The appeal of fast loss
Aggressive calorie deficits produce faster results on the scale in the short term, which is part of why extreme approaches remain popular despite consistent research cautions against them.
What the research shows about durability
Extreme, rapid weight loss -- as seen in research on reality-TV-style competitions -- is associated with larger and more persistent metabolic slowdown than gradual approaches, potentially making the lost weight harder to keep off.
Lean mass is the other cost
Faster weight loss, especially without adequate protein intake and resistance training, carries a higher risk of losing muscle alongside fat -- which further reduces resting metabolic rate and can make maintenance harder.
What "moderate but durable" looks like
A pace of roughly 0.5-1% of body weight per week, paired with adequate protein and resistance training, is commonly cited as balancing meaningful progress against muscle preservation and lower metabolic-adaptation severity -- trading some short-term speed for a plan more likely to hold over years.