Rest assured it happens to almost everyone — a combination of changing hormones, changes in lifestyle, and pesky genetics.
But when does an expanding waist start to become a health concern? And what can you do to stop it?
The circumference of your waist is important because it's a good indicator of how much abdominal, or visceral, fat you have — the fat you carry around your middle, including your belly.
A little bit of that sort of fat is useful because it protects your vital organs.
But abdominal fat also produces more of the inflammatory chemicals that interfere with the body's normal function than fat stored elsewhere on the body, like the buttocks or thighs.
"When you have a lot of visceral fat, it releases a lot of these inflammatory chemicals which have profound effects on other parts of the body," says public health expert Professor Tim Gill from the University of Sydney.
That includes an increased risk of developing chronic diseases like heart disease and type 2 diabetes.
In men, a waist circumference of 94 centimetres or more means you have an increased risk of chronic disease, according to the Department of Health. Measuring 102 centimetres or more means a "greatly increased risk".
For women, the corresponding figures are slightly lower: 80 centimetres and 88 centimetres.
In 2014-15 the average waist measurement for men was 97.5cm, while for women it was 87.5cm.
More than half of all Australian men had a waist size that placed them in the 'increased risk' category, while two in three Australian women fell into this group.
Is waist circumference always a good measure?
It's often thought waist circumference is unreliable for people of certain backgrounds who may have larger or smaller frames.
But Professor Gill says waist circumference is less affected by factors like ethnicity, height and muscularity compared to other measures like body mass index (BMI).
"Of course, you can see in very large Polynesian men they're likely to be bigger around the waist," he says.
"But in general, if your waist circumference is going up, you're putting on fat, and you're putting on the worst type of fat."
And though a bodybuilder may have large biceps, they usually have a trim waist too, Professor Gill says.
"You have a look at their waist, it's actually quite small. That's because they are burning up all their fat stores and working on putting on just muscle," he says.
What about BMI?
Your body mass index (BMI) is a ratio of your weight and height — a number that gives a rough idea of the amount of fat you carry.
A BMI of between 18.5 and 25 is deemed healthy — lower or higher than that and you're thought to be underweight or overweight.
Professor Gill says on a population level, BMI can be a reliable predictor of health — those with BMIs in the healthy range have less chronic disease than those deemed overweight or obese based on their BMI.
But when you look at an individual's BMI, it's more likely to be distorted by factors like height, frame size or ethnic background than waist circumference.
"It can certainly be distorted by the muscularity of a person — a bigger muscle-bound bodybuilder, or a Polynesian person, they're going to have less fat compared to the typical European person with the same BMI," Professor Gill says.
And those with lighter frames can have a lower BMI but still have large stores of fat on their body, Professor Gill says.
The 'middle-aged spread'
It's a frustration for countless men and women entering middle age. The beer gut, the spare tyre, the muffin top … there are plenty of names for it and none of them are flattering.
This "middle-aged spread" — the weight we gain in our 30s and 40s, often around the middle — is a widespread phenomenon, and occurs because of a combination of physical and lifestyle changes.
As we age our sex and growth hormones (the ones that maintain muscle mass) tend to drop off, and with less energy needed for our muscles, we store the excess as fat.
That's especially the case for men whose sex hormones decline gradually as they age, and women who have gone through menopause.
The fat stores are more likely to be around our middle — because those same growth and sex hormones that work to keep it out of there are the ones being lost.
It's a vicious cycle, Professor Gill says, because the more abdominal fat you have, the more it interferes with the remaining sex hormones, making keeping the weight off even harder.
Most people are also more sedentary as they age, and in some cases eat more — another ingredient in the recipe of middle-aged weight gain.
And that beer belly that can arise around the same time? Professor Gill says the excess kilojoules from large amounts of drinking play a role, but associated lifestyle factors are also to blame — smoking, sedentary behaviour and poor diet.
Some of us are blessed with genes that make us less likely to succumb to middle-aged spread. But consume more energy than you burn, and you're likely to get a bigger tummy no matter what your genes, Professor Gill says.
Losing the weight
So how do you give the spare tyre the boot?
Start small, says Professor Gill. Losing even a little bit of weight — as little as five per cent of your overall body weight — can have a "profound effect" on your health risk and your waistline.
"That's the good thing about the waist circumference measurement, it tends to move quicker than your BMI," he says.
The other good news is the fat stores around our abdomen are "active" — meaning fat moves more quickly into and out of the area compared with other parts of the body.
So though it can be easy to put on, it should come off at a faster rate too: "You lose weight proportionally faster from your abdomen than from other parts of your body," Professor Gill says.