There is now concrete evidence that fat is being used as a fuel during and after resistance training.
Ormsbee, M.J., et al. 2007. Fat metabolism and acute resistance exercise in trained men. Journal of Applied Physiology, 102, 1767–72.
Am I burning fat while doing resistance exercise? This is a question that clients regularly ask personal trainers and group fitness instructors. Resistance training, because of its chief role in maintaining and/or increasing lean body mass (muscle), is an essential component of any weight management program.
We know that muscle contributes significantly to resting metabolic rate, which is the energy expended to maintain all bodily functions at rest. We also know that a guiding principle of weight management is the attainment and maintenance of a “negative” energy balance (i.e., more calories burned than stored) over extended periods of time. However, what physiological function does weight training actually provide to fat metabolism during and immediately following an exercise session?
Surprisingly, this investigation led by Ormsbee and colleagues (2007) is the first study to examine the specific effects of resistance exercise on adipose-tissue fat metabolism. This research team also examined the extent to which the body uses fat as a fuel during and after a resistance training session.
Fat is stored in the body in the form of triglycerides. Triglycerides are made up of three fatty-acid molecules held together by a molecule of glycerol. The mobilization of fat refers to the initial process of releasing fat from storage sites (adipocytes) in adipose tissue. Lipolysis follows, which is the progression of reactions that biologically “disassemble” triglycerides into three fatty acids and glycerol, which are then released into the blood. The metabolism of fat describes the complete biological breakdown, or oxidation (loss of electrons), of fatty acids into energy that can be used by the cells of the body.
At the start of exercise, the adrenal medulla (in the kidneys) secretes epinephrine and norepinephrine, which are part of the body’s “fight or flight” autonomic response to physical stress (such as exercise). Epinephrine and norepinephrine activate hormone-sensitive lipase (HSL), which is a specialized enzyme of fat metabolism. When HSL is stimulated, it acts to break apart triglycerides in the manner defined above (lipolysis). HSL actions can be inhibited by the hormone insulin (which regulates blood glucose level). Therefore, during exercise the rate of lipolysis is largely regulated by the balance between the stimulating effect of epinephrine and norepinephrine and the inhibitory effect of insulin.
The subjects of this study were eight physically active males in their mid-20s. The volunteers answered a health history and physical activity questionnaire, which showed that they had been participating in resistance exercise more than 3 days a week for the last 2 years. The researchers chose this specific population of (active) exercisers because there is evidence that the lipolytic response to catecholamines (the chemical compound group of epinephrine and norepinehrine) may be compromised somewhat in inactive and overweight/obese populations (Bennard, Imbeault & Doucet 2005). Subjects were also free from any existing acute or chronic illness or from any known metabolic, cardiovascular or pulmonary disease. None were taking any medications or supplements, and all subjects were nonsmokers.
The subjects made three separate visits to the exercise physiology laboratories. During the first visit, the researchers collected baseline information, including the subjects’ height, weight, body composition, and 10-repetition maximum (10-RM) for all weight training exercises in the study. During the second and third visits, the participants were randomly assigned to either a resistance training day or a nonexercise control day. It should be noted that the participants abstained from vigorous activity, alcohol and caffeine 48 hours prior to each scheduled testing day. Also, at least 7 days passed between the second and third testing days.
The subjects were weighed on an electronic scale, and height was determined with a standard stadiometer (a measurement device with a movable horizontal board that comes in contact with the head). Seven skinfold measurement sites (chest, midaxillary area, triceps, subscapular region, abdomen, suprailium and thigh) were measured and used to calculate body density and estimate body fat percentage. The subjects’ 10-RM was assessed for the following exercises: chest press, lateral pull-down, shoulder press, leg press, leg extension and leg curl.
During and immediately after each testing trial, the subjects had microdialysis probes inserted into abdominal adipose tissue to measure lipolysis. Microdialysis is a technique used to determine the chemical components of the fluid in tissues. A tiny sterilized probe is inserted into the fat tissue. The tube is made of a semipermeable membrane that allows specific molecules to pass through it. In this study the researchers measured glycerol, as it is an index of lipolysis.
The substrate (i.e., fat and carbohydrate) energy expenditure before, during and after the resistance training and control trials was measured with indirect calorimetry. With this laboratory technique each subject wears a mouthpiece (attached to gas analyzers) for the collection and measurement of oxygen consumption and carbon dioxide removal. These are the primary gases exchanged during respiration (i.e., the interchange of gases in the alveoli of the lungs). Since fat and carbohydrate liberate energy when the cells utilize them, the energy expenditure can be measured (indirectly) and the specific contributions of fat and carbohydrate can be determined.
Subjects were instructed to fast 10–12 hours before reporting to the lab on the day of testing, as different foods might inhibit or accelerate certain steps of metabolism. Once at the lab, each volunteer had a microdialysis catheter (a thin, flexible tube) placed into his subcutaneous fat tissue. Subjects then underwent resting indirect calorimetry. In their two experimental trials they were randomly assigned to do either a resistance training workout or no exercise (control). On the resistance training day, the volunteers performed 3 sets of 10 repetitions using a load of 85%–100% of their 10-RM on the chest press, lateral pull down, leg press, shoulder press, leg extension and leg curl. Rest periods were kept to 90 seconds or less between all sets and exercises. Every step of the testing protocol was the same for the control day, except that the subjects did not participate in the resistance exercise; they were kept resting in a supine position during that time. Immediately following the exercise session or the controlled rest period, subjects underwent indirect calorimetry for 45 minutes. Microdialysis continued for 5 hours following the exercise or control phase.
The subjects were instructed to record their dietary intake for 2 days prior to the first test session (either a control day or a resistance training day). They were also instructed to replicate this 2-day dietary intake for the second testing session so that diet could not affect the study results.
There are some very practical and important findings from this original investigation. First, energy expenditure was elevated for 40 minutes after the resistance training workout and was approximately 10.5% higher than during the corresponding 40 minutes on the control day (see Figure 1). This effect confirms research shown in other studies (Bennard, Imbeault & Doucet 2005).
Second, and perhaps more meaningfully, microdialysis data indicated that glycerol levels (the marker for lipolysis) were raised 78% during and 75% after the resistance training as compared with their levels during corresponding times on the control day. In addition, the indirect calorimetry data showed that fat oxidation was 105% higher after the resistance training than it was after the control session (see Figure 2). Thus, fat was definitely being used above resting values as a fuel (in conjunction with carbohydrate) during and after the resistance training bout. The hypothesis is that enhanced lipolysis during and after exercise is due to the increased levels of epinephrine and norepinephrine (Ormsbee et al. 2007; Bennard, Imbeault & Doucet 2005). In addition, previous research (Bennard, Imbeault & Doucet 2005) has shown that growth hormone (a powerful activator of lipolysis) is elevated after exercise and thus also contributes greatly to postexercise fat oxidation.
This study is the first to show directly that resistance exercise increases adipose-tissue lipolysis and thus helps to improve body composition. This boost in lipolysis is apparently due to the excitatory effect of resistance training on specific hormones (e.g., epinephrine, norepinephrine and growth hormone). As this study design was completed with trained male subjects, it is hoped that the methods and procedures will be completed with other subject populations (e.g., females, untrained persons, youth, seniors, overweight, etc.) in future research.