|Yes! You do Burn Fat During Resistance Exercise
Lawrence Herrera and Len Kravitz, Ph.D.
Ormsbee, M. J., Thyfault, J. P., Johnson, E. A., Kraus, R. M., Choi, M. D., and Hickner, R. C. (2007). Fat metabolism and acute resistance exercise in trained men. Journal of Applied Physiology, 102, 1767-1772.
Am I burning fat while doing resistance exercise? This is a question that clients ask personal trainers and fitness professionals regularly. Resistance training is an essential component of any weight management program due to it's chief role in maintaining and/or increasing lean body mass (muscle). Muscle contributes significantly to resting metabolic rate, which is the energy expended to maintain all bodily functions at rest. And a guiding principle of weight management is the attainment and maintenance of a 'negative' energy balance (i.e., burning more calories than storing) 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 Ornmsbee 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 the body uses fat as a fuel during and after a resistance training session.
Fat Metabolism 101: The Principle Physiological Functions
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' the triglyceride into three fatty acids and glycerol, which are released into the blood. The metabolism of fat describes the complete biological breakdown or oxidation (which means 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 are major stimulatory hormones of hormone sensitive lipase (HSL). When HSL is stimulated, it acts to break apart the triglyceride in the manner defined above called lipolysis. HSL actions can be inhibited by insulin. 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 8 physically active males in their mid-twenties who gave their written consent to participate before beginning the investigation. 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 exercisers because there is evidence that the lipolytic response to catecholamines (combined name for epinephrine and norepinehrine) may be compromised somewhat in overweight/obese populations (Bennard, Imbeautl and 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 had three separate visits to the exercise physiology laboratories. During the first visit, baseline information including height, weight, body composition and 10 repetition maximum (10-RM) for all weight training exercises was collected. 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 two experimental testing days.
Body Composition and 10-RM
The subjects were weighed on an electronic scale and height was determined with a standard stadiometer (measurement device with movable horizontal board which comes in contact with head). Seven skinfold measurement sites (chest, midaxillary, tricep, subscapular, abdominal, supraillium, 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.
Microdialysis and Resistance Exercise
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 semi-permeable membrane which allows specific molecules to pass. 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 mouth piece (attached to gas analyzers) for the collection and measurement of oxygen and carbon dioxide, the gases that are exchanged during respiration (oxygen being consumed while carbon dioxide is expired). Since fat and carbohydrates liberate energy when they are utilized by the cells, 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 the day of testing as different foods might inhibit or accelerate certain steps of metabolism. Once at the lab the subjects were inserted with the microdialysis probe in subcutaneous fat tissue and underwent resting indirect calorimetry. The subjects were randomly assigned to either do a resistance training workout or no exercise (control) on their 2 experimental trials. On the resistance training day the volunteers performed 3 sets of 10 reps using a load of 85-100% of the subjects 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 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 the subject underwent indirect calorimetry for 45 minutes. Microdialysis was continued for 5 hours post the exercise or control phase.
The subjects were instructed to record their dietary intake for 2 days prior to the first test session (control day or resistance training day). They were instructed to replicate this 2-day dietary intake for the next testing session so that diet could not affect the study results.
There are some very practical and important findings from this original investigation. Energy expenditure was elevated approximately 10.5% higher for 40 minutes after the workout day as compared to the control day. This effect confirms research shown in other studies (Bennard, Imbeautl and Doucet, 2005).
Secondly, 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 corresponding times on the control day. In addition, the indirect calorimetry data showed that fat oxidation was 105% higher after the workout day as compared to the control session. Thus fat is definitely being used above resting values as a fuel (in conjunction with carbohydraes) during and after the resistance training bout. The enhanced lipolysis during and after exercise is hypothesized to be due to the increased levels of epinephrine and norepinephrine (Ormsbee et al., 2007; Bennard, Imbeautl and Doucet, 2005). In addition, previous research (Bennard, Imbeautl and Doucet, 2005) shows that growth hormone (a powerful activator of lipolysis) has been shown to be elevated after exercise and thus also contributes greatly to this post-exercise fat oxidation.
Essential Message for Personal Trainers and Fitness Professionals
This study is the first to directly show that resistance exercise increases adipose tissue lipolysis and thus contributes to improved 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.
Bennard, P., Imbeault, P., and Doucet, E. (2005). Maximizing acute fat utilization: Effects of exercise, food, and individual characteristics. Canadian Journal of Applied Physiology, 30(4), 475-499.
Lawrence Herrera is a senior University Studies student at the University of New Mexico (Albuquerque). His program emphasis is in exercise science, sports performance and nutrition. He is a certified personal trainer with the National Academy of Sports Medicine.
Len Kravitz, PhD, is the program coordinator of exercise science and a researcher at the University of New Mexico, Albuquerque, where he won the Outstanding Teacher of the Year award. In 2006 he was honored as the Can-Fit-Pro Specialty Presenter of the Year and as the ACE Fitness Educator of the Year. He was recently presented with the 2008 Can-Fit-Pro Lifetime Achievement Award.