Every popular diet—low carb, low fat, intermittent fasting, plant based—works for fat loss to the extent that it helps you eat less than you use, consistently. That is the uncomfortable, unexciting core of it. What differs between approaches is mostly how easy they make that for a particular person.
None of that means the body is a simple calculator. It is worth understanding where the simple model holds and where it gets fuzzy.
Maintenance is an estimate
Online calculators estimate your maintenance calories from weight, height, age, sex and an activity level you choose. They are reasonable starting points, but two people with the same numbers can differ by a few hundred calories a day, and the activity multiplier is mostly guesswork.
Treat the estimate as a hypothesis. Eat around it for two to three weeks, watch your weekly average weight, and adjust. Your own trend is better data than any formula.
The part people underestimate: everyday movement
Formal exercise is usually a smaller share of daily energy use than people expect. A larger and more variable share comes from non-exercise activity—walking, standing, fidgeting, housework, taking the stairs. It differs a lot between people and between days.
When you diet, this activity tends to drop without you noticing. You sit a little more and move a little less. A step count is a simple way to keep an eye on it: if yours falls from 9,000 to 5,000 a day during a diet, that alone can absorb a meaningful part of your deficit.
Adaptation is real, but modest
As you lose weight you need less energy, partly because a smaller body costs less to run and move. Beyond that, the body does make some additional adjustments during a prolonged deficit. For most people these are modest, and they are not a reason fat loss becomes impossible.
That is why a plan that worked at 200 lb (91 kg) often needs adjusting at 185 lb (84 kg). Nothing is broken; the numbers have simply changed.
Why 'my metabolism is broken' is rarely true
When weight stops moving, the most common causes are ordinary: portions creep up, weekends look different from weekdays, cooking oils and drinks go uncounted, daily movement falls, or water retention is masking fat loss. None of these is a character flaw; they happen to almost everyone.
Some medical conditions and medications do affect weight and appetite. If you suspect that, or you have unexplained changes in weight, energy or appetite, speak to your doctor rather than cutting food further.
What to do this week
Pick a starting calorie estimate and track your weekly average weight and your daily steps. Give it at least two weeks before changing anything, and when you do change something, change one thing by a small amount.
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