It's actually complicated.

I spoke with a PhD physical therapist about this (his undergrad work was in exercise physiology), and at about 40, all other things being equal, you're going to start losing muscle mass. By "all other things being equal", I mean that even if your diet is identical, you exercise at the same intensity, and through some previously unknown magic (e.g., drugs) you have identical hormonal levels to your 18yo self, you're still going to end up losing muscle mass and strength when you get somewhere in your 40s. He explained the basic outlines of the mechanism, but I simply don't have the science background to understand it, or to explain it.

That loss of muscle mass means that you're simply not going to be burning through calories. Muscle burns more calories per kilogram than fat does.

So, that's part of it; there's just a certain level of decline that's going to happen with age, and there's nothing you can do to prevent it.

But the other part is that activity levels do tend to sharply decrease for men (and women) outside of their 20s, while what they consume does not. Once you start having kids and/or other responsibilities, it's hard to find time to maintain the same level of physical activity that you might have previously had.

In my case, children soak up time and when I do get out for a walk I have to keep pace with a young child instead of marching up a mountain, I'm no longer working on my feet all day, and I spend my free time sitting down and eating chips because the day is exhausting instead of taking acid and going to a rave.

beer

guys that don't drink a lot of beer are by far much better off than men who do and have the same physical activity. probably just my own observation and not scientific but it does have something to it.

Calories in vs calories out is true at any age.

In the 30s, life is starting to stack up. Career, kids, etc. These things easily dominate your schedule and can keep you from eating well, or hitting the gym or sleeping as much as is needed.

Also, years of beer can catch up to you and there's so many calories in that.

Also get your blood work checked. If your hormones aren't where they need to be, it may be harder to build and maintain muscle. Muscle burns more calories at rest than anything else so slipping muscle quantity can result in a snowball effect of weight gain and unhealthiness.

Beer.

The hidden issue is general aches and pain. That noise an old guy makes when he stands up, means he doesn't get up, and doesn't move around as much. But it's real easy to sit down with a pint and a burger.

Contrary to popular belief, your metabolism does not really slow down as you age.. From the time that you are 20ish to around 65ish, there is no inate change in how your body uses energy.

People use "metabolism" as some magic word, but it's literally just how many calories you burn by moving around a doing stuff and keeping your body temperature constant. Your metabolism slowing down would just mean that you are doing less stuff.

Some people don't like that information because they like to think that getting out of shape is due to factors outside of their control (and it definitely is for some people due to all sorts of life circumstances), and the fact that it is in your control makes them see it as a moral failing.

I see it more optimistically: when you age, you aren't fighting the tide. You do have control over your body composition. It gets inherently a little harder once you get up to your mid sixties, but even then, the difference is not as large as you'd expect.

Mid 50s here. One of the biggest factors for me has been how long it takes to recover from the wear and tear of being active. In my 20s I could be sore in the afternoon from exercise in the morning but be fine by evening. Now 48 hrs is typical. At my age I'm only focusing on maintenance and injury prevention.

It's all about time and commitment. When you're younger, you tend to be more active and need extra exercise less. As you get older, your need to move in order to survive diminishes. If you don't compensate for it with good exercise habits, then you're going to gain weight.

Alcohol and sugary drinks. Stick to water and the occasional tea or coffee, so long as you don't add sugar. Coke and its ilk are liquid death, avoid at all costs. Giving up those high sugar content drinks will be hard if you are used to them, though...

For me, it’s just a complete change to my exercise routine. In my 20s, with no kids, I was running about 5k (3 miles or so) most days, with some weight lifting mixed in. These days, with a full time job, two young kids, and two dogs, plus all of the other responsibilities that seem to come with adulthood, I’m lucky to squeeze in a 15 minute bike ride here and there.

Even with all that, I try my best to eat as healthily as possible. Over the last 5 years I’ve gained about 8 pounds, which could be worse all things considered.

Oh theres so many reasons. The highlights?

Theres a strong probability that you are time poor, this kicks your ass in two ways. You get a lot less incidental movement. You drive more and walk less, order things to be delivered instead of going and getting them. Secondly sacrificing sleep which leads to...

Testosterone dropping, not only does your testosterone start to fall in your 30s, low sleep has a terrible impact on T production. Which means a whole bunch of lovely side effects including lower lean muscle mass, depression and fatigue. Oh also being overweight also fucks your T levels...

Lets throw in the fact that most of us have indoor jobs that are very sedentary, which ruins your vitamin D levels. Which leads to fatigue, depression, poor sleep (theres that sleep again) and a bunch of other shit.

The thing is its actually not that hard to stay in shape. Its about a sustained conscious effort and finding things you like that work FOR YOU. Theres no secret diet or workout plan. Calculate your Macros and eat accordingly, find an exercise you LIKE doing and do it often. Dont have a vision board or a body goal, learn to love the process and its the easiest thing you can ever do.

