Should I work out on an empty stomach? Is it safe? Will it help me lose weight?
The idea that working out in a fasted state i.e. before you have eaten helps you burn fat is rampant on the internet. Is this true and what’s the evidence behind it?
Your body mainly runs off carbohydrates; it is its primary and preferred fuel source. Your body actually stores this carbohydrate in the form of glycogen in your muscles and that is what will be used in preference to other energy sources first. The glycogen in our muscles can actually power us through many hours of walking and about 90 minutes of jogging (depending on the person).
When the glycogen stores become limited or run out, that's when your body usually turns to other energy sources such as fat, and protein. The more 'trained' you are, the better you are at burning fat. Plus women naturally are actually better at utilising fat as an energy source (we also have a higher fat percentage in our bodies; this is entirely biologically necessary).
Exercising when fasted, provided you go about it sensibly, is usually completely safe and fine. There are some dangers of working out in a fasted state which are linked to very high intensity or very long duration. This is because it can become dangerous if the body is forced to use protein as its energy source. This involves breaking down muscle, and can lead to a condition called rhabdomyolysis. Most people don’t have to worry about this, but it is worth at least being aware of.
There’s a growing body of evidence behind the effects of fasted vs pre-fed exercise, and like many issues that are painted black and white, it comes down to the individual and their goals. If you eat before exercise, then this is linked to increased performance, especially aerobically and for greater duration. If you don’t eat before exercise that it is possibly linked to increased insulin sensitivity. This warrants further research but could be interesting in the future for those are either pre-diabetic or type 2 diabetic.
If weight loss is your goal then the evidence suggests currently that exercising fasted vs. pre-fed makes no overall difference to body composition changes - to those who are implementing a calorie deficit. So for weight loss, it's not the magic bullet! When making changes to your diet, take it slow and steady. Because some individuals can feel light-headed when exercising fasted or heavy and nauseous when exercising after eating, it really does make sense to listen to your body and figure out what works for you.