One of the most common reasons a diet appears to "stop working" is an inaccurate TDEE estimate rather than a failure of willpower — if the starting maintenance number is wrong, every deficit or surplus calculated from it is wrong too, no matter how disciplined someone is about following the plan. This is why tracking your body weight trend over 2-3 weeks at a consistent, known calorie intake is generally more accurate for finding your personal TDEE than relying on predictive equations alone.

Elite athletes often undergo metabolic testing to measure resting metabolic rate directly, combined with activity monitoring and training-load tracking to estimate total daily energy expenditure more precisely, since even small energy deficits during heavy training can gradually impair performance, recovery, and health.

This is especially important for young athletes and female athletes, though Relative Energy Deficiency in Sport (RED-S) can affect males too. Consistently eating below true energy needs while training hard raises the risk of RED-S, which can impair hormonal function, bone health, immune function, recovery, and performance — and because it develops gradually, it often goes unnoticed until symptoms become more significant.