Why am I gaining weight on a diet?
Last updated September 3, 2026.
Gaining on a diet usually traces to portion drift, weekend compensation, liquid calories, water shifts masking fat loss, or an overly aggressive cut backfiring; medical causes (thyroid, medications, PCOS) are the rule-out. The audit that finds it: track everything, including weekends, drinks, and cooking oil, for 2 weeks. Scale noise deceives: new exercise inflames muscle and stores water for weeks, and daily weigh-ins chase water, so weigh weekly under identical conditions and watch waist and clothing fit as parallel signals. Crash approaches backfire because the body defends: you move less, hunger hormones rise; the CDC's 1-to-2-pounds-a-week gradual pace beats it and keeps the loss off. If honest tracking still shows gain after a month, take it to a clinician: thyroid function, medication review (steroids, some antidepressants, beta blockers), PCOS screening, sleep, and perimenopause belong on that list.
What to do
- Track 2 weeks honestly: weekends, drinks, and oil included.
- Weigh weekly, same conditions: daily weight is mostly water.
- Gradual beats crash: 1-2 pounds a week sticks better.
- Watch the waistband, not just the scale.
- Still gaining after a month: thyroid and med review with a doctor.
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