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Does exercising on an empty stomach increase fat loss?

By Jill Coleman

The most frequently asked question in fat loss: should you eat before a morning workout or not? Ask some fitness experts and surprisingly, many of them are fairly impassioned on the subject. However, the experts’ opinions are diverse on the topic, most likely because the ever-evolving research is mixed. However, let’s try to make sense of this quandary first by evaluating an individual’s goal and second by examining an individual’s preferences.

Many dedicated exercisers like to work out early in the day; in fact, it’s been shown that people who exercise first thing in the morning have a higher level of exercise adherence. However, there exists much debate over whether a pre-workout meal will ultimately benefit or hinder exercise outcome. Unfortunately, this debate is not easily solved, simply because the outcome is not defined. Is the goal performance? Fat loss? Muscle gain? For our purposes, let’s create two distinct goals and evaluate each. The first is performance, including both training for physical competition like a road race or triathlon, as well as training for muscle strength or size. The umbrella of “performance” covers both of these types of training because they both require the same hormonal and caloric environment to be attained. More on this later. The second goal to be addressed is fat loss (i.e. decrease in body fat %). Once the goal is defined, it becomes easier to steer the debate. Likewise, it will depend on the mode of exercise: weight training versus cardiovascular exercise.

Morning Cardio for Fat Loss

The old bodybuilding mantra declares that doing morning cardio on an empty stomach will help burn more fat than eating beforehand. Let’s begin by dissecting this statement and evaluate its truthfulness. It’s true that 90% or more of exercisers want to lose fat and improve body composition. And it seems logical enough that doing cardio on an empty stomach would facilitate this goal since if there is nothing on the stomach to burn, like a breakfast, the body must resort to using its fat stores for energy to exercise. Unfortunately, it is not this clear cut. It is important to evaluate the hormonal situation of the body upon waking. The body has just undergone an overnight fast and during the night, the body uses both its blood sugar and some of its sugar stores (glycogen) to do work, like repair muscles, digest food, detoxify the body and boost immunity, etc. Luckily, the body also uses fat to propel this work too; you are always using both fat and sugar as energy all the time, the ratios of which are determined by intensity of movement and available substrate. Upon waking, several important catabolic hormones are usually elevated because they assist in breaking down sugar stores, fat and even a bit of muscle to continue to make sugar available to do the work described above. These catabolic hormones are cortisol, glucagon and adrenaline. On the other hand, human growth hormone (HGH), an anabolic hormone is also elevated during sleep since this is the most optimal time for muscle building. So, what does this hormonal soup mean for potential fat loss efforts? Well, it really depends on the substrates available, particularly the amount of stored carbohydrate that is left in the muscle and liver to be used for energy to propel exercise.

In a recent study, published September 2008 in the Journal of Physiology and Pharmacology, researchers wanted to see what substrates would be used to propel exercise if participants underwent glycogen-depleting exercise prior to bed (90 minutes of aerobic activity sustained at 70% HR max). After exercise, they went to bed without eating. Upon waking, three groups were formed: Group 1 consumed 1000 calories considered “high carbohydrate” 2 hours before morning exercise. Group 2 ate 1000 calories considered “low carbohydrate” 2 hours prior to exercise and Group 3 did not eat anything at all. All three groups then participated in graded exercise to exhaustion, with measurements of fat-derived energy substrate (i.e. fat stores) taken at intervals. The study showed that depletion of carbohydrate stores enhanced fat usage significantly at sub-maximal workloads.

Another study, published in the International Journal of Obesity in June 2006, was done on post-menopausal obese women. The study evaluated the effects of pre-workout meal timing on fat utilization during exercise. Each participant performed 2 separate exercise training sessions on non-consecutive days: the first session was performed 1 hour after consuming a standardized meal, while the second session was performed 3 hours post-meal. The exercise sessions were identical in intensity and workload, thus the only difference was the amount of time since eating. The relative amount of fat used during exercise was measured in each of the two protocols, and resulted in higher lipid oxidation (fat use) during the exercise session following a 3-hour pre-workout fast. These results support a longer period of fasting prior to exercising to maximize fat usage. The interesting thing was that the amount of calories burned in both protocols was the same, yet the semi-fasting state pulled more calories from fat. This begs the question—is a calorie really a calorie? More on this later.

Though there is much research that supports exercising on an empty stomach to increase fat oxidation, these studies tell us nothing about the individual tendencies of the exerciser. For example, many trainees cannot and should not exercise on an empty stomach because they experience lightheadedness, dizziness and even nausea as a result of low blood sugar. Interestingly, this occurs more with untrained exercisers hinting at inefficiency in fat and glycogen break-down. If you tend to get dizzy or lightheaded when working out intensely, eat a small meal 1-2 hours prior to exercise. If you must eat, what is the ideal combination? Carbohydrates alone will increase insulin production, an anabolic hormone which will help increase blood sugar to the detriment of fat burning; insulin shuts off lipolysis (fat burning) completely. If insulin is present in the blood stream, fat cannot be burned for energy. Eating amino acids (protein) will provide an energy source for exercise and certain amino acids—branched chain (BCAAs)—help prevent muscle break-down during exercise. BCAAs are metabolized in the muscle for energy and their breakdown aids in the carbohydrate production process to maintain energy and optimal brain function during exercise. Thus, the ideal combination for a pre-workout snack to aid in fat loss consists of a variety of amino acids mostly, with minimal carbohydrate input. Whey protein powder mixed with a cup of water is a perfect choice.

