It’s traditionally something bodybuilders do: fasted cardio, usually (early) in the morning, before breakfast. The idea behind this is that, deprived of food, the body must use body fat as an energy source. Still, the usefulness of fasted cardio is often questioned. A new meta-study may provide more clarity.

WHAT IS ‘SOBER’?

An empty stomach is not necessarily the same as feeling empty or hungry. We speak of sober if you have not eaten for a long time and your last meal has been completely digested. The latter can take up to six hours, especially with a high-calorie meal ^[ xi ] . If your last meal included just a few scoops of protein powder, for example, it will be fully processed within hours.

One of the hallmarks of a fasting state is that levels of the hormone insulin have dropped to a minimum (i.e., baseline level) ^[ vii ] . Insulin regulates the glucose metabolism in our body: it transports the glucose to the body cells, where it can be converted into energy.

WHY EXERCISE ON AN EMPTY STOMACH?

The idea behind sober sports is based on the fact that our body can draw from different energy sources. Fat is one of them, of course, but it’s not the source your body uses first. Carbohydrates, readily available as glucose or stored in the form of glycogen, are a faster fuel. This is because the combustion of carbohydrates requires less oxygen. And during intensive cardio your body has only limited oxygen available. The situation is different with low-intensity cardio, such as walking. There is more oxygen available, so your body will initially use your fat stores and not your glycogen stores.

Thus, glucose and glycogen are the primary energy source for moderate and high intensity cardio. The more the glycogen stores are depleted, the more the body switches to burning fat. If you have fasted for a while, the glycogen stores are limited and your body will switch to fat burning almost immediately. The lower insulin levels also contribute to this: the lower the insulin levels, the more your body will use fat as a primary source of energy instead of carbohydrates.

Sounds logical, right?

WHY THE THEORY IS WRONG

Yes, it is true that you burn body fat faster on an empty stomach. And vice versa, that you use more glucose if you have eaten before. But in the end, it makes no difference to the number of calories you burn. You don’t burn more calories when your body uses fat as an energy source.

Whether you actually lose fat and how much, depends on your energy balance over a longer period of time, basically over your entire day. Only with a total negative energy balance (a calorie deficit) will your body break down more fat (lipolysis) than it produces (lipogenesis). It’s the difference between fat burning and fat loss.

If you still have breakfast on an empty stomach after your cardio session , you will burn a relatively large amount of glycogen during your subsequent activities. If you first have breakfast and then exercise, you will still burn a relatively large amount of fat during your subsequent activities. Viewed throughout the day, for fat loss, it doesn’t matter which energy source you use more on one than the other ^[ v ] .

The above is confirmed by a meta-study on fasted cardio ^[ i ] , which is an analysis of multiple studies. Five pieces, that is. That does not seem like much for a meta-analysis, but the selection criteria were also quite strict. The total study population amounted to 96 healthy persons (60 male, 36 female), ranging in age from 21 to 27 years. Different forms of cardio were studied: both High Intensity Interval Training (HIIT) and the more traditional long-term cycling and treadmill walking. And so in those studies both empty and full stomachs were examined.

The researchers’ main conclusion based on the admittedly limited research material:

The findings support the notion that weight loss and fat loss from exercise is more likely to be enhanced through creating a meaningful caloric deficit over a period of time, rather than exercising in fasted or fed states.

For the same reason, many other claims about cardio are more myth than fact. That you should train in the fat-burning zone, for example, or that you only start burning fat after 30 minutes of work.

However, the study concludes that significantly more research is needed to draw definitive conclusions:

It remains unclear whether training in fasted compared to fed states leads to greater weight loss and whether this practice results in beneficial or detrimental changes in body composition.

POSSIBLE DISADVANTAGES OF FASTED CARDIO

With cardio on an empty stomach you do not burn more fat than with cardio on a full stomach. And the slogan ‘if it doesn’t help, then it doesn’t hurt’ may not hold true, especially for bodybuilders.

The greatest danger for them lies in the possible catabolic (muscle-depleting) effect of fasted cardio. Now that danger actually comes with every long-term cardio session, regardless of whether you do it on an empty stomach. Simply put: the longer the cardio lasts, the greater the chance that the body will also use that third energy source: proteins. And that can be at the expense of your muscle tissue. After all, if the amino acids – the building blocks of proteins and your muscles – are exhausted from your bloodstream, the body will break down your muscle tissue into amino acids. It’s one of the reasons why bodybuilders are often recommended short, intense forms of cardio, such as HIIT. But with pre-breakfast cardio, the catabolic risk may be even greater, as levels of the ‘stress hormone’ cortisol are highest in the early morning, just after getting up.^[ ii ] . As a result, your morning cardio session can release even more of the muscle-destroying cortisol than cardio at other times of the day.

