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Fat

The impact of fatness / weight / distribution of body fat

For adults, the World Health Organisation (WHO) defines weight categories by Body Mass Index (BMI) as follows:

  • Underweight is a BMI less than 18.5
  • Normal weight is a BMI greater than or equal to 18.5 and less than 25
  • Overweight is a BMI greater than or equal to 25 and less than 30
  • Obese is a BMI greater than or equal to 30

      • However take a look at the information from research studies on the rest of this page if you are interested in assessing your own risks.


BMI is generally considered appropriate for population studies but inappropriate in isolation for individual evaluation therefore to better understand the potential health impacts of different degrees of fatness it is informative to look at large population based research studies which consider other measures of fatness as well as BMI. Key extracts from such studies follow on the rest of this page. Considering other measures of fatness will enable you to understand if you have an appropriate amount of body fat mass consistent with not having excess visceral abdominal (belly) fat and to gain an insight into the risks of excess fat.

See the following diagram for an an illustration of the important role that decreases in waist circumference have for linking improvements in lifestyle behaviours with downstream reductions in the risk of morbidity and mortality followed by an overview diagram that illustrates the potential role of functional and dysfunctional adipose tissue contributing to increased cardiometabolic risk. This information is from a Consensus Statement from the International Atherosclerosis Society (IAS) and International Chair on Cardiometabolic Risk (ICCR) Working Group on Visceral Obesity that advocates waist circumference as a vital sign in clinical practice and details that the benefits associated with reductions in waist circumference might be observed with or without a change in BMI. Source:




                • The ability of subcutaneous adipose tissue (SAT) to expand through hyperplasia (generation of new fat cells) allows the safe storage of the excess energy from the diet into a properly expanding subcutaneous ‘metabolic sink’. When this process becomes saturated or in situations where adipose tissue has a limited ability to expand, there is a spillover of the excess energy, which must be stored in visceral adipose tissue as well as in normally lean organs such as the skeletal muscle, the liver, the pancreas and the heart, a process described as ectopic fat deposition. Visceral adiposity is associated with a hyperlipolytic state resistant to the effect of insulin along with an altered secretion of adipokines including inflammatory cytokines whereas a set of metabolic dysfunctions are specifically associated with increased skeletal muscle, liver, pancreas, and epicardial, pericardial and intra-myocardial fat. FFA, free fatty acid.



  • White individual waist circumference thresholds
                                                                    Waist circumference (cm) [a]
            BMI category (kg/m2)                     Women Men
            Normal weight (18.5–24.9)              ≥80         ≥90
            Overweight (25–29.9)                      ≥90         ≥100
            Obese I (30–34.9 )                      ≥105         ≥110
            Obese II and III (≥35 )                      ≥115         ≥125

    • Waist circumference thresholds stratified by BMI for white individuals; individuals with measurements higher than these values have a high risk of future coronary events (based on 10-year risk of coronary events or the presence of diabetes mellitus). [a] Waist circumference threshold indicating increased health risk within each BMI category.

  • Ethnicity-specific individual waist circumference thresholds (not considered in association with BMI)
                                                                        Waist circumference (cm) [a]
                                                                        Women        Men
            Japanese[b]                                          ≥90              ≥85
            Jordanian                                             ≥96              ≥98
            Chinese                                                ≥80              ≥80
            Korean                                                 ≥85              ≥90
            Tunisian                                               ≥85              ≥85
            Iranian                                                  ≥91              ≥89
            Asian Indian                                         ≥80              ≥90

    • [a] Waist circumference values for adults above which cardiometabolic risk is elevated. [b] Japanese waist circumference values are thresholds above which visceral adipose tissue volume is >100 cm3.


Central fatness and risk of all cause mortality (from a systematic review and dose-response meta-analysis of 72 prospective cohort published in 2020)

  • Conclusions: Indices of central fatness including waist circumference, waist-to-hip ratio, waist-to-height ratio, waist-to-thigh ratio, body adiposity index, and A body shape index, independent of overall adiposity, were positively and significantly associated with a higher all cause mortality risk. Larger hip circumference and thigh circumference were associated with a lower risk. The results suggest that measures of central adiposity could be used with body mass index as a supplementary approach to determine the risk of premature death.

    • Dose-response association of waist-to-hip ratio with risk of all cause mortality. Solid line represents non-linear dose response and dotted lines represent 95% confidence interval. Circles represent hazard ratio point estimates for adiposity categories from each study with circle size proportional to inverse of standard error. Small vertical black lines represent baseline adiposity category for each separate study.
    • For an online waist-to-hip ratio calculator see the following link:



    • Dose-response association of hip circumference with risk of all cause mortality. Solid line represents non-linear dose response and dotted lines represent 95% confidence interval. Circles represent hazard ratio point estimates for adiposity categories from each study with circle size proportional to inverse of standard error. Small vertical black lines represent baseline adiposity category for each separate study.



    • Dose-response association of waist circumference with risk of all cause mortality in women. Solid line represents non-linear dose response and dotted lines represent 95% confidence interval. Circles represent hazard ratio point estimates for adiposity categories from each study with circle size proportional to inverse of standard error. Small vertical black lines represent baseline adiposity category for each separate study.



