It is true that your metabolism slows down a little each decade, causing most women to gain about 1 pound per year due to age.  But by the time women hit premenopause (anywhere from 40-55 years old) and menopause and beyond (around 55+), it feels like a lot more than one pound per year.
Natural weight gain due to aging, while real, doesn't explain why most women over 40 experience a dramatic increase in weight, specifically, belly fat, which is very difficult to lose.
Clients are constantly telling me how they have a closet full of clothes that don't fit and nothing they seem to do (like going to super expensive pilates classes twice a week) is making a difference.
Something else must be going on, right?!
Remember our equation for a healthy weight from What Do Hormones, Weight Gain, and Metabolism Have to Do With Each Other:
[healthy hormones --> healthy cells --> healthy metabolism --> healthy you --> healthy weight]
If we start at the beginning, we can see that there must be hormones at play that would impact our cells, our metabolism, and eventually our belly's.
Well, there are two main hormones responsible for this new stubborn belly fat.
The first is estrogen, which most women jump to right away because it's consistent with their personal experience. But the second hormone is even more important and that hormone is insulin. 
Insulin is the hormone that tells your body to store fat. 
Estrogen is the hormone that protected you from the effects of insulin. 
Your declining estrogen and increasing insulin have to lead to the perfect storm (another one!) of you putting on weight and specifically you putting on fat around your middle.
To be sure, there is so much we don't understand about how estrogen impacts women's metabolisms, the mechanisms by which most of the changes associated with the midlife transition are not clear. Not by a long shot. 
Now, with that truth in mind, let's look at 3 reasons you have such stubborn belly fat and why it's so hard to get rid of... and then how to fix it.
#1: Lower estrogen levels appear to cause more fat to be stored. 
In 2009, researchers took two groups of mice and fed them the same rat chow and watched how their little mouse bodies accumulated and stored fat.
The difference between the two groups was that one group had their ovaries removed (which depleted all their estrogen) and the other group of mice still had their little mouse ovaries (and estrogen).
The clever researchers wanted to see what happened to the metabolisms (and weight) of the mice without estrogen compared to the metabolisms of mice with estrogen while both groups at the same diet.
Well, the poor mice with no estrogen burned fewer calories, even though they ate the same rat chow, and their existing fat cells became larger from storing more fat.
This study showed that the loss of estrogen has a direct influence on how much fat women store and how many calories they burn.
The most interesting finding was that the 'estrogen-less' mice also became very insulin resistant.
We'll come back to this very important finding later.
#2: Lower estrogen levels appear to increase the redistribution of fat to the belly and viscera. 
Over 65% of women aged 40-59 and over 73% aged over 60 have too much fat stored around their midsection. We call this abdominal obesity. When the fat is concentrated in the belly and not the thighs or hips.
It's the apple shape instead of the pear shape. 
A study at the Mayo Clinic in 2013 showed that decreasing estrogen levels in postmenopausal women caused more activity in two important enzymes (adipocyte acyl-CoA synthetase & diacylglycerol acyltransferase). Which caused women to preferentially store fat in their belly area (even in the dangerous deep intra-organ space) instead of their hips or thighs. 
There is also much discussion about certain estrogen receptors and changes in androgen hormones that lead to this belly fat, but again, there is so much about this we don't have all figured out. 
What we do know is that when estrogen levels decline, it causes more fat to be shunted and stored in the abdominal region. It impacts the distribution of your body fat. 
#3: Lower estrogen levels appear to increase insulin resistance.
This is the money shot right here.
Because you can't store body fat, in your belly or anywhere else, without insulin.
Insulin resistance happens after your cells have been bathed in insulin for too long from a metabolically damaging diet and lifestyle. Your cells are full of sugar and they just can't take in any more sugar.
So, your pancreas releases more and more insulin to do the job of getting the sugar out of your blood and forcing them into your cells and taking the excess sugar to be stored as fat. 
When you're insulin resistant, your insulin turns on and stays on too long.
Insulin resistance leads to stubborn weight gain because as long as your insulin is on, you are in fat-storage mode.
If you are insulin resistant, which is the MOTHER OF ALL METABOLIC DAMAGE, you will constantly be battling this fat accumulation process that you can't turn off.
At this time, we know that a lack of estrogen leads to insulin resistance, but the exact mechanism(s) are still being discovered.
Houston-based researchers note, "Estrogen may regulate insulin action directly via actions on insulin-sensitive tissues or indirectly by regulating factors like oxidative stress, which contribute to insulin resistance." 
Even if we don't have all the details figured out, we know that declining estrogen appears to lead to increased insulin resistance.
The next logical question would be, "If declining estrogen makes me gain belly fat and weight because I become more insulin resistant, can't you just give me estrogen and everything will ok?"
Well, no, not really. it's not that easy. 
Again, not by a long shot.
A better question would be, "How can I treat my insulin resistance since that is the root cause of my weight gain now that I've lost the protective benefits of my natural estrogen?"
Ah! Now you have options!
Options that don't include any medications, injections, patches, or creams and can be helpful at any age!
This is what repairing your metabolism is all about. And you don't have to be postmenopausal or have high fasting blood sugars to have insulin resistance.
Most of us have lifestyle and diet patterns that are causing our insulin to turn on and stay on way too long. Over time this causes insulin resistance or metabolic damage at the cellular level. 
You may have been slowly creating your insulin resistance for years with Diet-Cokes and low-fat living and then when you lose your protective estrogen, you've suddenly got a massive weight issue on your hands.
To wrap it up, losing most of your estrogen is a total drag on your metabolism and does have a direct impact on the way you store and carry fat. 
But replacing estrogen alone won't magically solve all your weight woes. 
To get rid of your belly fat you need to be focused on fixing your metabolic damage, your insulin resistance, and managing your insulin.
Then, and only then, will you be able to start melting away that stubborn and unwanted belly fat.
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 Wikipedia: Insulin. https://en.wikipedia.org/wiki/Insulin
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