Mercy's Dr. Susan Besser Discusses the Importance of Eating Properly After a Workout

November 29, 2017
Mercy Personal Physicians at Overlea - Overlea, MD

Dr. Susan Besser, in a recent interview with national MEN’S HEALTH magazine, responded to questions for a story concerning “Things That Happen to Your Body If You Don’t Eat After Working Out.” Dr. Besser is a primary care provider specializing in Family Medicine and sees patients at Mercy Personal Physicians at Overlea.

In terms of not eating after a workout, this can apply to people doing intermittent fasting with morning workouts and then not eating for hours or people who workout late at night and then go straight to bed without eating until morning. What are the pros and cons?

Firstly: this is a two part question. Exercising at night and then not eating until morning is different than exercising in the morning without eating for hours afterward. Here is why that is so: exercising at night theoretically is after a day of eating so there is some nutrition onboard already. After exercise, if you are going to bed straightaway, your metabolism slows as you sleep so you are not using as much energy as you would if up and about during the day. Thus, you have time to recover.

Alternatively, if you exercise in the morning and then don’t eat for a few hours, your body doesn’t have energy stores to work with and you are continuing to stay active, thus using more energy.

Think of it this way: if your body was your car, when do you put gas in it? Before a trip or as you are parking in the garage?

So, if you exercise at night (presuming you ate well during the day), there really aren’t any significant cons. But if you exercise in the morning without eating, the result is that your body is starving.  So, if there isn’t any available energy (like food), your body will take stored energy. First it will dig into glycogen. Glycogen is stored sugar (its stored in the liver)- and it’s a perfect short-term energy source.  Your body uses glucagon stores all the time to keep your body’s sugar level stable. Without it, your sugar will rapidly go up and down causing low blood sugar (and we all know how badly that makes us feel when it happens).

But, eventually your body runs out of glucagon. Then it has to move on to less readily available sources of energy- like fat (which is great if you are trying to lose weight and have extra fat on board) or muscle if there isn’t fat on board.

So, using the fat as a food source by itself can cause the body to become ketotic- that means the electrolytes in your body aren’t quite “normal”. That can cause problems such as muscle cramping or problems with kidney function. Now this doesn’t happen right away, it’s a cumulative problem (so just missing one meal won’t do it- it’s over time).

If the body decides it still needs energy after this- it turns to muscle. Obviously, if your body is degrading muscle to use for energy, you will lose muscle mass and get weaker. Pretty much the opposite of what you are trying to do with all that exercise!

Are there any pros or situations where this isn't bad? Can it ever be good if done right? Is there ever temporary, short-term benefit or a long-term benefit that exists? Why if so?

Are there any pros, you ask? Any benefits, short or long-term? Well, in the long run (in my opinion), probably not.  Its less of a problem as I said if the exercise is at night, but I wouldn’t recommend repeated exercising in the morning without eating afterward.  Yes, you can do it once or twice, the body is pretty resilient, but not over a long term.

How does it take energy from glycogen stores, if you don't eat after working out? What does this mean for the body? What is happening? Why is this bad?

Glycogen is meant to be used as a back-up if there are no other energy stores available. Glycogen will get replaced as you replenish your energy stores, but it takes time, so your body may be short-changed for a while. And if you really need the emergency stores to regulate your sugar and they are not there, you will get hypoglycemic.  Hypoglycemia can cause symptoms from weakness, sudden sweating, fatigue to syncope (passing out).

Can it can lead to dehydration (can be severe too and lack of electrolytes could cause irregular heart beat)? Why does this happen? What are tips to avoid this? Can you if you drink but don't eat?

Yes, if you don’t eat (or presumably drink) you will get dehydrated with exercise.  Just exercising, even under optimal conditions causes increase sweating and thus dehydration. If you don’t replace the fluid, it will cause electrolyte imbalances (worse if you don’t eat--back to that ketotic state I mentioned earlier). Electrolyte imbalance can definitely cause heart arrhythmias- but this isn’t going to happen unless you are chronically undernourished and then stressing the body with exercise. Or if you take a diuretic (prescribed for high blood pressure) which can cause electrolyte imbalance. That’s why doctors are always testing blood if you are on medications- to check you electrolytes (among other things).

Most of us won’t lose enough electrolytes with our regular exercise routines to cause imbalance –UNLESS you are on diuretics or have decided to do a “colonic cleanse” which also causes massive loss of electrolytes. (It’s a little different for elite athletes, they tend to “punish” their bodies more than most of us.

The best tip is to stay hydrated- water is fine, you really don’t need an electrolyte drink (unless you are doing a very long exercise routine in very hot weather/temperature). Both eating and drinking are important, of course, but you can go longer without food than without liquid- there are more food stores than liquid stores in the body.

How does it affect blood sugar? Why does it get too low blood sugar with faintness/passing out/etc.?

Sugar (glucose) is the first energy source. It is the easiest for the body to access. If there isn’t enough on board and your sugar drops, you aren’t getting the energy you need to function. Vital organs- like the brain- use a lot of sugar to keep running. If the brain runs out of sugar, it can’t immediately tap into other sources so it stops functioning in a optimal way- hence lightheadedness and fainting. The brain is trying to conserve energy so slows some of its functions. 

Will the result be an inability to repair muscle and that muscle will just continue to break down for days (or more) leading to a cycle, etc.? What does this mean?

The body will use whatever it can to maintain energy- including using parts (like muscles) to preserve itself. So yes, over time if you don’t offer other sources of energy, the body will continue to use muscle causing more muscle breakdown.  To stop this cycle--Eat! Offer your body better sources of energy and it will stop degrading itself. And yes, if you have more reasonable energy sources, the muscle will repair over time- but it will take more time to repair than it does to break the muscle down. So, don’t expect instant improvement if you have damaged your body. I can’t give specifics as to how long that is- it really depends on each individual- their basic health status and how much damage you have done.

Bottom line: treat your body like a temple--be good to it, and it will be good to you.

--Susan L Besser, M.D.Mercy Personal Physicians at Overlea

Dan Collins - Senior Director of Media Relations at Mercy Medical Center

Dan Collins, Senior Director of Media Relations

Email: Office: 410-332-9714 Cell: 410-375-7342

About Mercy

Founded in 1874, Mercy Medical Center is a university-affiliated medical facility named one of the top 100 hospitals in the U.S. by Thomson-Reuters with a national reputation for women’s health. Mercy is home to the nationally acclaimed Weinberg Center for Women’s Health and Medicine as well as the $400+ million, 20-story Mary Catherine Bunting Center. For more information visit Mercy online at, Facebook, Twitter or call 1-800-MD-MERCY.

News and Events