Known as fasted cardio, exercising on an empty stomach has been shown to be beneficial for weight-loss, but it is not without limitations.
Typically, when you start eating a meal, your body goes into what is called the fed state. This is the state when your body is digesting and absorbing food and it usually lasts for about four to six hours. Your body will usually go into a fasted state about six hours after your last meal, according to The Telegraph.
In this state, it’s much easier for your body to burn fat because your insulin levels are low.
“When the body is in a fasting state, several backup mechanisms ensure your muscles and brain get the necessary sugar for fuel. During exercise, after your body’s sugar stores are exhausted, the body then taps into stored fat and turns that into sugar or turns proteins from muscles into sugar,” Dr William Kormos, who specialises in general internal medicines, wrote in Harvard Health Publishing.
“This has led to the hypothesis that exercising when the body is already sugar-starved will lead to more rapid weight-loss,” he adds.
Several studies have suggested that when you exercise in a fasted state, you could burn up to 20% more fat than you would have otherwise, but this is not true for all types of workouts. Fasted cardio works best for low- to medium-intensity exercises and is not always recommended for HIIT (High-Intensity Interval Training) workouts.
“It’s important to note that if you are going to do a particularly strenuous workout, you should eat some carbs beforehand. That way, you’ll prevent your blood sugar levels from dropping too low, which can cause dizziness and nausea,” Max Lowery, author of The 2 Meal Day, Burn Fat and Boost Energy Through Intermittent Fasting, writes for The Telegraph.
When you start with fasted cardio, it’s best to keep to gentle aerobic activities like jogging, swimming, walking and cycling. And while it’s true that your workouts might seem a lot harder than they usually are, eventually your body will adjust.
Lowery explains that you shouldn’t always exercise on an empty stomach because this could decrease your body’s ability to use carbohydrates effectively in the long run and that’s not the goal. The goal is to teach your body to use carbs and fat as and when required.
With all this said, it’s important to also mention that there is a bit of scepticism around the fat-burning benefits of fasted cardio.
“Although the results sound promising, it is less certain that exercising on an empty stomach will help you lose weight. There are relatively few studies that have measured long-term weight loss or body composition in these circumstances,” Dr Kormos writes.
In his explanation, he cites one study of 19 men during the holy month of Ramadan when Muslims fast from sunrise to sunset. As would be expected, all the men lost weight, but the study also found that the men who had exercised during that time had lost slightly more weight and body fat.
“It is safest to say that these studies are suggestive, and it’s best to focus on regular exercise and not worry about pairing your workouts with fasts,” he adds.
Fortunately, fasted cardio isn’t intrinsically bad for you, so at the end of the day it all comes down to a matter of preference.
What is very important is to consider what you are going to eat after fasted cardio.
“Make sure you’ve got a large intake of protein to help with muscle recovery because there is going to be a lot of tissue breakdown when you are training. If you train on an empty stomach and you pile up on junk food after you work out, you’re going to put back in what you have worked out and the body will absorb twice as much,” says CrossFit trainer, Michael Mvenge.
If you start feeling dizzy and completely out of breath during your workout, make sure that you stop immediately.
“This means that you’ve pushed your body over its limit, so if you’re training on an empty stomach, take it gradually, there’s no need to rush,” Mvenge says.
Additional sources: Harvard Health Publishing, The Telegraph