Energy balance

Dramatic weight loss triggers a disproportionate, long-lasting reduction in resting metabolic rate (metabolic adaptation) that actively counteracts weight maintenance, making long-term maintenance of extreme weight loss biologically difficult without sustained environmental support.

If you have lost a significant amount of weight, expect your metabolism to slow down more than predicted by your new size. This is biology, not failure. To maintain your weight, you must sustain the lifestyle changes (diet and activity) that got you there, rather than returning to old habits. Monitor your weight frequently and be prepared to re-engage with your healthy behaviors if you see small gains.