The debate still continues over whether resistance training alone (i.e., in the absence of aerobic training and/or dietary intervention) can substantially decrease body fat over time and, if it can, precisely how and why it works. Issues such as lipid topography (where fat is deposited in the body), the type of fat cell and differences in metabolism and metabolic health, gender, age and genetics (such as leptin and UCPs or uncoupling proteins) compound the problem with the fat loss issue.
Explanation for the lack of consistency in research on resistance training and fat loss is often found in poor research methodology, very low sample sizes and differences in techniques used to measure fat, lean tissue and metabolic rate. The length of time of the exercise intervention in most studies is also often too short for any effective change in the research outcome.
Body Composition and Resistance Training
Body composition is comprised of fat mass and fat-free mass. Fat mass is made up of stored fat in adipose tissue (or sub-cutaneous fat) and essential fat which may be stored in muscle, bone marrow, liver, kidneys, spleen, heart and spinal cord. Fat mass also includes fat deposits stored around the organs of the body (called visceral fat). It is this visceral fat that is largely associated with health-related problems such as heart disease. Fat-free mass is made up of muscle mass, glycogen and water as well as bones and internal organs. Lean body mass (muscle mass) is made up of muscle, glycogen, water and essential fat stored as muscle triglycerides. Resistance training has been included in exercise programs on the assumption that it will increase fat-free mass (lean body mass specifically), which in turn is believed to lead to a decrease in fat mass. The reason for this is explained in the belief that increases in lean body mass will automatically lead to an increase in metabolic rate. An analysis of the research on resistance training and fat loss has shown that while fat loss has occurred to various degrees, it has not necessarily been the result of an elevation in metabolic rate and/or significant changes to lean body mass.
Energy Expenditure and Resistance Training
An excess quantity of fat is likely to be the result of a disturbed balance between energy intake and energy expenditure. Fat loss has traditionally been based on a model of energy intake and energy expenditure. To lose fat weight, according to the model, energy expenditure must exceed energy intake by bringing about a negative energy balance. Other models used to explain fat loss include:
- Rate of change in fat stores = Rate of fat intake - Rate of fat oxidation (Egger 1997).
Another model of fat loss, the "ecological" model, has suggested that:
- Equilibrium in fat stores = Energy/fat intake-Fat expenditure X Physiological Adjustment (Swinburn and Egger, 1997)
The most often used solution against a positive energy balance is a reduction of energy intake by changing the diet. Most studies investigating hypocaloric (very low calorie) diets, as a means of bringing about a negative energy balance, have shown a substantial decrease in fat mass but also an unwanted decrease in fat free mass and energy expenditure. This method has also produced a rebound effect with a considerable regain of fat weight when the person has resumed a normal diet. Some authors have argued that this rebound effect helps a person to maintain a set point or "settling point" for body weight (Keesey 1989).
To determine how resistance training may influence fat metabolism, it is important to understand the relationship between exercise and 24 hour energy expenditure. Twenty four hour energy expenditure has three separate components:
Thermic Effect of Activity (either incidental or planned), which accounts for about 15-30% of energy expenditure; Thermic Effect of Feeding, which accounts for about 10% of energy expenditure; and Resting Metabolic Rate (RMR), which accounts for about 60-75% of energy expenditure. Resistance training will marginally increase energy expenditure during the exercise bout; however, it is extremely difficult to estimate the actual energy cost (usually expressed in kcal.kg-1.h-1). The gross energy cost of exercise that can be maintained for more than a few minutes roughly varies between 2.0 kcal.kg-1.h-1 (leisure walking, leisure canoeing) and 20 kcal.kg-1.h-1 (running, cross-country skiing). In previously untrained subjects, the highest energy expenditure during walking has been estimated to be 9.6 kcal.kg-1.h-1 and in running 16.2 kcal.kg-1.h-1 (Saris and van Baak, 1994).