    • Dose-response association of waist circumference with risk of all cause mortality in men. Solid line represents non-linear dose response and dotted lines represent 95% confidence interval. Circles represent hazard ratio point estimates for adiposity categories from each study with circle size proportional to inverse of standard error. Small vertical black lines represent baseline adiposity category for each separate study.
    • For a video about ideal waist size see the following link:



    • Dose-response association of waist-to-height ratio with risk of all-cause mortality. Solid line represents non-linear dose response and dotted lines represent 95% confidence interval. Circles represent hazard ratio point estimates for adiposity categories from each study with circle size proportional to inverse of standard error. Small vertical black lines represent baseline adiposity category for each separate study.
    • For an online weight-to-height ratio calculator see the following link:



    • Dose-response association of body adiposity index with the risk of all cause mortality (Pnon-linearity<0.001, n=4). Solid line represents the non-linear dose response and dotted lines represent 95% confidence interval. Circles represent the hazard ratio point estimates for adiposity categories from each study with circle size proportional to inverse of standard error. Small vertical black lines represent baseline adiposity category for each separate study.
    • For an online body adiposity index calculator see the following link:



    • Dose-response association of A body shape index with risk of all cause mortality (Pnon-linearity<0.001, n=6). Solid line represents non-linear dose response and dotted lines represent 95% confidence interval. Circles represent the hazard ratio point estimates for adiposity categories from each study with circle size proportional to inverse of standard error. Small vertical black lines represent baseline adiposity category for each separate study.
    • For an online A body shape index calculator see the following link:



Association between body mass index and all-cause mortality among never-smokers (from a population-based cohort study of 3·6 million adults in the UK published in 2018)





    • Association between Body Mass Index (BMI) and all-cause mortality among never-smokers, by sex (A) and age (B). The research models were also adjusted (in addition to age, sex and smoking status) for alcohol use, diabetes, index of multiple deprivation but not for fitness (additional grid lines added to the original to make the charts easier to read and green shading added to show the "normal" weight BMI range which rather questions the appropriateness of the lower end of the "normal" weight BMI range).



Body-mass index and all-cause mortality (from an individual-participant-data meta-analysis of 239 prospective studies in four continents published in 2016)

    • Association of body-mass index with all-cause mortality, by geographical region. In these prespecified analyses of almost 4 million adults, the HRs for overweight and for obesity grade 1 were broadly similar across different geographical regions (Europe, North America, east Asia, and Australia and New Zealand; numbers of deaths in south Asia were too small to be reliable), but the HRs for underweight and grade 3 obesity appeared somewhat higher in Europe than in east Asia.


    • Association of body-mass index with all-cause mortality, by baseline age group. Combining all regions, the HRs for overweight and obesity were higher at younger ages than older ages, and in men than women; this finding held in each major geographical region. In each region, BMI was non-linearly associated with all-cause mortality, with nadir at BMI 20·0 kg/m2 to less than 25·0 kg/m2 and excess mortality in underweight, overweight, and at BMI 18·5 kg/m2 to less than 20·0 kg/m2, at the lower end of the WHO-defined normal range. The nadir depended on age, and was BMI=22 kg/m2 for baseline age 35–49 years, BMI=23 kg/m2 for baseline age 50–69 years, and BMI=24 kg/m2 for baseline age 70–89 years.


    • Association of body-mass index with all-cause mortality, by sex.


    • Association of body-mass index with mortality, by major underlying cause. For each major cause of death, BMI was non-linearly associated with mortality in each major region we studied. Above 25 kg/m2, BMI was strongly positively related to coronary heart disease, stroke, and respiratory disease mortality, and moderately positively related to cancer mortality; these findings were broadly similar in Europe, North America, and east Asia. Within WHO's wide normal BMI range (18·5–<25·0 kg/m2) the main geographical difference was that, in east Asia, mortality from coronary heart disease had its nadir at 18·5–<20·0 kg/m2, lower than in other regions. In all regions, underweight was associated with substantially higher respiratory disease mortality and somewhat higher mortality from coronary heart disease, stroke, and cancer. HRs comparing underweight versus normal-weight cardiovascular disease mortality were more extreme in Europe than elsewhere.



BMI and all cause mortality (a systematic review and non-linear dose-response meta-analysis of 230 cohort studies with 3.74 million deaths among 30.3 million participants published in 2016)

  • Conclusion: Overweight and obesity is associated with increased risk of all cause mortality and the nadir of the curve was observed at BMI 23-24 among never smokers, 22-23 among healthy never smokers, and 20-22 with longer durations of follow-up. The increased risk of mortality observed in underweight people could at least partly be caused by residual confounding from prediagnostic disease. Lack of exclusion of ever smokers, people with prevalent and preclinical disease, and early follow-up could bias the results towards a more U shaped association.

    • Non-linear dose-response analysis of BMI and all cause mortality among never smokers, healthy never smokers, all participants, current, former, and ever smokers.



Relation of body fat mass and fat-free mass to total mortality

  • Conclusions: Fat mass and fat-free mass show opposing associations with mortality. Excess fat mass is related to increased mortality risk, whereas fat-free mass protects against risk of mortality. These findings suggest that body composition provides important prognostic information on an individual’s mortality risk not provided by traditional proxies of adiposity such as BMI.
                • Linear and spline functions with corresponding 95% CIs from Cox proportional hazards regression for the relations of fat mass index (kg/m2), fat-free mass index (kg/m2), and BMI (kg/m2) to total mortality in participants <65 y (n = 14,087; 95% CI with shaded lines) and ≥65 y (n = 2068; 95% CI with gray filling).




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