Is it bad to workout on an empty stomach?
Does training on an empty stomach burn more fat?
This is another one of those big fitness questions that draws a lot of controversy. The logic of it seems simple, if you workout without outside fuel (food) in your system, then your body will need to tap into fat stores to fuel your workout. There are studies that confirm this to be true and studies that say it doesn't work. Some believe that not only does training in the fasted state actually decrease fat loss, it also causes your body to burn muscle for fuel.
Scary, but fortunately that hasn't been shown to be true. Many studies have looked at Muslim athletes during Ramadan to see if fasting negatively impacted their training. These athletes do not eat or drink anything from sun up to sun down during Ramadan so this extreme example is an excellent time to really see the effects. It turns out that even body builders do not lose muscle while training during this period and that fasted training does cause a greater decrease in body fat percentage.
Not only burns more fat, but protects against weight gain
Additionally, fasted training causes metabolic improvements and improves your insulin sensitivity throughout the rest of the day. It has been shown that training in the fasted state first thing in the morning is a good way to protect against weight gain when eating large meals. This is why I recommend starting off a big eating day (like Thanksgiving) with a workout on an empty stomach to protect against some of the damage you plan on doing later. Of course, this type of training can also really help your weight loss efforts.
Great for weight loss, but tough
If you are looking for a time to maximizing the amount of fat you burn from your workout, then you should get going first thing in the morning before you eat. Your body basically burned up all the sugar in your system while you slept. If your body has no sugar to burn during your workout then it has no choice but to start burning fat.
If you eat something with carbohydrates before you start, then your body will burn through all those carb calories before it even starts mobilizing stored fat. It is not that your body prefers carbs to fat, it just prefers to grab what is already floating around in your blood rather than having to pull it out of your fat cells.
While this will help you burn more fat, it is going to make your workouts feel a little more difficult (especially in the beginning as you adjust to this new form of training). The criticism of fasted training that you won't be able to achieve the same level of performance is correct, but most people that are trying to lose weight aren't trying to maximize their athletic performance. If you're training for a sport or event, you probably should eat before you train to properly fuel your performance, but for those trying to shed the extra pounds, fasted training (especially first thing in the morning) is a great way to get the scale moving.