ML Coaching
Fitness myths are stubborn. They're almost like zombies – no matter how many times you kill them, they keep coming back. Take the myth that carbs and insulin are responsible for fat gain, for example: it persists despite overwhelming evidence against it.
It's just as widespread a belief that cardio on an empty stomach boosts fat loss, even though there's more and more evidence pointing the other way. In fact, there's no evidence supporting it at all.
Even so, plenty of people swear by fasted cardio. I'll show you here that there's no reason to train that way. There's no proven mechanism explaining how fasted cardio would maximize fat loss.
How we lose body fat
We lose body fat by creating an energy deficit. That happens when we burn more energy than we take in. If you don't supply your body with enough energy through food, it has to get that energy from somewhere else – your body fat and protein stores (like muscle). Ideally, we want our body to pull everything from fat stores and nothing from muscle. But for the sake of illustration, let's assume 90% comes from body fat and 10% from muscle mass. If we have an energy deficit of 500 calories a day, 450 calories come from body fat and the remaining 50 from muscle. If we maintain that calorie deficit, we lose fat over time (and a little muscle mass).
For fasted cardio to increase fat loss, it would have to…
Let's say we create that 500-calorie daily energy deficit through a combination of diet and regular cardio. If you now decide to switch to fasted cardio, it would have to boost your fat loss through one of these three mechanisms:
1. Increased calorie burn
In this case, you increase your calorie burn to create a bigger deficit while eating the same amount. Say your energy deficit goes from 500 to 600 calories: 540 calories would now come from fat and 60 calories from muscle.
2. Reduced calorie intake
Instead of burning more calories, in this case you'd eat less. If fasted cardio somehow suppressed your appetite, you'd consume 100 calories less than usual, pushing your deficit from 500 to 600 calories a day. Just like in the first example, 540 calories would now come from body fat and 60 calories from muscle mass.
3. Increase fat loss while preserving lean muscle mass
Your energy deficit stays at 500 calories, but now 95% of it comes from body fat (475 calories) and 5% from muscle (25 calories).
These are the only three hypothetical scenarios in which fasted cardio would boost fat loss more than regular cardio. The question now is whether any of these scenarios actually holds up. Luckily, there are scientific studies that can answer that for us.
Does fasted cardio increase your energy expenditure?
The answer is no. Researchers compared the 24-hour calorie burn of 60 minutes of fasted cardio with cardio done after participants had eaten a meal, and found no difference.
This graph shows that fasted cardio doesn't increase energy expenditure. That proves the first mechanism doesn't hold up.
Does fasted cardio have an appetite-suppressing effect?
Here too, the answer is no. In fact, the study shows that appetite is actually lower after cardio done on a full stomach. Calorie intake over a 24-hour period was even lower than with fasted cardio. So we can cross the second mechanism off our list too.
Does fasted cardio have a redistribution effect (more fat loss and less muscle loss)?
The main reason many people do fasted cardio is that you burn more fat during the session than you would if you'd eaten a meal beforehand. But that's short-sighted. First of all, the fat you burn during a workout doesn't matter for your overall fat loss. Otherwise, you'd never lose body fat by doing things like interval training, which relies primarily on carbs. But we know you can lose just as much fat through carb-burning exercise as through steady, fat-burning exercise – as long as the calorie deficit is the same. It's not about where your energy comes from during training, but about the deficit over a 24-hour period.
The increased fat oxidation during fasted cardio also doesn't explain where that fat actually comes from. Half the fat you burn during sustained cardio comes from intramuscular triglycerides (fat droplets stored in your muscle tissue), NOT from body fat. The more you train, the more your body relies on these triglycerides. Training also increases the oxidation of dietary fat that you eat after your workout. That works similarly to a low-carb or ketogenic diet, where burning dietary fat is used as an energy source. So there's no evidence that fasted cardio speeds up body fat loss, even if fat oxidation is briefly elevated in the short term. If anything, fasted cardio at most improves the use of intramuscular triglycerides and dietary fat as an energy source.
As the graph below makes clear, fasted cardio can improve fat oxidation over a 24-hour period, but it does so by sparing carbs, NOT protein or muscle mass.
Put simply – when you do fasted cardio, you deplete your muscle glycogen (carbs stored in your muscle tissue) by around 18%, depending on the duration and intensity of your training. Any carbs you eat for the rest of the day are then used to refill that muscle glycogen instead of being burned. That's why you burn fewer carbs over a 24-hour period, as the graph makes clear. Because your body is storing those carbs, it's in a "positive carbohydrate balance."
It's important, though, not to forget that our carb stores are limited. Over a period of several days or weeks, your body can't keep storing more and more carbs, because there's a limit. So over time it adapts, increasing the rate at which it burns carbs and reducing the rate at which it burns fat. That's why it's important to look at the consequences over a longer period.
When it comes to long-term body fat loss, what matters is how your body uses protein, not carbs. Studies show that fasted cardio has no protein-sparing effect.
Actual results
There still aren't many studies out there, but a few small-scale trials confirm the facts above – something I've also observed myself as a personal trainer in Zurich.
In a 4-week study with 20 young women, where half the participants did an hour of fasted cardio three times a week and the other half did regular cardio, no significant differences were found. Another 6-week study showed the same results.
Train in whatever way works best for you
Research has shown that cardio on an empty stomach has no advantage over regular cardio. In the end, it's up to you which form of training you prefer. Some people prefer fasted cardio, others would rather eat a meal beforehand. Just try out what works best for you.
If you want to optimize your training and get the best results with the least effort, feel free to send me an email. As part of my personal training in Zurich, I'll build an individual training plan for your needs. I'll also explain which fitness myths are actually backed by science and tested, and which ones are just stubborn myths.