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Listen to Dr. Neal address this topic on Episode 110 of the podcast Optimal Health Daily.

Being in a fasted state simply means you haven’t consumed any foods or drinks containing any significant calories for a while. How long is “a while”?

Number of Hours in a Fast

Well, it kind of depends on who you ask and the situation involved. For example, before you go in for routine blood tests like getting your cholesterol and blood sugar levels checked, your doctor may recommend that you fast for at least 8 hours before going to the lab and having your blood drawn. Before going in for a body composition analysis with one of those Bod Pod machines (which basically tells you how much fat vs. muscle you’re carrying on your body), they may tell you to fast for 4 hours beforehand. Then in other cases, fasting may last days and even weeks. Most of the studies on fasting and exercise address fasting for anywhere from 8-12 hours – this is sometimes called “the overnight fast” since some of us lucky enough to get this many hours of continuous sleep technically won’t be eating or drinking during these hours. Fasting for 8-12 hours is usually what folks are talking about in this context.

Fasted Cardio or Fasting Before Exercise

Why would working out on an empty stomach potentially be a good thing? Researchers are discovering that by not eating before you perform aerobic activity (like running or jogging, cycling, etc.), your body is forced to burn fat as its major source of energy. If, however, we eat something before we go for our run, we will burn less fat for fuel. For most of us, one of our goals is to lose body fat, or at least keep it to a low percentage of our overall body weight. So burning more fat during a workout sounds like a great thing… and it is! Brazilian researchers recently performed a meta-analysis on how much fat was burned when exercising in a fed vs. fasted state. A meta-analysis basically means that they looked at a bunch of studies that were already published on this same topic and tried to find what the overall consensus was. Sure enough, they found that the body burned more fat as fuel when folks hadn’t eaten over the past 8-12 hours.

But (you know there had to be a “but” somewhere), here’s what the researchers also said and I quote:

It is necessary to take care when prescribing this strategy in practice… these findings should not be extrapolated as long-term effects, especially with the aim of reducing body fat, as there is insufficient evidence of effectiveness and safety.

Basically, these researchers are saying, yes, by performing aerobic or cardiovascular exercise in a fasted state will make the body burn more fat as fuel. BUT… they admit that before you go and try this for yourself thinking it’s going to help you eventually cut down your body fat, think again. There are no studies that look at whether this actually translates to the loss of actual body fat. So far, researchers are finding that the body uses more fat as fuel, but fat doesn’t always come from stored fat in your fat cells. We have what are called “free fatty acids” floating around our bloodstream constantly. Who’s to say the body’s not using those free fatty acids as fuel instead? In fact, the researchers argue that this is likely the case. Rather than shrinking our fat cells, the body may just be burning the fat that’s floating around anyways. You may think, “Well, isn’t that still a good thing? If we’re removing fat from our bloodstream, won’t that reduce our risk for having a heart attack or stroke?” Maybe, maybe not. We need some fat in our bloodstream, too! Plus, we have to weigh the potential risks and benefits of fasting before a workout.

Risks of Fasting Before a Workout

Can you think of any situations where by not eating before a workout, the individual would be placing themselves at risk for some serious complications? The first thought that comes to my mind would be swimming. I would never recommend someone jump in a pool and start swimming laps after having fasted for 8-12 hours. Why? Well, when an individual’s blood sugar gets too low, fainting happens. If you faint in a pool, you drown.

I wouldn’t recommend this for pregnant women… or those with diabetes… or, children and adolescents… or the elderly for that matter. In fact, I would be cautious before recommending anyone try this! Having low blood sugar is actually far more dangerous in the short-term than having high blood sugar. Again, this is because if the brain doesn’t get enough fuel in the form of food, fainting and even falling into a coma can occur. Likely, even before fainting occurs, the person may feel weak, nauseated, may have difficulty speaking clearly, may begin suffering from cold sweats, and on and on.

Should You Fast Before Exercising to Burn More Fat?

While studies in the short-term are finding that we may burn more fat during our cardio workouts while in a fasted state, to me, I would caution anyone before trying this themselves. At the very least, be sure you have some carbohydrate-rich foods nearby in case you do decide to try it and experience any of the symptoms I just mentioned. And remember, just because you’re burning more fat during your workout, doesn’t mean you will actually lose real body fat. The researchers themselves said that…

Listen to Dr. Neal address this topic on Episode 110 of the podcast Optimal Health Daily.