More recently, emphasis has been placed on the amount of fat to carbohydrate used as a proportion of the total energy expended during an activity. This concept differs from the model that emphasizes the total amount of kilocalories expended during exercise. In this regard, it is likely that resistance training may well be less effective than aerobic training in fat metabolism, given the fact that resistance training, for the purpose of changing body composition, relies mainly on anaerobic metabolism, while aerobic exercise relies on aerobic metabolism.
So, if resistance training does not expend a great amount of energy during the exercise bout, and what is expended is predominantly carbohydrate, how could it be successful in reducing fat? One answer seems to be found when we look at what happens immediately after exercise or during the period of excess post-oxygen consumption period (EPOC).
Resistance Training and EPOC
Resistance training may well be effective in fat metabolism during the period of EPOC. This may occur in two ways: 1) via an increase in Metabolic Rate and hence energy expenditure, which has been "kick started" during the exercise bout and 2) via a change in the fat to carbohydrate "mix" as a contribution to either elevated energy expenditure or "normalized" expenditure (i.e., energy expenditure which has returned to pre-activity level).
On the evidence, it would appear that, for sedentary exercisers, metabolic rate is elevated for no longer than 20-30 minutes after exercise. If that is the case, the important issue lies in whether resistance training can actually increase the rate of fat use post exercise.
There is some evidence to show that lipid oxidation can vary from pre-exercise to post-exercise from between minus 24% to plus 35% (as measured by respiratory quotient or RQ) following a weight training program (Ludo 1994, Parker 1998). This author has found an increase, on average, of up to 20% increase in lipid oxidation for at least two hours post resistance training (Parker, 1999). Other authors have found no significant differences in RQ during sleep when comparing body builders with a control group (Bosselaers, 1994). Evidence from Ludo et al. (1994) has demonstrated that weight training can moderate substrate utilization during sleep towards an RQ ratio of 0.78-0.79. Any change in RQ has been explained by an increase in catecholamine (epinephrine and nor-epinephrine) levels (Bielinski, et al. 1985; Tremblay et al. 1985), but this requires further extensive investigation.
Metabolic Rate and Resistance Training
As people get fatter, they are likely to lose fat free mass, partly because their spontaneous, incidental and/or planned activity declines or ceases. As a consequence, their energy expenditure declines. This decline in energy expenditure is often explained by a decrease in resting metabolic rate (RMR) (Keim 1990, Mole 1990). RMR is the rate at which an individual’s body uses energy at rest, usually in a 12-hour fasting state. The assumption has been that if fat-free mass can be restored, RMR would increase, hence increasing energy expenditure.
Technical and methodological differences, as well as inter-individual genetic differences, often explain the inconsistency of findings of resistance training on RMR. Inconsistency may also be accounted for by measures used to assess fat-free mass. Skeletal muscle mass, for example, has a lower metabolic rate than other components of fat-free mass (e.g., internal organs). The length of time it takes to change body composition (especially lean body mass), particularly in obese clients, also varies considerably.
So, can resistance training increase metabolic rate over time? Table 1 is an overview of some studies that have investigated this question. From this table, it would appear that while resistance training can produce a reduction in fat, it is not necessarily because of an elevation in metabolic rate over time.
Resistance Training and Gender Differences
It is now evident that there are gender differences that may affect fat loss (Poelhman, et al. 1995; Williford et al. 1989; Wilmore 1992). Generally, it appears that men have an advantage in losing fat as a result of exercise (Anderson et al. 1991; Ballor and Keesey 1991) and that women need a greater training stimulus to induce fat loss (Williford 1993). The major factors associated with fat loss, which appear to be influenced by gender, include body composition, fat topography and adipose reactivity, substrate utilization and response to exercise, hormonal differences, age and genetic factors. In pre-menopausal women, for example, it appears that resistance training may well be better than aerobic training in reducing sub-cutaneous fat and visceral fat and that aerobic training is better in reducing overall fat (Parker, 1999).