Of course you can also opt for HIIT, where the increase in cortisol levels is largely compensated by the increase in growth hormone and testosterone levels. But that brings us to another disadvantage of fasted cardio: you can train less intensively. Glycogen can be burned both with the intervention of oxygen (aerobic) and without the intervention of oxygen (anaerobic), while oxygen is always required for the combustion of fat. Because oxygen is only available to a limited extent, there is also a limit to the intensity of your training. To put it more simply: on an empty stomach you can only train at a low intensity. Never mind that you could finish a grueling HIIT session before dawn. That is almost impossible, because with a high-intensity exercise such as HIIT you also mainly use glycogen as fuel (and creatine phosphate), and not a drop of fat. And – you already knew – that supply is almost exhausted before breakfast.

A third disadvantage lies in the EPOC (Excess Post Oxygen Consumption), also known as the afterburn. This is the amount of calories burned after a workout for recovery. There is research suggesting that this amount is higher if you have a meal a few hours before training ^[ viii ] .

One last, not unimportant disadvantage of fasted cardio: it is certainly no fun.

ALLEGED OTHER BENEFITS OF FASTED CARDIO

Is cardio on an empty stomach now completely written off? Maybe not. There may be two specific benefits to be attributed to this form of cardio. Besides the fact that you burn calories with it, of course.

“YOU BURN MORE STUBBORN FAT”

Although you do not burn more fat with fasted cardio, you may associate a relatively large amount of stubborn fat with it. That is the fat on the abdomen and lower back in men, and on the hips and thighs in women. These parts of the body give up their fat stores less quickly, so that the fat disappears completely there. Research shows that the blood flow in these places is better when you train on an empty stomach ^[ ix ] . This allows the so-called catecholamines to reach your fat reserves more quickly, which ensures better mobilization of those ‘difficult’ fat cells ^[ x ] .

Stubborn fat is a thorn in the side of many, so it would be nice if you lost it faster with fasted cardio. After all, you are often lean enough in other places and you can’t wait for that fat layer over the lower part of your belly to come. Is that possible with fasted cardio? It sounds a bit like burning fat locally and therefore something that is too good to be true. More research is needed to determine whether cardio on an empty stomach can actually help with this.

“YOUR BODY LEARNS TO CONSERVE GLYCOGEN”

2017 research ^[ iii ] reveals a previously unknown potential benefit of fasted cardio. According to the study, cardio on an empty stomach increases the enzymes responsible for fat regulation and fat burning. Over time, your body would thus ‘learn’ to use fat as fuel more efficiently. This will make it faster to use fat as an energy source and save glycogen.

In particular, individuals who have great difficulty losing fat, despite proper training and diet, could ‘reprogram’ their bodies with fasted cardio to use fat more quickly as an energy source ^[ iv ] . For bodybuilders, this could mean that glycogen stores are saved during the cut, which are important as fuel for strength training.

Sounds good, but this aspect of fasted cardio has not yet been sufficiently explored and therefore requires more research.

IN SUMMARY

1. With cardio on an empty stomach you do not burn more calories than with cardio on an full stomach. It is true that you use more fat as an energy source during fasted cardio, but the actual fat burning depends on your total energy balance over a longer period of time.

2. When you consume a relatively large amount of fat during a cardio session, for example because it is low intensity and/or because you train on an empty stomach, your body will use a relatively large amount of carbohydrates (glucose/glycogen) at another time. Seen throughout the day, for fat loss, it makes no difference which energy source you use at what time.

3. In short, with a negative balance (a calorie deficit) you lose fat, regardless of how that deficit came about.

4. Fasted cardio may even be detrimental to bodybuilders, as levels of the muscle-breaking hormone cortisol are highest in the early morning. In addition, you can only train with limited intensity with fasted cardio.

5. Yet fasted cardio may have its benefits. This way you could burn stubborn fat faster. And you could ‘teach’ your body to use fat as fuel more efficiently. However, more research is needed on these purported benefits.

Should you do fasted cardio?