here’s what happens when you fast
When we stop eating, our body first gets energy from recently eaten food, presented as glucose in our bloodstream. Once that’s gone, we draw energy from our liver and muscle glycogen stores. These stores may take anywhere from 12-36 hours to deplete after fasting begins, depending on the amount of stores available and amount of energy you expend while fasting (aka daily activities and exercise). Also of note, the higher your insulin resistance (such as due to obesity and diabetes), the greater the time it takes to flip the switch as well.
Once glycogen stores are depleted, the metabolic switch gets “flipped.”
Now, your body breaks down both fat and muscle for energy. Yep BOTH.
Fat gets broken down into free fatty acids that travel to the liver and are metabolized to produce ketones. Ketones are then the body’s energy source for both your muscles and brain during your fast.
(This is the basic principle for the keto diet as well. However, the difference is that with intermittent fasting you produce ketones due to fasting from all foods. With keto, you produce ketones due to very low levels of carbohydrates and thus a lack of glucose. Since intermittent fasting has less restrictions on WHAT you eat, I’d hypothesize that it’s easier to adhere to than the keto diet – which I’m not a huge fan of.)
At the same time, muscle protein is broken down into glucose.
So the hierarchy of energy use is as follows:
- Glucose from recently eaten food
- Glycogen stores from liver and muscles –> glucose
- Fat + muscle –> ketones + glucose
But THIS is where it gets interesting.
A review of multiple intermittent fasting studies showed that your body may retain more muscle with intermittent fasting than with daily calorie restriction alone.
Yes, I still use the word may here.
The review found that intermittent fasting led to a weight loss of 90% fat and 10% muscle, whereas continuous calorie restriction weight loss was comprised of 75% fat and 25% muscle. The review couldn’t find a conclusive reason why and did note that the studies use different methods to assess the results. But even so, this suggests some pretty promising muscular benefits of intermittent fasting.
2. How can you maximize muscle maintenance while intermittent fasting?
Although intermittent fasting seems to naturally preserve more muscle, there are a few things you can do to help with muscle retention.
Eating plenty of protein will provide the building blocks your muscles need to stay strong.
Performing strength training is also a great way to preserve muscle. Not only will the effort burn fat, it will help your muscles counteract the fasting and stay strong.
For best results, aim to train all muscle groups 2x per week.
3. Is intermittent fasting more effective than other diets?
Well, first of all, we need to acknowledge that intermittent fasting only means limiting your eating to a specific window of time. It doesn’t necessarily mean you’re restricting calories or avoiding certain foods.
So…is it more effective than continuous calorie restriction?
It may be for you! The honest truth is that we don’t really know. Some studies show that it is, and others show no significant difference. The main thing you need to consider is if it would work for your lifestyle and be sustainable.
Ultimately, whatever is sustainable will be most effective for you.
For example, if you wake up at 5am for work and are always hungry first thing, then it probably won’t be a good option. Sure, you could play around with the timing. But if you wake up hungry and go to bed hungry, you’ll be miserable and won’t continue.
4. What’s the most effective method of intermittent fasting?
Another interesting part of intermittent fasting is just how many different time splits there are.
Here are the most popular ones:
- 16:8 (16 hour fast, 8 hour eating window). People often eat from 12pm-8pm.
- Alternate day fasting (eating nothing or greatly reduced calories every other day)
- 5:2 (eating nothing or reduced calories – around 500-600 kcal – on 2 non-consecutive days of the week, and eating normally the rest of the week)
- The timing of your meals is not quite as strict here. Some either eat 3 small meals on fasting days, or delay their first meal and just eat lunch and dinner (arguably just 2 meals closer together may be more effective since you’ll be actually fasting for part of the day).
While the 16:8 split is arguably the most well-known, some dietitians recommend that the 5:2 may be the most effective intermittent fasting schedule for the long-term.
This is because rather than restrict your calories every day, you only do so 2 days per week. Many people not only find this more sustainable, it also prevents your body from “getting used to” a lower amount of calories and adjusting your metabolism to retain fat.
Ultimately, it’s whatever works for you that will be the most effective. If you can’t fathom eating only 500 calories in a day, then the 16:8 split will likely work better.
5. Is intermittent fasting safe for everyone?
Many registered dietitians agree that a few groups should avoid intermittent fasting. It’s not a great option for those who:
- Are pregnant
- Are trying to become pregnant, especially with fertility issues
- Have type I diabetes or experience drops in blood sugar
- Have a history of an eating disorder
- Are undernourished or underweight
- Are under 18 years old
For these groups, the risks tend to outweigh the benefits. If you fall into one of these categories, then it’s especially important you consult your physician before altering your diet.
If you aren’t currently eating a healthy diet, remember that intermittent fasting isn’t a magical practice that will undo the effects of junk foods. Focus on eating more whole, unprocessed foods such as fresh vegetables, fruits, grains, nuts, and lean meats/fish.
6. How much should you eat while intermittent fasting for fat loss?
If doing the classic 16:8 intermittent fasting for fat loss, you’ll be more successful if you limit your calories. It’s recommended to aim for no less than 1200 calories/day for women and 1800 calories/day for men when restricting calories daily.
For alternate day fasting, in which you eat nothing/very little on fasting days, you can be a bit less restrictive on non-fasting days. However, you shouldn’t binge eat or you could negate your fasting entirely.
The 5:2 day split involves eating normally on 5 days (once again, not binging) and eating 500 calories for women/600 calories for men on fasting days.
Got any other questions about intermittent fasting for fat loss? Drop me a comment below and I’ll do my best to answer them. And before you leave, be sure to snag my FREE Weight Loss Checklist: