Why do I store fat in my legs?

As a coach specialising in fat loss, the vast number of my clients have been female. This is not unusual and is in fact the norm in the industry – women are far more likely to seek a personal trainer whereas men tend to avoid asking others for help unless they really need to – but it certainly brought up a bunch of patterns pretty quickly.

The pattern I want to talk about today is that of female pattern fat storage, or more specifically, why women tend to store fat in their bum, thighs, pelvis and upper arms – in comparison to men who tend to get a pot belly. There are a ton of ways that you could approach this, but I’ll be adopting a stance grounded in evolutionary biology because I think it’s the most compelling explanation, and also just because it’s kinda interesting.

So what am I talking about?

Obesity is on the rise. According to a report from the UK Parliament, 27% of Britons are obese, and 36% are overweight (1), meaning that being overweight is now the norm. The numbers skew slightly between the sexes though, with 41% of men being overweight vs 31% of women, while 27% of both sexes were obese. This indicates that, although there is a notable difference it’s not huge and obesity is a problem across the board.

What isn’t obvious from these statistics, though, is what people look like at a given bodyweight, or even at a given bodyfat percentage. Why the difference?

The first thing that we need to discuss when talking about male and female fat, is that women simply have more of it. According to one study which compared men and women of healthy bodyfat levels (2), men of a healthy bodyweight were around 20% bodyfat and women at the same BMI were 33% (Interestingly this was for white and black individuals, Asians were at 23 and 35% respectively for men and women). This is generally accepted for people who don’t exercise, so looking at American Council Exercise guidelines(3), the numbers are as follows:

Table 6.14
General Body-fat Percentage Categories

Classification	Women (% fat)	Men (% fat)
Essential fat	10-13%	2-5%
Athletes	14-20%	6-13%
Fitness	21-24%	14-17%
Average	25-31%	18-24%
Obese	32% and higher	25% and higher

These numbers show the disparity between BMI measurements and other metrics of overweightness, with female fat levels of 33% being overweight by one metric and healthy by another, but the numbers do line up pretty damn well, and they do so exactly for men. What’s useful to note here is that a healthy weight woman will store as much fat as an obese man – despite looking the same level of visual leanness, and despite having a drastically different level of disease risk. This second fact is important, and we will return to it.

So men and women are both equally susceptible to fat gain (the level of overweightness is likely to only be disparate due to environmental or sociocultural factors), but at a given level of leanness women have more fat, and a woman with more objectively measured fat than a man is likely to be in better shape aesthetically and metabolically.

But why do we look different?

Fat storage in humans follows generalised sex-specific patterns. The below graph taken from “Sex differences in fat storage, fat metabolism, and the health risks from obesity: possible evolutionary origins” (4) illustrates this well. The graph compares overall fat mass, and fat mass stored proportionally in the legs between overweight and healthy weight men and women.

This shows that at a man who is obese, will store less fat in his legs than a woman who is a healthy weight. This also shows that women increase leg fat more preferentially than men do as they go from lean to obese. A question remains, though – where does the male fat go?

The answer – it goes around his organs.

Here you see the same comparison, but this time looking at abdominal fat, both subcutaneous (under the skin) and visceral (around the organs). A healthy weight man has more visceral fat at both points, and the healthy male level isn’t all that far away from the obese female level. The impact of this will be returned to shortly.

To summarise, in general women tend to store fat on their thighs, buttocks, pelvis and upper arms. Men tend to store fat viscerally, and this occurs even before puberty, with female babies having more subcutaneous fat than male babies – indicating that this is not necessarily entirely hormonal, though the rate at which adolescents gain fat is far from even, with adolescent girls gaining fat at around twice the rate of boys, even when both maintain a healthy weight (with a lot of this extra fat going to pelvic areas and breast tissue).

The effect happens across the spectrum of fatness, too, so overweight men and women may have the same waist circumference (an important measure of CVD risk), but a woman will have on average 1.8kg more subcutaneous fat than the man, meaning he has far more visceral fat.

Why does this matter?

Bodyfat is not all equal. Two terms that are useful here are central adiposity and peripheral adiposity. Central adiposity refers to fat within the abdomen whereas peripheral adiposity is spread elsewhere and mostly subcutaneous. Peripheral adiposity leads to what is often called ‘wobbly bits’. Think stomach rolls, wobbly legs and bingo wings. Visceral fat – the beer belly – is a little more sinister. Central adiposity is associated with diabetes, CVD, Hypertension and dyslipidemia (5). In fact, central adiposity is one of the main predictors for diabetes (6) while higher degrees of peripheral adiposity at a given bodyfat percentage is associated with reduced comparative health risks (7).

This should not be taken to say that peripheral adiposity is healthy, though. Metabolically healthy folks who are overweight but have most of their fat stored subcutaneously are still at higher risk than those who are of a healthy weight (8). Just a lower risk than those who have most of their fat stored viscerally. The same warning must be mentioned for those who are of a healthy weight, but who have a lot of abdominal fat (these individuals usually, but not always, live a very sedentary lifestyle as exercise can prevent this happening(9)) because their disease risk is a lot higher than their BMI would have you think.

With the health implications of male pattern fat storage put aside, female pattern fat storage is still a problem for many women due to the aesthetic effect, and the prevalence of cellulite*. One thing that needs to be noted is that fat that is easy to store generally becomes difficult to lose – meaning that the places that often store fat first are the places that often lose it last. Many women will have highly defined upper bodies long before they get lean lower bodies, and men are the opposite – with many of us having defined legs and lovehandles at the same time.

So why does it happen?

Women generally metabolise fat different to men. Women produce a greater amount of VLDL (the kind of fat that leaves the liver and goes into circulation for use) than men, but the serum levels are pretty similar, that means that the extra fat is being stored (and at baseline, released) more easily in women.

The places that serum fat (either as VLDL or from meals) is stored is different in men and women, too, with fat cells in the areas associated with either sex being more receptive. Interestingly, when fatty acids are released in women more are released from abdominal subcutaneous fat than anywhere else – which is important, because it shows that fat loss will happen in the abdomen first, but fat gain will happen evenly across the subcutaneous fat in the abdomen, buttocks and legs. If, therefore, women yo-yo diet, their legs will gradually store more and more fat at a given bodyweight. One more reason to make a lifestyle change than simply to diet!

A lot of the difference (though not all, as mentioned) appears to be hormonally regulated. Testosterone tends to reduce fat gain, with hypogonadal men storing more fat, primarily subcutaneously (10) and those with elevated testosterone being better able to lose fat. This isn’t all roses, though, because elevated testosterone is one reason for additional visceral fat, even in women who have elevated levels (11).

Estrogen seems to lead to preferential subcutaneous fat storage. Women who have lower estrogen levels generally store more visceral fat than do women who utilise HRT during menopause (12).

So it’s mostly hormonally driven, with estrogen making certain areas more receptive to fat storage, and testosterone doing the same to other areas. Therefore storing fat in your legs and buttocks as a woman is ENTIRELY normal, and similarly storing fat around your abdomen (stomach, lower back) is totally normal for men, too. It’s also true that these places will always be the last places to get lean, no matter what you do, so the answer to ‘what can I do about my legs/butt/bingo wings is pretty straight forward (though of course not easy, or courses like The BTN Academy wouldn’t be around to teach nutritionists and PT’s exactly how to help clients manage it) – you need to lose weight. Strengthening your legs, arms and other areas will not spot reduce fat but it can certainly help to improve overall body composition, so resistance training alongside weight loss is the way to go if getting rid of wobbly bits (or a beer belly) is your aim!

That leaves a big question, though:

So what’s the point?

As I mentioned in the intro, there are some interesting evolutionary hypotheses for why this is all the case. First of all, an important point in the conversation around women having more bodyfat than men is that bodyfat is not inert – it’s not just a sack of stored energy. Bodyfat produces hormones including estrogen (in fact postmenopausal women get ALL their estrogen from their adipose tissue) so it would make logical sense that women would have more of it at baseline. This also explains why men who gain a lot of fat start to develop some female-like characteristics in terms of fat storage, such as gluteal and pectoral fat (or man-boobs, if you rather).

Estrogen isn’t all that adipose tissue produces, though – it also produces (or stimulates the production of) the hormones leptin and insulin. Leptin is closely associate with appetite; as your fat cells get fuller your body produces more of it, and this in turn reduces calorie intake (or at least that’s the idea, we can override this with pizza and other high-calorie delicious things quite easily). Insulin, on the other hand, up-regulates the cellular processes that drive nutrient storage from the blood.

Again, though, not all fat is the same. Excessive visceral fat is associated with elevated insulin, which is one reason that this male-pattern fat storage is associated with insulin resistance and diabetes. Subcutaneous fat, however, is associated with increased leptin. To consider the purpose of these it’s a good idea to think evolutionarily again.

Being overweight or obese was not really a factor, historically, and so it can be argued that there is no adaptive reason for men to store more and more visceral fat as we become overweight. Remember, for much of the history of mankind and our numerous predecessors, feeding was done intermittently, with many hours and even days between meals. That means that there is a HUGE advantage to being able to store excess energy which counteracts the potential negative effects of gaining fat.

Male fat storage, rather than being adaptive, is just a reflection of the fact that testosterone has the adaptive capability to reduce subcutaneous fat unless you become obese (which, thanks to estrogen production, would be a problem) but the fact that it doesn’t have an ability to do the same to visceral fat as this has never been a problem before.

Estrogen, on the other hand, leads in a roundabout way to increased levels of leptin. Leptin is not just a hormone that regulates weight, though – leptin has a number of other roles:

• Leptin accelerates puberty. Overweight girls start menstruating at a younger age (13) and mice given exogenous leptin attain sexual maturity faster (14). This potentially explains why girls reach maturity faster than boys; something which has a fairly obvious adaptive value for a reproductive species that tends to have one child at a time, as it allows for a greater reproductive window in life.
• Leptin is important for folliculogenesis (15, 16) (the maturation of the ovarian follicle, or the ‘finishing of an egg’) indicating another reproductive benefit of women having more fat at a given bodyweight, and more fat that is able to produce leptin at a given bodyfat percentage
• Leptin receptors are found in numerous foetal tissues and so leptin seems important for foetal development, with leptin levels being a predictor of foetal size at a given age (17)
• Low leptin levels are associated with miscarriage, because low leptin indicates low bodyfat, which would represent a poor environment for childbirth evolutionarily. Along with this, fertility at a low BMI is impacted for women first. Although men DO eventually become infertile as bodyfat gets very low (or at least unable to function sexually), women experience amenorrhoea at a far higher comparative BMI (20)

Subcutaneous fat, therefore, seems adaptive in women because it provides the leptin that assists with fertility. That’s not all it does, though.

Women are more able to utilise free fatty acids for fuel than men, who tend to use glucose more readily at a given intensity of exercise (18, 19). That means that women naturally have a greater storage of usable energy, and a greater ability to actually use it – sorry dudes, but from an evolutionary perspective this makes sense, because women are far more important for the species. Men are somewhat disposable as far as propagating the species, though, but women certainly are not. A man can impregnate 50+ women in a month (if he really put his mind to it) but women can reproduce around once per year – that increased involvement places them at a far higher value, and so their survival has been selected for more preferentially. In short, if men died off because their fat storage ability and fat oxidising propensity wasn’t good enough, it didn’t really matter because the species still carried on, if women died because of the same conditions then those genes didn’t make it to the next generation. Over millions of years that ‘selective pressure’ makes a lot of difference.

In short, women who stored more fat in these areas are more able to produce children and give birth. The obvious benefit here is due to a surplus of energy that can be used during times of famine, and the connection between leptin levels (a marker for fat storage) and fertility is a clear sign of the link here. This is not something that has gone unnoticed in men, with there being a cross-cultural preference for women who have a hip:waist ratio that most closely resembles a reproductively healthy balance between central and peripheral fat (21).

Final thoughts

So there you have it. There is a clear evolutionary benefit to female fat storage patterns, with male storage patterns generally being a side product of the other effects testosterone has, in combination with the impact of simply holding too much fat. In order to lose ‘wobbly bits’ or a ‘beer belly’ the answer is not to look to hormonal interventions (as has been claimed by some, who will ‘diagnose’ hormonal issues from these totally normal storage patterns), it’s just to lose weight and stay active.

If you have a genuine endocrine problem, of course (you may be hypogonadal for instance), then things are different – speak to your endocrinologist about this.

Of course, once you get above a healthy bodyfat level, regardless of sex, the additional fat storage becomes maladaptive. Women start to store more visceral fat, and they store additional subcutaneous fat which – although it’s not AS dangerous – can still contribute to CVD. Men, however, start off with a potentially problematic level of visceral fat and this only gets worse – as does their sex hormone profile thanks to elevated estrogen.

So stay a healthy weight, avoid repeatedly gaining and losing fat, and stay as active as you can. But relax – there’s a damn good reason that cake goes to your hips.

At the BTN Academy we always stand by the importance of individualised approaches. Men and women are different, and no two men and no two women will ever be the same – that’s because every single person is an individual with different genetics, personality traits, lifestyle variations, habits, wants, desires and goals. That’s why on the BTN Advanced Nutrition Coaching Course – Active IQ Level 4 starting in November we have dedicated modules to things like male vs female nutrition, nutrition for numerous different athletes, and nutrition for a host of other specific situations that are likely to crop up during typical coaching practice.

If you want to work in nutrition – hell, if you want to know as much about nutrition as you can, then you need to learn to work out different needs and then cater to them.

Join our Advanced Nutrition Coaching Course – Active IQ Level 4 here and we will show you how.

References

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3. https://www.acefitness.org/
4. Power, M. and Schulkin, J. (2007). Sex differences in fat storage, fat metabolism, and the health risks from obesity: possible evolutionary origins. British Journal of Nutrition, 99(05).
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6. Racette, S., Evans, E., Weiss, E., Hagberg, J. and Holloszy, J. (2006). Abdominal Adiposity Is a Stronger Predictor of Insulin Resistance Than Fitness Among 50-95 Year Olds. Diabetes Care, 29(3), pp.673-678.
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