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I started jogging about 18 months ago. My intention wasn't to lose weight but to get some kind of regular activity in for general fitness now that I work mostly from home. However I knew that my weight was the top end of "healthy" so I figured losing a bit would be a good side effect.

I only check my weight rarely but to my surprise it has simply gone consistently up since I began running. At first I put this down to increased muscle mass over fat. But it just kept goikg up and I've now put on about 6.5kg in those 18 months.

My running is not particularly arduous. I do 5k, on average twice a week. My usual route does include some climbs but nothing really vicious and I can now run it in about 28 minutes (I'm almost 50). I can do it im 26 minutes on the flat.

All my clothes still fit and I haven't noticed my waistline expanding. Indeed I've got the occasional complement from people that I'm "looking well". Is it really possible I've put on all those kilos purely from muscle, or am I also likely gaining fat somewhere? How can I tell?

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Any increased level of activity will build some level of muscle in order to better support the new activity. Hill running specifically can build better running as your legs need to be able to push your weight up the hill with more explosive force.

Having said that, all of the 6.5kg is probably not muscle.

Another adaptation is your muscles learn to hold more glycogen. Glycogen is a fuel source necessary for producing fast energy, and the more you have the more prolonged your exercise can lead.

Glycogen holds water. So more glycogen means more overall water retention in the body.

Those could potentially account for a large portion of the 6.5kg increase in 18 months. Especially if the weight gain was higher in the first 9 months than it was in the last 9 months. It's hard to tell. Everyone is different.

Beyond that, it's possible to out-eat the activity level you're producing. Increased activity can lead to increased appetite, and it's common for people to overeat as a result. The weight gain appears to be relatively slow, but it's something to keep an eye on.

It's really difficult to tell what-is-what without using some special machines. You probably don't have anything to worry about at the moment if you don't see any physical changes. So just keep doing what you're doing. The muscle mass and glycogen adaptations should settle down, and your weight should stay fairly consistent. They could start trending up again if you increased your activity even more. The rate of fat gain would also slow down as your caloric demands increase as your weight goes up.

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    13 lbs in that time frame is more than would be expected from muscle/glycogen. I suspect your out eating analysis is the culprit. Or there is other activity that is not outlined.
    – JohnP
    Apr 21, 2023 at 14:02
  • Yes. I would say there was some fat gain. Though it sounds like from the OP's description that it has been pretty small so far.
    – DeeV
    Apr 21, 2023 at 17:43
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It doesnt sound like you are running enough to lose weight, especially if you have been running the same weekly amount for 18 months. You may just be running enough to keep your weight gain moderate. On a 5km run you wont burn many calories, roughly 300, that’s not many compared to how easy it is to consume 300 calories. Running to lose weight doesn’t typically come with the caveat that you’ll have to put on weight for two years while you gain muscle and then you start losing the weight you wanted to lose.

If you are interested in finding out what is going on then you could try a number of approaches:

  • Stop running for a month and measure your weight at the start and end. See if it increases by more than your current trend.
  • Increase your weekly running total. Depending on how much you increase your total you may just slow the rate or reverse the rate altogether. It may take a couple of months to notice any difference.
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  • Thanks for this. As I said my goal wasn't to lose weight in the first place so that's fine: I'm just surprised I could have put on that much weight from muscle alone. My running app says that at my pace and with hill I'm burning about 400 kcal per run but still, as you say, that's not much in terms of what I'd normally eat in a day.
    – Bob Tway
    Apr 20, 2023 at 13:48
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Have you measured your weight before and after eating on multiple occasions? If it were me, I would measure my waist and perhaps take the British Heart Foundation's advice: "Calipers are the cheapest, easiest and most portable method to measure body fat in specific areas. Using at least three spots on your body – chest, abs and thigh are often used – pinch the skin, pulling the muscle away from the fat and measure the fold with the calipers. Always test on the same side."

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