Where your fat sits matters more than what you weigh, and thin people with a gut may be in worse shape than heavier people with a normal waist.

• People at a normal weight but carrying belly fat had 2.42 times the death risk of overweight people whose fat was spread more evenly, in a study tracking over 15,000 men.
• Muscle acts like a suitcase for sugar — with too little of it, sugar gets packed into the liver and around the organs instead, causing inflammation and blood sugar problems.
• A simple check: your waist should be under half your height, so someone 160cm tall wants a waist below 80cm. Britain's NICE guidance adopted this in 2022 and said in 2025 to stop diagnosing on BMI alone.
• Poor sleep piles fat onto the belly fast — healthy young people sleeping four hours a night for two weeks gained barely any weight but added 11% more organ fat.
• The fix is sleep, easy aerobic work you can still talk through two or three times a week, weight training to build muscle, and eating more starch on active days than on sitting-all-day ones.

Outlook: Expect waist-to-height ratio to keep displacing BMI as the standard health screen, which will flag a lot of "normal weight" people as at risk.