The final word: if you can exercise effectively on an empty stomach first thing in the morning, do it. Research shows that the recruitment of fat for energy is substantial. However, be smart. If you are dragging and can barely make it through, there is an obvious energy problem—consider a high-quality protein source prior to exercise. If energy remains low, slowly add in quality carbohydrates until you find the optimal ratio.

Performance Outcomes on an Empty Stomach

Many who advocate eating before morning exercise cite detriment in performance outcomes on an empty stomach. For race or triathlon preparation, the research hints that indeed this is the case. Trainees who eat before high-intensity exercise can potentially train longer and report a lower rate of perceived exertion. For race prep, eat a small amount of both carbohydrate and protein. This combination improves energy and prevents the potential for an insulin spike and corresponding energy nose-dive. Try a ½ cup (dry) old fashioned oats made with water, and then mixed with 1 scoop whey protein 1-2 hours before exercise. The timing will depend on the individual as many cite not being able to stomach a meal before intense training, while others simply wake up too early to fit it in. Remember, morning cardio junkies will get up as early as 4am to get their cardio fix, which means a pre-workout wake-up call at 3am? No thanks! The bottom line: it depends on your unique fitness level, preferences and quality of energy.

The hormonal responses to weight training, however, are much different than that of cardiovascular activity. When performing long duration cardio activity, the body burns both fat and sugar calories during, however there is minimal after-burn associated with this type of exercise. Weight training, on the other hand has been shown to increase fat-burning and muscle-building hormones HGH and testosterone significantly, as well as impact post-workout caloric and fat burning. This phenomenon is called EPOC (excess post-exercise oxygen consumption) and it has been shown nearly conclusively that weight training increases EPOC over steady-state cardio. The more intense the weight training, the higher relative proportion of carbohydrates is used for energy during. This situation is favorable in terms of fat burning post-workout, because when sugar stores are depleted during exercise, fat stores are utilized to replenish them! This effect can last up to 2 days following a tough weight training workout.

So what does this mean for pre-workout intake? Remember in the beginning when we grouped weight training in the “Performance” category? It’s because the above scenario calls for intensity and sugar burning during exercise. Therefore, increase intensity and give the body the fuel it wants by eating a small amount of carbs before a weight-training workout to maximize the fat-burning after-effect (and shut down cortisol production which will pull from muscle). However, to avoid an energy crash, remember to pair it with protein.

This point it illustrated in a study published in November 2006 in the journal Medicine and Science in Sports and Exercise. It looked at nutrient timing and its effects on muscle hypertrophy, strength gains and body composition. The 10-week study used a protein/carbohydrate dietary supplement. Participants were split into two groups, and were assigned to either a) drink the supplement immediately before and after an intense weight training bout or b) drink the supplement in the morning and late evening. Upon completion, all participants’ strength, hypertrophy and body composition were measured. Participants who consumed the beverage before and after the weight training work outs demonstrated a significant increase in lean body mass (hypertrophy) and strength, as well as an improved body composition as determined by a significant increase in muscle mass.

The bottom line? For performance, whether aerobic or muscle gain, give yourself the opportunity to get the best, most intense workout possible by providing energy in the form of healthy carbs along with enough amino acids to prevent a large insulin spike and prevent muscle break-down. But remember, exercise for performance and exercise for fat-loss are very different and each individual should decide to eat or not to eat based on their specific goals.

The Caloric Perspective

Many fitness experts will cite calories as an explanation for why it does not matter if one eats before or after morning exercise. The idea is that a calorie is a calorie, and when exercising, you burn the same amount of calories whether you eat beforehand or not. And since, many argue, weight loss is the culmination of calories “in” versus calories “out,” it should not matter when you eat them. However, as we have shown here, it is in fact not calories that matter, but instead the hormonal situation and available substrates in a “fasting” versus a “fed” state. Doing cardio in the absence of carbs will increase fat burning, even if the caloric expenditure is the same. As for weight training, eating some carbs (along with protein) will powerfully impact intensity and thus EPOC potential.

Thus, the final message: it’s up to you! Take a look at the science, but more importantly take a look at your individual tendencies and preferences. If you can do it effectively, morning cardio on an empty stomach will increase fat burning, period. Eat a small amount of healthy carbs and lean protein before weight-training and long-duration, performance-oriented exercise.