Wednesday, 9 July 2008

True or False? Exercising on an empty stomach will help you burn more fat

Generally False… for a few reasons:

Weight and body fat loss or gain is determined by caloric balance. If, over the course of the day, you eat more calories than you spend, you’ll gain weight and body fat. If you spend more calories than you eat, you’ll lose weight and body fat. Your weight is not determined simply by what you burn during exercise.

It’s commonly believed that if you exercise on an empty stomach, you’ll burn more fat since your carbohydrate stores are almost empty. As such, your body will use up your fat stores to fuel your workout.

Before we discuss that, here’s an overview:

Fuel for exercise

When we eat carbohydrates (grains, breads, pastas, rice, cereal, fruits, sweets) it gets stored in our liver- to control blood sugar- and in our muscles- to fuel our exercise. The stored form of carbs is called glycogen. If you run out of glycogen when you exercise, you will “hit the wall” and have to stop.

Glycogen is one of the only fuels we can burn during anaerobic exercise- exercise that doesn’t require oxygen, i.e. short bouts of high intensity activities like sprinting, hockey, weight lifting. When there’s oxygen present, i.e. when you do aerobic activities like walking, running, biking, fat can be used as a source of fuel. However, to burn fat, you need to have some glycogen.

It takes 24-48 hours for your muscle glycogen stores to fully replenish- even if you’re eating a high carb diet. An endurance athlete will use up all of their muscle glycogen stores in 1-3 hours of continuous moderate-high intensity workout (depending on fitness level) if they don’t take in any carbs during exercise.
A weight lifter will deplete as much as 26% of their overall muscle glycogen during high-intensity strength training- however, research has shown that muscle glycogen depletion is localized in the muscles that are worked. As a result, if you train your legs, you may have lost 26% of your overall muscle glycogen but the glycogen in your leg muscles can be totally depleted.

So, if you didn’t deplete your glycogen stores in the last 2 days and have been eating a high carb diet and drinking enough fluids, when you wake up in the morning, regardless of whether you eat or not, your muscle glycogen stores will be full. However, your liver glycogen stores will be pretty much empty meaning your blood sugar may be low. The reason we tell people to have a pre-exercise snack or meal is to replenish liver glycogen stores which will maintain blood glucose levels, helping you feel more alert and energetic. However, this snack is not really used as fuel. When you workout, you’re burning what you’ve eaten and stored over the last 24-48 hours.

A single carbohydrate-rich meal will quickly restore your liver glycogen stores: an energy bar, a glass of OJ and a toast, a bagel, a sports drink, a meal supplement like ‘Boost’, a small bowl of cereal.

Burning fat

As for burning more fat when you exercise on an empty stomach- some small studies have shown that this is possible… but there’s a tradeoff: you can run low on energy. A 2000 study published in the International Journal of Sport Nutrition and Exercise Metabolism found that performance of moderate-to-high-intensity exercise lasting 35-40 min was significantly improved in those that consumed a moderately-high carbohydrate, low fat, low protein meal 3-hr before exercise compared to a similar meal consumed 6 hr prior to exercise. Various other studies have demonstrated the same thing. Exercisers are not able to exercise as hard or as long when exercising on an empty stomach.

If you’re not able to work out as long or as hard, you’re going to burn less total calories. So, regardless of the fact that you may be burning a little more fat calories, the total calories burned will be less and you will not lose more body fat. Moreover, because your blood sugar will fall as a result of your liver glycogen depletion, chances are you’ll be famished after your workout. Hunger is a side effect of low blood sugar. Experts point out that you’re more likely to overeat post-workout and therefore consume more calories, leading to weight and body fat gain!

Bottom line

The research has consistently shown that, for both endurance and power athletes, performance will suffer if you don’t consume enough carbohydrates during the day (you should be aiming for more than 50% of your total calories coming from carbs) since muscle glycogen is the limiting factor for any type of activity.
As for pre-exercise, it’s recommended you have a small snack or meal to top off your liver glycogen stores and ensure peak performance or peak calorie-burning.


Girard Eberle, Suzanne. Endurance Sports Nutrition. IL: Human Kinetics. 2000.
Kleiner, SM; Greenwood-Robinson, M. Power Eating: Build Muscle, Boost Energy, Cut Fat, 2nd ed. IL: Human Kinetics. 2001.
Maffucci, DM; McMurray, RG. Towards optimizing the timing of the pre-exercise meal. Int J Sport Nutr Exerc Metab. 2000 Jun; 10(2): 103-13.
Smith, Heidi. Nutrition for the Long Run. Copyright “October 2003” by Heidi Smith.

1 comment:

Anonymous said...

excellent read sybil!

Though I must say I strongly believe timing plays a roll in the ammount of fat lost. Though I'm sure there are no or not many studies proving this(i doubt many have even asked the question and gone out to prove or disprove it) it does not mean it's not true.

Take a post workout of meal. Eating quickly after heavy weight lifting will build more muscle then say waiting 4 hours post workout. Or even the sumo wrestlers blog you posted. How it was all about timing and how meals were planned so that they got as big as possible. So I don't see how fat loss would be any different. True if you are not following then meal plan you lay out and say go binge after then yeah you will not lose more fat but done properly it is effective.

Given you eat then amount of cals you are aiming for in a given day. I would say a routine as follows would lead to maximum fat loss and muscle building(note this goal is not the typical goal of most atheletes, and in their case they would want to eat before workouts to maximize performance):

-am empty stomach: 30-40 min cardio low intensity HR 130-140 BPM
-immediately after: high protein high good fat meal. Ex protein shake and some fish oil
-meals through out the day(every 3 hours or so): balanced eating
- pm weight lifting: highest daily amount of carbs in this meal, with quality protein, low or no fat

given eating like this vs eating the same exact food but with scattering it differently i think your results would be less total fat loss. This is assuming the meal plan is correctly planned to lose weight.

And obviously not everyone wants to go to gym 2 times a day but this is just for arguement sake that fat loss can be optimized!