Cardiovascular training

Is Cardio on an Empty Stomach Bad for You?

Max. D Gray
By Max. D Gray. Updated: March 16, 2023
Is Cardio on an Empty Stomach Bad for You?

There are many approaches to achieve your fitness goals, many of them contradictory. Diet is one of the most controversial issues when it comes to working out. Some say you need to eat before, others afterwards. There is even the claim that working out on an empty stomach is the best way to burn fat. The theory is that working out on an empty stomach uses up fat reserves to provide energy instead of carbohydrates and hormones which are otherwise used. For example, once you wake up in the morning, levels of glucose and insulin are very low.

However, is the above theory accurate? Or, is cardio on an empty stomach bad for you? oneHOWTO explains that, although the theory is based on some truth, working out on an empty stomach can be harmful.

  1. Metabolism and weight loss
  2. What happens when you do cardio on an empty stomach?
  3. The best way to do cardio and burn fat

Metabolism and weight loss

Before explaining why doing cardio on an empty stomach is unhealthy, it is important to know how metabolism and losing weight are linked. Fat is stored in our bodies in the form of triglycerides which accumulate in adipose tissue. This is the way the body acquires the energy it requires to move around during the day. However, when we do not use enough energy to burn these accumulate these triglycerides, we end up with a surplus.

We acquire triglycerides through diet, being transported through the body via lipoproteins in our the bloodstream. What interests us in terms of a cardio workout is how we can release these triglycerides in the form of fat burning. To do this, we have to break them down because they cannot pass through cell membranes as they are.

For the lipoproteins to carry the fats, they require enzymes which break them down into free fatty acids and glycerol. The enzyme we need to break down triglycerides is called Sensitive Lipase Hormone (HSL). Insulin regulates the action of this enzyme. It can slow down the mobilization of fats, by affecting cacholamines (adrenaine and norepinephrine) which makes the fat move. The higher the insulin levels in our body, the more difficult fat burning will be.

The process of separating triglycerides we have just described is called hydrolyzing. Once hydrolyzed (broken down), triglycerides pass into the blood bloodstream through a protein called albumin. They are then taken to the parts of the body where they are required. This is the important part in terms of fat burning and cardio workouts. FCarnitine palmitoyltransferase I (CPT1).

What happens when you do cardio on an empty stomach?

Now that we know the chemical process of burning fat, we can look at how this affects a cardio workout. Since high insulin levels makes burning fat more difficult, people who support doing cardio on an empty stomach claim that not eating food before exercise is the ideal time to do it. For example, you have less insulin when you wake up, which is why many people exercise before breakfast.

While this theory is not incorrect in terms of insulin and fat burning, the question over whether to do cardio on an empty stomach is not as straightforward as ‘empty stomach = low insulin = good for weight loss’. There are other factors to take into account.

Cardiovascular exercise carried out on an empty stomach means fat may be burned, but this is not all. High intensity cardiovascular exercise can also mean you reduce muscle mass. This is because when there are low glucose levels, the body needs energy from other places, in this case the muscles.

Another factor is that we need energy to carry out exercise. It is not as simple as using fat for energy, we need to keep topping up our energy levels in a healthy way. If we don't, we can become lightheaded, exhausted and even nauseous. We also simply won't be able to exercise as effectively, meaning we don't get a proper workout. When you exercise on an empty stomach you may burn more fat but you will also oxidize much less during the day.

Doing cardiovascular exercise on an empty stomach is known as ‘fasted cardio’. If we carry this out too often, we can end up making our body get used to burning fat deposits alone rather than taking it from glucose also. In these cases, our body might try to put on more fat to meet our energy needs, something which can serious affect our metabolism.

Fasted cardio on an empty stomach will burn fat in two ways depending on the type of exercise:

  • Low-intensity exercise: this will burn fat, but only in a limited way since the exercise is low-intensity. This means it will not benefit you particularly in terms of weight loss.
  • High-intensity exercise: you will use more carbohydrates (e.g. glucose) for high-intensity exercise, but if the body cannot find it due to having no food for fuel, it will turn to using up proteins in the muscles.

If we want to burn calories, we still need to have energy to do it. This is why we go over the best way to use cardio exercise to lose weight in the next section.

Is Cardio on an Empty Stomach Bad for You? - What happens when you do cardio on an empty stomach?

The best way to do cardio and burn fat

None of this is to suggest that cardio is not a great way to burn fat. However, we need to use our diet as a way to achieve weight loss, not stop eating altogether. To lose weight while doing cardio, it is recommended we have a low-calorie diet. In this way, we can burn more calories than we can consume, still burning fat, but also using more readily available energy sources.

If you are still convinced that doing cardio on an empty stomach is a good idea, then you will need to consult a doctor. They will be able to advise you on whether your metabolism will be suited to it and also provide more specific dietary advice. It is likely, however, they will recommend you do eat something before exercising.

The most important thing to keep in mind is that weight loss does not happen overnight. Our metabolism changes throughout the day and there are better times to exercise than others. It is not simply that our insulin levels affect weight loss. We also need to factor in other metabolic processes. It is more important to have moderate to high intensity exercise on a regular basis and maintain a hypocaloric diet than it is to workout on an empty stomach.

If you need more help, this article on how to plan your cardio workout can help you to know when is the best time to work out. You can also make your workout more fun with this cardio Zumba workout below:

If you want to read similar articles to Is Cardio on an Empty Stomach Bad for You?, we recommend you visit our Fitness category.

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