Research Review: Sex differences in fat loss | Precision Nutrition

Research Review: Sex differences in fat loss

By Helen Kollias

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When it comes to fat loss, men are from Mars and women are from Venus. Or, more precisely, men are from planet Viscera and women are from planet Subcutaneous. But they can agree on losing the same amount of fat overall.

Hopefully, a long time ago you realized that boys and girls were different. Or at least you realized which bathroom you could go into without being thrown out.

And I think most people would agree that men and women tend to carry most of their fat in different places. No, I don’t mean a man bag versus an evening clutch. What I mean is that generally, men carry most of their extra fat around their belly while women tend to carry their extra fat around their hips and lower body.

A while back I reviewed a study looking at different areas of fat and how a specific type of fat, visceral fat, was particularly bad for you. In the same review I talked about apple-shaped versus pear-shaped people.

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What do the different types of fat look like?

Just so you get an idea of different fat and how best to measure it, Figure 1 below shows two magnetic resonance images (MRIs) comparing obese men and women.

It may be hard to orient yourself – the image is around the level of the belly button, but it is cross sectional. You know, like the lady being sawed in half at a magic show – if she was in fact being sawed in half. The MRI is much less messy and everybody stays in one piece.

The spine is at the bottom, so the people are shown in cross-section, lying on their back, facing up.

Labelled are the subcutaneous fat (fat under the skin) and the visceral fat (surrounding the internal organs) cross-section.

figure-1-mri-of-fat
Figure 1 – Cross-sectional abdominal MRIs from (1)

Research question

Normally, women tend to be pear shaped and men tend to be apple shaped. What happens when men and women become obese? Is this still true? And do obese men and women lose weight from the same places?

This week I look at a study that tries to figure out if obese men lose fat differently then obese women.

Kuk JL, Ross R. Influence of sex on total and regional fat loss in overweight and obese men and women. Int J Obes (Lond). 2009. 33:629-634

Methods

This study was pretty straightforward: it compares how body fat is distributed in obese men and women and how it changes after some sort of weight loss program.

So what do you need?

  1. Participants: The participants were 153 men (81) and women (72) with a starting BMI over 27 (considered obese).
  2. Body fat distribution measurement before and after the weight loss: They had a whole-body magnetic resonance imaging (MRI) scans before and after a weight loss intervention. A total of 41-47 images to figure out how much fat was where.
  3. Anthropomorphic measurements: Despite the long name this is just weighing the participants on a scale and measuring their waist and hip circumference.
  4. A weight loss method: In this study they used three methods: calorie restriction, exercise or both (calorie restriction & exercise).
  5. Time: It takes a bit of time to lose weight so there were 12-16 weeks between the before and after measurements.

Why all the and/ors in this study?

You may be wondering why exercise or diet (calorie restriction) or exercise and diet. And what’s up with 12 – 16 weeks?

Despite all the and/or statements in the methods this is not an ingredient list for low-fat cookies. This study is actually a combination of results from other studies. You see, there were a few other studies comparing different types of fat loss and body fat distribution data, so the researchers decided to put them together to compare what happened.

Does it matter that this is a mix of different studies?

Yes and no.

Let me explain.

Yes: if everybody didn’t go through the same thing that may change the results.

No: if there are an equal number of participants in each group then any difference would be lost by averaging.

And when comparing the groups in the other study there were no differences between groups – so while not perfect, chances are this study represents what would have happened if everybody (all 153) did the same thing to lose weight.

Data recycling

It looks to me that this group of researchers initially did a study and realized there were differences between men’s and women’s distribution of fat loss. Then the researchers either did another study looking at this specific question or they did some more studies with similar designs and put them all together.

So the data may have been used in other research articles – data recycling.

Results

People lost weight on the weight loss interventions! Well most people – the range was +1.3% to -19.1%. Meaning somebody, I’m not saying who, gained 1.3%. But I’m getting ahead of myself…

In the beginning

Before I get into the difference between men and women, and the weight loss, I should tell you where they started.

The men and women had the same BMI: 32ish. I say 32ish because the men had a BMI of 31.9 and the women had a BMI of 32.8, but the difference isn’t enough to worry about.

Even though the men and women were close in BMI the men were heavier (and taller). And the men had bigger waists and higher waist-to-hip ratios – more on why later. Meanwhile, the women had more fat overall.

The researchers tried to get men and women closely matched: they tried to make everything but body fat distribution the same, but in order to get comparable BMIs the same the men had to be taller & heavier and the women had more fat (and less muscle).

As I mentioned before, the guys’ bigger bellies and higher waist-to-hip ratios were both indicators of visceral fat, but you would think they both would be good indicators of abdominal fat too. If you measure around your belly then you are measuring both (visceral & abdominal fat). Right? Yes, you are, but according the MRI scans waist girth is a really good predictor of visceral fat.

What was really interesting was even though the guys had more visceral fat and a bigger belly the women had more abdominal fat! So it looks like women can have more abdominal fat without having the more problematic visceral fat.

Weight loss

Comparing men and women after the weight loss intervention found that there was no difference in total fat loss – zippo, nadda, none. Both men and women lost about 6 kg (14 lb) of fat as figured out by the MRI.

Something that is kinda hard to explain is why they lost 7 kg (over 15 lb) as measured by the scale. Where the heck did that 1 kg disparity between MRI and scale come from? Did they lose muscle? Water? Glycogen?

All are possible, and while you may say 1 kg isn’t that much, it is about 15% of the total fat lost. Imagine that 15% going missing if you lose, say, 100 lb of fat. That’s 15 lb MIA.  (Note: for those who are offended by imperial units you can use kilograms for our example – but boy did you lose a lot of fat!)

Body distribution Men Women Observations
Lower body 34 lb 41 lb Women lost more lower body fat than men – but they also started with more
Upper body 31 lb 33 lb No difference between sexes
Abdominals 18 lb 17 lb No difference between sexes
Viscera 17 lb 9 lb Men lost more visceral fat – but they started with more

So the good news is that you can, in theory, spot reduce – if you have a lot of fat (11.2 kg of lower body fat or 3.5 kg of visceral fat) to start with.

figure-2-reduction-in-sub-cut-fat-men-and-women
Figure 2 – Contribution of visceral and regional subcutaneous fat loss in relation to total fat loss

Conclusion

The main finding in the study is there are sex differences in visceral fat and lower body fat loss. Men lose more visceral fat and women lose more lower body weight (and total subcutaneous) as a percent of weight loss. But whether this is because of sex differences or starting fat distribution is impossible to know.

No MRI? Not to worry

If you don’t have a MRI in your basement you can use a cloth measuring tape to find out if you have too much visceral fat. Good old waist-to-hip ratio is pretty good at indicating whether you have too much visceral fat. If your waist girth goes down then your visceral fat goes down (more in men, but it still goes down in women).

Bottom line

The main point of the study was to see whether men and women had different types of fat and whether it was in different places – the answer to both was yes.

The next question was whether men and women lose different amounts of fat – the answer to that is nope.

So no difference in fat loss, but where you’ll lose it and exactly what type seems to be gender specific.

References

Click here to view the information sources referenced in this article.

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