Keeping BMI between 20–25; losing weight if overweight

Cost: There is no monetary cost. However, diet and exercise require daily time commitment. This is the most demanding guideline in the whole chapter: maintaining weight loss long-term is harder than losing it in the first place.
In plain terms: Mortality is lowest when BMI falls within 20–25. At BMI 27.5–30, risk rises by roughly 20%; at 30–35, it increases by about 45%; at 35–40, it nearly doubles. East Asian individuals are especially sensitive to excess weight — for every 5-point rise in BMI, risk climbs by roughly 40%.
Benefit: This analysis combined individual data from 239 longitudinal studies (no subgrouping was performed). Only three groups remained in the final dataset: non-smokers, individuals without chronic illnesses at enrollment, and those who survived at least five years after enrollment. Results showed that mortality risk was lowest at BMI 20–25. Between 25–27.5, risk rose by about 7% (HR 1.07); at 27.5–30, it climbed by 20% (HR 1.20); at 30–35, by 45% (HR 1.45); at 35–40, by 94% (HR 1.94); and at 40–60, it was 2.76 times higher (HR 2.76). For East Asian populations, each additional 5 kg/m² of BMI above 25 raised mortality risk by roughly 39% (HR 1.39).
Evidence grade: A
Sources:Global BMI Mortality Collaboration (2016). Body-mass index and all-cause mortality: individual-participant-data meta-analysis of 239 prospective studies in four continents. Lancet. https://doi.org/10.1016/S0140-6736(16)30175-1；Flegal KM 等 (2013). Association of all-cause mortality with overweight and obesity using standard body mass index categories: a systematic review and meta-analysis. JAMA. https://doi.org/10.1001/jama.2012.113905（争议方）
Notes: This topic remains debated. Some researchers argue that being slightly overweight actually extends lifespan; their pooled data indicated a roughly 6% lower mortality risk for overweight individuals and 5% lower risk for those with mild obesity. The main disagreement centers on whether to exclude three specific data groups: smokers, individuals with pre-existing illnesses, and participants in the first few years after enrollment. Exclusion is recommended because severely ill patients often lose weight first. After removing these groups from the analysis of 239 studies, the elevated mortality risk associated with excess weight reappeared. It is also important to note that no randomized trials have proven that weight loss itself reduces overall mortality. The benefit figures above result from cross-sectional comparisons between people of different weights, not from direct proof that losing weight yields such reductions. Those aiming to lose weight need not obsess over meal timing; neither breakfast consumption nor the 16:8 intermittent fasting method offers any extra advantage — see Section 6, Item 26 for details.