Keep BMI between 20–25; lose weight if you’re overweight

No monetary cost. However, diet and exercise require daily time and effort. This is the toughest guideline in the whole chapter: maintaining weight loss is harder than losing it in the first place.

Data from 239 longitudinal studies were pooled and analyzed (no subgrouping was performed). Only three groups remained in the final analysis: non‑smokers, individuals without chronic disease at enrollment, and those who survived at least five years after joining the study. Results showed that mortality risk was lowest at BMI 20–25. Between 25–27.5, risk increased by about 7% (HR 1.07); at 27.5–30, by roughly 20% (HR 1.20); at 30–35, by about 45% (HR 1.45); at 35–40, by nearly 94% (HR 1.94); and at 40–60, it was 2.76 times higher (HR 2.76). For East Asians, each additional 5 kg/m² of BMI raised risk by about 39% (HR 1.39).

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）

This topic remains controversial. Some researchers argue that being slightly overweight actually extends lifespan; their pooled data show a roughly 6% lower mortality risk for overweight individuals and a 5% reduction for those with mild obesity. The main disagreement centers on whether to exclude three types of data: smokers, patients already suffering from disease, and participants in the first few years after enrollment. Those groups are excluded because severely ill patients tend to lose weight first. After removing these three categories from the analysis of 239 studies, the elevated mortality risk associated with excess weight reappears. It is also important to note that no randomized trials have proven that weight loss itself lowers overall mortality. The benefits described above result from comparing people of different weights, not from a guarantee that losing weight will produce those exact reductions. Those aiming to lose weight need not obsess over meal timing; neither breakfast nor the 16:8 intermittent fasting regimen offers any extra advantage — see Section 6, Item 26 for details.