Weight lost only on non-exercise days - eh?

In recent times I have struggled to lose any lbs, even when in theory, with a net difference of over a 1000cal-per-day, I should be able to lose 2lb-per-week with no difficulty.

That certainly occured early-on in my diet, but those days appear to have gone.

I started at 252lb and the target is 196lb. I'm halfway. The first stone (14lb) was a doddle, the second one extraordinarily hard. MFR has set a profile for 1400cal-per-day plus exercise, which used to be tough, but I now find easy to achieve. Indeed I find I can easily live on no-exercise days on 1400 calories and couldn't imagine ever being able to eat the 2500 cal that allegedly an adult male requires to maintain weight.

Exercise is entirely cardio - and comprises swimming 50 lengths, four-times-per-week, mostly front crawl, in a race with a swim partner, and 30-45 minutes fast walking 5-days-a-week. Rather than over-estimate, I've valued that at 500 cals worth, as the effort required has decreased as I get fitter, but not lighter. MFR provides a figure of over 800 calories burnt for this.

In the past I always ate back exercise calories, but find now I can consume day 1500-1700 cals and swim 50 lengths at high speed with no detrimental impact - which of course is supposely impossible. Doing so sees no weight loss, day-on-day-out, when actually exercising. I'm certainly not in starvation mode as I wouldn't have the energy to keep up the exercise.

I've tried to figure-out how this can be so, and decided it is a combination of dieting - seeing carbohydrates are burnt more efficiently (there's no scientific evidence for this), mixed with a slow respiration (I have a nurse-checked at-rest-pulse of 52 bpm & when relaxed only respire 4-5 times-a-minute). Again a slow respiration has no connection with preventing weight loss.

What I am finding though is that as soon as I stop the exercise - say after maintaining that regime for 3 or 4 days, I do lose weight. I try not to weigh myself everyday, but the first day after a cycle of exercise, even on an unintended 'pig out' day (say a restaurant visit with friends), I will lose weight, sometimes even 2lb in a day, but only for one day.

If I stop exercising and just consume 1400cal (I can't do less) then weight-loss just flatlines after that first day, seemingly permanently. In essence I appear to be able to live quite happily on 1400 calories without any fear of weight loss, down from the (myth?) of 2500 calories previously required.

If I keep up the exercise days I find I need to increase consumption to 1600-1700 calories (still with a net difference of -1500 cal-per-day over what I should be consuming) but weight-loss plateaus again. It has been months since I consumed anything near 2500 cal in a day (even on a 'pig out') so in theory I should always be losing weight. Not happening. I never gain weight though.

A weekly graph with calorie consumption would show exercise days at no more than 1500-1700 cal consumed, with weight-loss flat-lined, and two other lines with calorie consumption at 1400cal and an initial weight-loss followed by a plateau.

Without exercise I will consume 9,800 cal-per-week, a shortfall of 7,700 cal - enough for a loss of 2lb-per-week, and I've not achieved less that 7,700 for 4 months. Adding exercise of 4 x 500 cal (and not counting the other walking days) sees a shortfall of over 8,800 per-week; enough to lose 2.5lb-per-week. Doesn't happen. And that's the minimum; some weeks I hit a shortfall of over 12,000 calories and lost (sometimes) half-a-pound, and never more than 2lb when I stopped the exercise. That shouldn't be possible.

I've researched this subject but can't find anything that addresses or explains it, other than the frequent comments from many that they 'diet but don't lose weight'. I never used to believe such tales, but now I do, utterly.

Other than the annoyance of not losing weight I feel fine. No loss of energy and fitter than ever before. Yet I consume a minimum of -1100 calories-per-day under the amount a non-dieter would be expecting. How can this be?