Why We Gain Weight – Does It Happen with Age?

By Jiri Kaloc

Most people think that weight gain is a part of ageing but not all old people are overweight. So, what’s the deal? When do we start gaining weight and what are the most important events that cause weight gain? Let’s dig into how your weight should or shouldn’t change throughout your life.

When do we gain weight?

First, let’s get the statistics out of the way. A commonly cited review published in March 2013 by the Agency for Healthcare Research and Quality looked at 58 publications that included 555,783 patients and concluded this:

“Adults tend to gain weight progressively through middle age. Although the average weight gain is 0,5 to 1 kg per year, this modest accumulation of weight can lead to obesity over time.”

If you want to put some numbers to it, then we could say that obesity incidence peaks between years 40 to 59. But interestingly, it then decreases slightly after age 60. That’s probably because people above 60 sometimes lose weight due to illness.

Does your metabolism slow down?

Now you might be thinking that it’s all because of your slowing metabolism. As it turns out, research shows that that’s unlikely. Your metabolism is very stable from your 20s all the way to your 50s. It only starts slowing down in your 60s, which is when most people already are fat. If you want to read more about how metabolism changes, check out our article.

Why do we gain weight with age?

If it’s not the metabolism itself, why does the average person gain weight with age? One big reason is the changing lifestyle as we go from an active 20-year-old to a sedentary elder. But there’s one more factor built into our physiology. Women and men are affected by different hormones but they result in the same negative influence on weight.

For women, it’s menopause. It happens most often between ages 45 and 55 and causes a significant drop in the hormone oestrogen. This drop makes the body much more likely to store a few extra kilos of fat around the belly. This shift in fat storage may make the weight gain more noticeable.

For men, it’s the significant drop in testosterone that can do something similar. Testosterone levels begin to gradually decline around age 40 at a rate of about 1-2% per year. That can lead to lower muscle mass and higher fat mass. This results in lower BMR and fewer calories burned at rest, which often leads to weight gain if no action is taken to compensate.

Big lifestyle changes

We can see that age can play some role but that still doesn’t explain why even younger people gain weight or why some older people manage to maintain a healthy weight. It turns out that big lifestyle changes are often the main driver of rapid weight gain. Here is why.

Your body tends to protect your current weight and current fat storage. It’s not easy to lose or gain weight with small changes. That’s because your body has tools to adjust your balance. If you start eating just a bit more, maybe your NEAT increases and you start fidgeting more, run warmer or just have more energy and are more active and burn off the small excess of calories. And similarly, if you undereat a little bit, your body simply downregulates some hormones to make you less active, run colder, and burn a bit less energy to maintain balance.

When the changes to your calorie equation are bigger, that’s when your body struggles to compensate and the result is often weight gain. Here are the most common reasons why people gain weight that are not related to age.

  • HolidaysResearch shows that 52% of the average yearly weight gain happens over just a few weeks during the holidays. The reason is a high availability of palatable foods, disrupted routines, and a lot of social cues to eat.
  • Getting a sedentary job – Going from active university life to sitting in an office all day working overtime to kick-start your career is the perfect situation that reduces your calorie expenditure dramatically and starts a process of gradual weight gain.
  • Injury or illness – Anything that takes you out of your exercise routine long-term reduces your calorie output and can be a strong enough impulse to cause weight gain.
  • Starting a family – The increased stress, poor sleep, and limited time for exercise and food preparation can easily break the balance of your calorie equation in favour of weight gain.

And how do genetics figure into this? Let’s explore the influence of genetics compared to lifestyle in the next article.