Contrary to popular belief, resistance training reduces body fat percentage, body fat mass and visceral fat in healthy adults to a similar percentage as cardiovascular exercise, found a meta-analysis by the University of New South Wales in Sports Medicine.
“People think that if you want to lose weight, you need to go out and run,” says senior author of the study Dr Mandy Hagstrom, exercise physiologist and senior lecturer at UNSW Medicine & Health.
“But our findings show that even when strength training is done on its own, it still causes a favourable loss of body fat without having to consciously diet or go running.”
Up until now, the link between strength training and fat loss has been unclear. Studies have investigated this link in the past, but their sample sizes tend to be small, a side effect of not many people wanting to volunteer to exercise for months on end. Smaller sample sizes can make it difficult to find statistically significant results, especially as many bodies can respond differently to exercise programmes.
“It can be really difficult to discern whether there's an effect or not based on one study alone," says Hagstrom. "But when we add all of these studies together, we effectively create one large study, and can get a much clearer idea of what's going on."
She and her team pulled together the findings from 58 research papers that used highly accurate forms of body fat measurement (like body scans, which can differentiate fat mass from lean mass) to measure the outcomes from strength training programmes. Altogether, the studies included 3000 participants, none of whom had any previous weight training experience.
While the strength training programmes differed between the studies, the participants worked out for roughly 45-60 minutes each session for an average of 2.7 times per week. The programmes lasted for about five months.
The team found that, on average, the participants lost 1.4% of their total body fat after their training programmes, which equated to roughly half a kilo in fat mass for most participants.
While the findings are encouraging for fans of pumping iron, Hagstrom says the best approach for people wanting to lose fat is to stick to eating nutritiously and have an exercise routine that includes both aerobic/cardio and strength training.
“If you want to exercise to change your body composition, you've got options,” says Hagstrom. “Do what exercise you want to do and what you're most likely to stick to.”
Busting the fat loss myth
Part of the reason many people think strength training doesn't live up to cardio in terms of fat loss comes down to inaccurate ways of measuring fat. For example, many people focus on the number they see on the scale – that is, their total body weight. But this figure doesn't differentiate fat mass from everything else that makes up the body, like water, bones and muscles.
“Generally, we don't gain any muscle mass when we do aerobic training,” says Hagstrom. "We improve our cardiorespiratory fitness, gain other health and functional benefits, and can lose body fat.
“But when we strength train, we gain muscle mass and lose body fat, so the number on the scales won't look as low as it would after aerobics training, especially as muscle weighs more than fat.”
The research team focused on measuring how much the total body fat percentage – the amount of your body that's made up of fat mass – changed after strength training programmes. This measurement showed fat loss appears to be on par with aerobics and cardio training, despite the different figures on the scales.
“A lot of fitness recommendations come from studies that use inaccurate measurement tools, like bioelectrical impedance or scales,” says Hagstrom. “But the most accurate and reliable way of assessing body fat is through DEXA, MRI or CT scans. They can compartmentalise the body and separate fat mass from lean tissue.”
While this study didn't show whether variables like exercise duration, frequency, intensity, or set volume impacted fat loss percentage, the team hopes to next investigate whether how we strength train can change the amount of fat loss.
As part of their study, they conducted a sub-analysis comparing how different ways of measuring fat can influence a study's findings. Interestingly, it showed that when papers used more accurate measurements like body scans, they tended to show lower overall changes in body fat.
“Resistance training does so many fantastic things to the body that other forms of exercise don't, like improving bone mineral density, lean mass and muscle quality,” says Hagstrom.
“If you're strength training and want to change how your body looks, then don't focus on the number on the scale too much, because it won't show you all your results. Instead, think about your whole body composition, like how your clothes fit and how your body will start to feel, and move, differently.”
Study details
The Effect of Resistance Training in Healthy Adults on Body Fat Percentage, Fat Mass and Visceral Fat: A Systematic Review and Meta-Analysis
Michael A. Wewege, Imtiaz Desai, Cameron Honey, Brandon Coorie, Matthew D. Jones, Briana K. Clifford, Hayley B. Leake, Amanda D. Hagstrom.
Published in Sports Medicine on 18 September 2021
Abstract
Background
Resistance training is the gold standard exercise mode for accrual of lean muscle mass, but the isolated effect of resistance training on body fat is unknown.
Objectives
This systematic review and meta-analysis evaluated resistance training for body composition outcomes in healthy adults. Our primary outcome was body fat percentage; secondary outcomes were body fat mass and visceral fat.
Design
Systematic review with meta-analysis.
Data Sources
We searched five electronic databases up to January 2021.
Eligibility Criteria
We included randomised trials that compared full-body resistance training for at least 4 weeks to no-exercise control in healthy adults.
Analysis
We assessed study quality with the TESTEX tool and conducted a random-effects meta-analysis, with a subgroup analysis based on measurement type (scan or non-scan) and sex (male or female), and a meta-regression for volume of resistance training and training components.
Results
From 11,981 records, we included 58 studies in the review, with 54 providing data for a meta-analysis. Mean study quality was 9/15 (range 6–15). Compared to the control, resistance training reduced body fat percentage by − 1.46% (95% confidence interval − 1.78 to − 1.14, p < 0.0001), body fat mass by − 0.55 kg (95% confidence interval − 0.75 to − 0.34, p < 0.0001) and visceral fat by a standardised mean difference of − 0.49 (95% confidence interval − 0.87 to − 0.11, p = 0.0114). Measurement type was a significant moderator in body fat percentage and body fat mass, but sex was not. Training volume and training components were not associated with effect size.
Summary/Conclusions
Resistance training reduces body fat percentage, body fat mass and visceral fat in healthy adults.
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