How to healthily lose a lot of weight without messing my metabolism?

Azdak

Any notable loss of weight will have an effect on “metabolism” (it’s really more an effect on TDEE, but we’ll use the shorthand for now).

If you lose weight, it’s going to happen, so I would not let it dictate your approach. At your weight, there is no reason to starve yourself. If you are following an exercise program (which includes strength training), have your macros straight, there is no reason why you can’t lose steadily on 1600-1800 cals per day (sometimes a little more). Since you seem to be comfortable at that level, that should work. At your weight, you can handle a higher daily deficit without adverse effects.

Worrying about what will happen somewhere down the road when you lose 100+ pounds is kind of putting the cart before the horse. When you get there, your body will be different, your physical abilities will be different. You will have different options.

While a lowered “metabolism” is a likely part of any substantial weight loss (at least temporarily), most people don’t regain weight due to a “damaged metabolism”—unless they have lost substantial lean mass. They regain weight because they go back to their old habits.

AnnPT77

I think I understand where you're coming from, and your doctor too, about being cautious about exercise initially, while you are still quite obese and having health issues from that.

As you are able to work in some exercise safely, I think that will be helpful to your goals . . . not only because you can eat back some exercise calories for higher satisfaction with eating, but also as a strategy to possibly limit the "metabolic slowdown" you're concerned about.

Strength exercise may be particularly helpful, certainly to preserve the muscle you have, but also - since you're new to exercise, and still overweight - you may be among the small subgroup that can hope to gain a little muscle even in a moderate calorie deficit. You might find strengthening exercise more doable sooner than exercise involving impact (like walking), if that is part of your doctor's concern about your exercising now. Definitely check in with him or her about what it's safe to do when.

Sometimes water exercise is encouraged as a starting point while still obese, as the water's buoyancy and support can be helpful. If you have access to a pool, this could be another possibility to ask your doctor about. Here (USA), a few gyms even offer special pool classes for obese beginner exercisers.

This (link below) is a blog post that helped me understand a little more about the effect of weight loss on calorie needs, in part just by clearly explaining some of the terminology and issues.

http://www.myfitnesspal.com/blog/heybales/view/reduced-metabolism-tdee-beyond-expected-from-weight-loss-616251

It also discusses a study that compared several weight loss strategies in terms of their effect on post-weight-loss calorie needs. I would say it was too short/simple/small to be definitive, but there are interesting aspects that influenced my thinking about what to do myself - strategies that I thought might help, and probably wouldn't hurt me.

I think others have given you wonderful advice about losing at a healthy/sustainable rate, using diet breaks, etc., too.

Future healthy-weight you will require fewer calories than current you needs to maintain, of course. But there are many reasons to believe that your future maintenance calories may not be punishingly low, especially with your commitment to take steps now to minimize that risk.

You may even be positively surprised: As a not-tall (5'5") older (62) woman with a narrow build (goal weight 120), sedentary outside of intentional exercise (though I do exercise), I ended up in a better spot than I'd worried I might after weight loss. You may well also!

Best wishes!

Edited: typos