Does Cardio After an Overnight Fast Maximize Fat Loss?

Does Cardio After an Overnight Fast Maximize Fat Loss?

A common weight loss and fat burning strategy is to exercise on an empty stomach, for example early in the morning.

From a biochemistry and metabolic standpoint, this is an intuitive concept. When you wake up after having not eaten for 8 or more hours, your circulating insulin levels have dropped and you should be in a so-called fasting state. Theoretically, you are tapping into whatever is left of your overnight glycogen stores and shifting over to free fatty acid metabolism for energy production. This leads to the conclusion that a hard workout or cardio workout in the morning should be an efficient fat burning workout.

The question becomes, does the evidence support that notion?

An important starting point is that total body metabolism is a dynamic balance of anabolic and catabolic activity that is always in flux based on metabolic demand, energy input and output. This does not occur in a vacuum at 7:00 AM on a treadmill. A better conceptual model is that it energy balance occurs on a constantly moving continuum, balancing between insulin and glucagon. When you exercise, you are still in a metabolically active state hours after and similarly, when you eat, you are in a postprandial state for hours. How long and how intense these periods last depends on many variables.

There are a couple of studies that have looked at fasting and cardiovascular exercise.

One group (Horowitz et al) had subjects cycle at varying intensities on separate occasions where they either ate high glycemic meals before training or fasted for 12 hours. Although fat breakdown was suppressed more in the fed state at low-intensity (22%), there was no difference in fat oxidation between the groups until 90 minutes (they biked for 2 hours). In the moderate-intensity arm, there was also no difference in fat oxidation.

Deighton et al had individuals run on a treadmill after overnight fasting or after eating breakfast for 1 hour at 70% of VO2Max. They found that 60 minutes of treadmill running did induce a negative energy balance, however there was no difference when performed before or after breakfast.

Another study (Febbraio et al) looked at pre-exercise and during exercise carbohydrate consumption on fat oxidation. The subjects cycled for 2 hours and either received a placebo, carbs before, or carbs during exercise. The authors found no evidence of impairment of fat oxidation associated with the carbohydrate consumption.

These studies suggest that although more fat may be broken down, there is not good evidence that the body actually uses it for fuel. Free fatty acids that are not oxidized will be stored and turned back into adipose tissue.

Other studies have looked at post-exercise oxygen consumption after eating or not eating. One group (Lee et al) gave subjects high glucose milk in 4 different experimental conditions: either low intensity or high intensity exercise with or without milk. (4 groups: low-intensity long duration exercise with milk , low-intensity long duration exercise without milk , high intensity short duration exercise with milk , and high-intensity short duration exercise without milk.) What they found was that milk consumption actually increased postexercise oxygen consumption, a proxy for metabolic activity, compared to those who were exercising in a fasted state.

The type of adipose tissue should also be considered. A significant amount of energy stores come from intramuscular triglycerides (IMTG).  As the name implies, this is fat energy stored in muscle and not contributing to what individually generally consider “fat” that you would find in your love handles, abdomen or butt. This fat is stored in muscle and is not the fatty tissue that affects your body composition. The more we train, the more intramuscular triglycerides we build up. it may be as high as 80% of our energy reserves during cardiovascular exercise.

One must also consider protein breakdown. In the fasting state, a glycogen depleted athlete will generate energy from protein breakdown. One study estimated that 10% of total caloric cost comes from protein breakdown in the fasting state. In individuals seeking to build muscle mass, this may be counterproductive. Generally this isn’t a huge deal as long as you consume fast digesting protein such as whey.

Lastly, fasting has deleterious effects on energy level and this can influence productivity and efficiency while exercising. Studies have shown (Shabort et al) that a pre-exercise meal allows individuals to train more intensely compared with individuals who fast beforehand. Personally, I usually have a protein shake about 30 minutes prior to exercising and another while at the gym.

In conclusion, we can make a few assertions. Conceptually, doing cardiovascular fitness in the fasting state should lead to more fat burning as there is more circulating free fatty acids and low insulin state. Unfortunately, it’s not that simple and the data really doesn’t support that notion. One could also argue the data is inadequate and that is a fair criticism. If you were forced to argue that fasting cardio helps or doesn't help you burn fat, you can make a better case that it doesn’t help.

The limited research suggests that (a) there is no difference in fatty acid oxidation whether you eat breakfast or not, (b) eating breakfast actually increases post-exercise oxygen consumption, (c) fat burned may be intramuscular TAG and not general body adipose, (d) protein breakdown while fasted may be counterproductive to goals and (e) Eating a pre exercise meal increases training intensity compared to fasting.

Finally, Like all things; we need better data.


Schoenfeld, B. (2011). Does Cardio After an Overnight Fast Maximize Fat Loss? Strength and Conditioning Journal, 33(1), 23-25. doi:10.1519/ssc.0b013e31820396ec. (Note that much of the inspiration of this article came from the original one by Schoenfeld).

Horowitz JF, Mora-Rodriguez R, Byerley LO, and Coyle EF. Lipolytic suppression following carbohydrate ingestion limits fat oxidation during exercise. Am J Physiol Endocrinol Metab 273: E768–E775, 1997.

Febbraio MA, Chiu A, Angus DJ, ArkinstallMJ, and Hawley JA. Effects of carbohydrate ingestion before and during exercise on glucose kinetics and performance. J Appl Physiol 89: 2220–2226, 2000.

Lee YS, Ha MS, and Lee YJ. The effects of various intensities and durations of exercis

with and without glucose in milk ingestion on postexercise oxygen consumption. J Sports Med Physical Fitness 39: 341–347, 1999.

Schabort EJ, Bosch AN,Weltan SM, and Noakes TD. The effect of a preexercise meal on time to fatigue during prolonged cycling exercise. Med Sci Sports Exerc 31: 464–471, 1999.

Deighton, K., Zahra, J., & Stensel, D. (n.d.). Appetite, energy intake and resting metabolic responses to 60 min treadmill running performed in a fasted versus a postprandial state. Appetite., 58(3), 946-954.

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