Well, as I write this, it's 1:45 AM, I'm dead tired, and was supposed to go to the gym tomorrow morning, but instead of sleeping I'm sitting up with a squirming 14 month old who keeps farting himself awake because, despite my best efforts to feed him a balanced diet, he eats nothing but bread and dairy. So, I'd say my biggest contributing factor to weight gain is, "insomnia-induced lethargy caused by baby farts."

Hormones. Specifically your thyroid can dramatically change your weight by Hormones production. In my case my thyroid has almost completely shutdown. I could just eat a carrot every day and still gain weight without synthetic hormones. I believe it can sometimes go the other way make it hard to gain weight. I strongly encourage having hormone levels checked and balanced. I wish I'd known about this 20 or 30 years ago.

Can only speak to my own experience but mainly, it was having kids. I went from working four nights a week with plenty of time to eat properly, exercise plenty and ensure my mental health was doing okay.

Suddenly I've got two young kids (5 & 2) and I've had to switch to a 45hr a week day job that's got significantly less walking than my previous one. I don't see my partner (she works when I'm on my weekends) and so we sort of just eat whenever the kids go to sleep, which usually means we eat poorly, or a takeaway. And on top of that my mental health is in the toilet so I find it really hard to even want to take care of myself at times. Why spend an hour of my very limited free time going for a run or cooking a healthy meal when I could sit down, order food and play videogames before going to bed to begin another day if bullshit.

You have less time and more commitments. You move a lot less. You get comfortable with your job, and life, and slowly lose muscle while gaining fat. With every pound of muscle you lose, it's easier to gain more fat. It becomes a vicious cycle. Add old injuries into the mix which make it easier to be sedentary, and you're in trouble. The solution is simple, move more than you do now. I kept hiking, and rock climbing all throughout my 30's and now 40's, and forced myself to stay fit. I also lift weights. As a result, I'm not fat, and neither are any of the friends in my hiking group. But I have other friends who just work, then go home and drink beer, and the pounds have crept upwards over the decades and now they are very fat. Don't let that be you. Keep moving!

Gaining weight means you're eating too much food.

The less you move your body, the less your body will be able to move. Do some weight lifting and stretches. You don't need to be going for a physique, just build/maintain some strength.

And time makes it difficult.

I don't really keep track of it (I am a mostly-stable skinnyfat), but the most recent thing is the local trail closed for maintenance (after I bought a cheap+geared ebike and rode 131 miles) 6 months ago. Just the section I'm near, blocking the village I'm in off from both directions and no clear set end date. Feels like some real Truman show shit. Before that happened I was at least doing something and did notice bike riding became significantly easier after the longest trip I made, probably gone now.

I also have untreated health issues (that is not new, and it's mostly not things that are likely to be fixed easily/at all except maybe some of the symptoms after long deliberation/trial-and-error) such as not sweating enough to stay cool, POTS (heart rate easily climbs, putting me out-of breath), lack of energy etc. (I'm also in a semi-rural area, shut-in)

Also poor sleep and probably cortisol, and I need to eat more vegetables though that varies a lot. And yeah I often eat too much (some of it is garbage frozen/canned stuff) and even occasional carbs/sugars probably stay for a while since I'm not burning them. I'd go for meal replacement for at least some meals if it wasn't so expensive (also online purchases/shipping), I liked the flavor of original Soylent when I tried it a while ago (I tried DIY powder for lower cost, I wasn't good at it).

Pastries. And it takes like 1hr of gym to deal with one delicious croissant.

For anyone in the US, you can ask your primary care doctor for a referral to a Registered Dietician if you are having issues with excess body weight, limited muscle gain, or other diet related health concerns.

Body weight really is all about calories in vs calories out regardless of age. Eating bulky vegetables is often recommended because they are filling without being calorie dense (as well as the vitamins). Low intensity activity (standing, walking, gardening) is associated with the largest decreases in body weight because it does use some calories but doesn't increase appetite as much as high intensity exercise. Although some exercise is still good to keep up muscle mass and heart health. Yes, as you age it can be more difficult to create/maintain muscle mass as your biology changes, but there are always health benefits to eating healthy and staying active.

I really like the Nutrition Source for science-based nutrition information in case anyone is interested!

I'd guess it's mostly genes. And not that it's pre-determined by genes and there's nothing you can do about it but for some people it simply just is easier to stay lean with little effort compared to someone else. I don't eat particularly "healthy" but I eat a lot at once and then I might go the rest of the day without eating. I regularly skip meals aswell. When you add in the regular gym workouts I've been doing for over a decade plus weekly mountain biking trips I guess it's no wonder I've still got that "swimmer body" in my thirties. Also I never really "got into shape" either - I never just got out of it.

The smallest brisket in the supermarket weighs at least 4kg.