People who have lost a lot of weight often note it gets harder drop more weight after the initial loss. A recent study in the Journal of Strength and Conditioning Research examined one reason why that might be.
The Hypothesis
One hypothesis is that people with more fat also have a greater ability to burn fat. For example, if one woman had ten pounds of lean body weight and weighed 125lbs on the scale (twenty percent body fat) and another woman had the same lean body weight but weighed 135lbs (26% body fat), it is believed that the second woman will have a greater ability to burn fat.
There are a few reasons to think this might be true. First of all, it seems that macronutrient availability influences energy expenditure. This seems to be true regardless of how those substrates are available in the body. For example, consuming more of a particular macronutrient in your diet can increase your body’s willingness to expend energy from that nutrient.
Secondly, people with greater body fat levels have higher resting levels of energy expenditure. All of this points to the fact that when more energy sources are available, more energy gets spent.
Study Design
To test this hypothesis, fourteen women were tested for their body fat levels. Their fitness was tested via VO2 max, and their substrate utilization for both fat and carbohydrate was examined. Having the values for lean body mass, the researchers expanded the results by determining the substrate utilization rates relative to lean body mass.
Each of the women had similar amounts of lean body mass and similar macronutrient consumption in grams per pound of bodyweight. Both of these factors, if majorly different, could have influenced the results, so they were factored out.
Results
The researchers found that peak fat utilization occurred at around 55-60% of the women’s VO2 max. For most people, that pace would correspond to a heart rate averaging between 130 and 140 beats per minute, depending on age and weight.
As for body fat levels and fat burning, there was no significant correlation, but this is worth some discussion. In statistics, “significant” would mean that higher rates of fat burning could reliably be explained by some feature of the women, such as greater levels of body fat. However, just because that factor wasn’t found, doesn’t mean it doesn’t exist.
In statistics, a non-significant trend can still give you information, although that information might not be so reliable. When you look at the actual numbers here, there does seem to be a trend. The women with greater body fat burned on average about 100mg more fat per minute, which is about 25% more. It was also 2mg per kilogram of lean bodyweight more per minute for the higher fat group.
This trend seemed to hold across the studied variables. Combine this trend with the fact that none of the women were actually obese and the range in body fat levels was fairly small (from 18.6%-30%), and you wonder if a larger study might not have given stronger evidence.
To add fuel to the fire, the women were statistically considered as two groups, rather than individuals, and several of the women had body fat levels close to the cutoff point. For example, there was a woman in the lower fat group with 24.5% body fat where the cutoff was 24.9%. So, despite the statistically-correct conclusion, an analysis of the evidence shows a strong possibility that having more fat does indeed mean a greater ability to burn fat.
References:
1. Ashley Blaize, et. al., “Body fat has no effect on the maximal fat oxidation rate in young normal and overweight women,” Journal of Strength and Conditioning Research, DOI: 10.1519/JSC.0000000000000512
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