The Effects of Resistance Training on Energy Expenditure (EE), Substrate Utilisation and Metabolic Rate: Summary of Studies
|Ludo etal (1994) (Longitudinal Intervention Study)
||21 males, 25-45 yrs, not exercised for 2 yr prior to study
No control group
|12 wks 2.d.wk-1, 14 exercises, periodised program. Wks 7-12 training. program changed for 7 exercises
||SMR measured in respiratory chamber, RQ.used to determine energy substrate utilisation.
||(i) RT has no effect on SMR but can increase fat utilisation during sleep.
(ii)Weight training > FFM and <FM
(iii) No relationship between changes to body composition and changes to SMR or RQ
|Bosselaers et al.(1994)
|9 non-obese males and 1 non-obese female formed strength-trained group
10 lean men and women formed control (non-strength trained) group.
|Habitual training routine maintained up to experiment then Ss put on simulated activity (unstated).
||24-h EE measured by indirect calorimetry in respiratory chamber.
||No differences in 24-h EE between bodybuilders and control when adjusted for differences in FFM.
|Broeder et al. (1992) (Prospective RCT Study)
||Resistance-trained group(RT) (13)
Endurance-trained group(ET) (15)
Control group (19)
|4 day split routine using 11 exercises with abdominal exercises performed each workout day; 3 sets 10-12 reps per set per exercise.
||Hydrostatic weighing determined body density
2 determinations of RMR by indirect calorimetry
|No change in RMR for both groups but may attenuate RMR.
< fat weigh > FFW in RT group;
< fat weight and maintenance of FFW in ET group
(Prospective Intervention Study)
|12 RT women and men, aged 56-80 yr
||12 wk, 3.d
wk-1, 3 sets, 80% 1RM
-tometry and respiratory chamber/ventilated hood.
|Unchanged body weight, > FFM,
< fat mass
Increase in RMR (p< 0.02)
|Ballor and Poehlman (1992)
(Cross Sectional Study)
|82 young women divided into sedentary group (SED) (48), aerobically-trained (AT) (21) and resistance -trained (RT) (13)
||RT program included 3-5 sets per exercise , 10-20 reps per set, 65-80% of 1RM
||Ventilated hood following 10-12 h fast
||< FW in AT and RT groups compared to SED group.
>RMR in AT and RT groups compared to SED group
When adjusted for differences in FFM and FM, >RMR in AT group
No difference in RMR between SED and RT groups.
- Ballor, D and Poehlman, E., (1992) resting metabolic rate and coronary-heart risk in aerobically and resistance trained women. Am J Clin Nutr., 56:968-74
- Broeder, C, Burrhus, K., Svanevik, L. and Wilmore, J., (1992) The effects of either high-intensity resistance or endurance training on resting metabolic rate. Am J Clin Nutr., 55:802-810
- Bouchard, C, Shephard, R, Stephens T, Sutton, J and McPherson, B. (1990) Exercise, Fitness and Health. Champaign, Illinios: Human Kinetics.
- Butts, N. and Price, S., (1994) Effects of a 12 week weight training program on the body composition of women over 30 years of age Journal of Strength Conditioning and Research 8 (4).
- Campbell, W., Crim, M., Young, V., and Evans, W., (1994) Increased energy requirements and changes in body composition with resistance training in older adults. Am J Clin Nutr 60:167-175
- Parker, R (1999) The effects of resistance training on lipid oxidation in pre-menopausal, obese sedentary women aged 30-55 years. (Awaiting Publication)
- Poehlman, E., (1989) A review: exercise and its influence on resting energy metabolism in man. Med Sci Sports Exerc Vol 21 5:515-525.
- Stone, M, Fleck, S., Triplett, T. and Kraemer, W. (1991) Health and performance-related potential of resistance training. Sports Med. Vol 11 4:210-231.
- Wilmore, J., (1984) Morphological and physiological differences between man and women relevant to exercise. Int Jnl Sports Med 8:193-194.