Does Exercise slow down metabolism?

[Branc93]

I'm 5"7, 27 year old male, 119kg / 265 pounds (obese). 

Now if I sleep and sit most of the day my fitbit versa records around 2,700-2,800 calories burned in a day. I'm eating around 1600 calories a day and I've lost almost 16kg in 9 weeks. That's a 1,100-1,200 caloric deficit according to my fitbit, fair enough. The problem is if I incorporate exercise in my daily routine (5-6 days a week lately).  It's not uncommon for me to reach a 3,500 calories burned in some days... Basically some days I do 1hr moderate stationary bike which my fitbit says burns around 850 Cals and some other days I do 30 min HIIT workout which m fitbit says burns around 450 Cals give or take. 

What can I do to avoid damaging / slowing down my metabolism? I'm afraid to eat more even if it's 100-200 cals per day and to be honest I feel pretty satisfied food and drink wise with 1600 Kcal in a day. 

[shipo]

Easy answer, no, exercise does not slow your metabolism down, quite the opposite actually, it speeds it up.

On the other hand, eating so few calories per day is a sure fire way to slow your metabolism down.  Here's the thing, while I'm well over twice your age, I'm about your same height and was about your same weight as recently as last December (not to mention a few other times in my life), and, at least for me, it took years to gain that weight; while we can lose it quicker than we gained it, losing it too quickly can be counter productive (i.e. slowing your metabolism down).  Speaking strictly from personal experience, I would lengthen your weight loss timeframe and maybe halve your daily calorie deficit.  Once you start shedding pounds and gaining strength and endurance, you can increase your workouts AND your calorie intake, and still lose weight.

In my case, broken legs (I've had three now) are frequently the precursor for weight gain, each time I've been able to eventually shrug off the debilitating effects of the injuries and start running (my exercise of choice), and once that ball gets rolling, the weight starts falling off.  True story, back in 2013 (about 10 years after my second broken leg) I was "260 something" (never was brave enough to find out what that something was) when a job change allowed me enough time in my daily schedule to start working out again.  I started running in April of that year, a whopping quarter of a mile at a time; then a half mile, then a mile...  When I'm getting back in shape I practice "LSD" (Long Slow Distance), and by August I was running six to eight miles at a crack, four or five days per week; also in August I attended a dinner with about a dozen of my former co-workers (from the company I left in March), and when I walked into the restaurant, a group of them walked right past me and didn't recognize me.

As with many things in life, sustainable weight loss is a marathon, not a sprint.  Take it slow and steady and you'll not only take it off, but you'll be better positioned to keep it off.  Good luck in your journey and keep us posted.

[Heybales]

Ditto's to Shipo.

Now - to the estimated Fitbit calorie burn for the exercise.

HIIT workouts (which really aren't HIIT but that's another topic) are interval by nature, perhaps even anaerobic.

All HRM's use a calorie burn calculation that is ONLY a best estimate (and even that can be ruined) for steady-state aerobic exercise.

Intervals is opposite of steady-state, and anaerobic ain't aerobic.

The calorie burns are inflated, easily by 100% if still getting fit.

The bike could be better fit unless you are doing Spin classes that are again interval, then inflated.

But that calorie burn if true would mean you avg 236 watts for the whole 60 min - highly unlikely.

So while it's true a bigger deficit that the body finds unreasonable WILL cause it to slow down many body functions and movement to conserve calories and lessen the amount of deficit, you aren't making it as unreasonable as you might think.

But you are increasing the deficit on exercise that will do nothing to help retain muscle mass - you are losing some already - you'll lose more with bigger deficit. You'll regret that now and later.

Especially if this extreme diet causes the normal 80% failure to reach and maintain goal weight, and you have to do it again next year. With less muscle mass even harder.

[emili]

Nine weeks is a long time to go without an increase in calories. Doing it for a short period of time, isn't going to do anything to your metabolism. However the longer you go, the more likely you will. I also agree about muscle loss. Right now you are young and it doesn't seem like a big deal. But the older you get, the more difficult everything becomes. I would reevaluate what your are doing. Slowing down weight loss a bit isn't a terrible thing. It is actually a good thing. It increases your odds for long term success. Just think about it...