Optimal BMI for Longevity

Consider this the disclaimer that BMI isn't the be-all-and-end-all of health.

Conventional Wisdom

Conventional wisdom divides the BMI range into four categorires:

< 18.5Underweight
18.5 - 25Healthy
25 - 30Overweight
> 30Obese

I'll refer to these throughout but in quotes (e.g. "healthy") to indicates that I'm using these conventional thresholds.

Caloric Restriction

Long-term caloric restriction seems, to me, equivalent to reduced weight and BMI. Unfortunately, most of the studies are either in animals or focus on overweight people. The one study I found that included "healthy" BMI individuals only examined one metric on that group split away from the overweight group. That metrics was a depression questionnaire and they found improvements among the overweight group but regressions among the "healthy" weight group Martin.

Other than that disclaimer, the RCTs are generally positive. 3 of the 4 RCTs in monkeys found mortality improvements, with the 1 of 4 finding no effect. Human trials haven't been run with enough people or for enough years to find mortality differences, but generally find improvements on various health metrics for their (typically overweight) subjects.

In short, caloric restriction RCTs in monkeys and humans provides some evidence that, when able to eat whatever they want, both monkeys and humans consume more food than is optimal. There is weak evidence that the health benefits reverse or a neutralized in the "healthy BMI range. the evidence we have from the caloric restriction RCTs is extremely weak but supports the general conclusion.

Longitudinal Analysis

Many longitudinal studies find that people with "overweight" (or even "obese"!) BMIs live longer than people in the "healthy" range. However, there are obvious problems with this evidence: it's based on correlations rather than causation.

You might be tempted to say that we can use it bound the optimal BMI, but the issue is there are strong confounders in both directions. On the one hand, we can posit that a lower BMI is a proxy for caring about health, in which case, we'd expect correlations to overestimate underestimate the optimal BMI. On the other hand, when people near death, their health problems often cause weight loss; likewise, smokers tend to have lower weight Fontana.

Still, after controlling for some of these factors, optimal BMI estimates revert to the healthy range: 22.5 - 25. Measures of morbidity (rather than mortality) correlate positively with BMI even within the healthy range.

That being said, the review admits the optimal BMI may be higher among the critically ill population for reasons that aren't fully known.

The review notes that a potential causal mechanism is that fat cells release hormones that cause inflammation, which is known to... TODO

Observational data from healthy individuals practicing long-term DR with adequate nutrition (at least 100% of the RDI for each nutrient) show that even in these lean participants (mean BMI 19.6 kg m−2; ~12% body fat), DR induces beneficial changes against aging-related pathologies. In these individuals, DR caused rapid and profound improvements in all the major risk factors for CVD.

They end up concluding:

There is ample convincing evidence that overweight and obesity exert adverse influences on health and life expectancy to justify public health campaigns to prevent excessive weight gain and to promote the maintenance of a healthy weight throughout the lifespan.

Martin, C. K., Bhapkar, M., Pittas, A. G., Pieper, C. F., Das, S. K., Williamson, D. A., ... & Stewart, T. (2016). Effect of calorie restriction on mood, quality of life, sleep, and sexual function in healthy nonobese adults: the CALERIE 2 randomized clinical trial. JAMA internal medicine, 176(6), 743-752. https://doi.org/10.1001/jamainternmed.2016.1189 Fontana, L., & Hu, F. B. (2014). Optimal body weight for health and longevity: bridging basic, clinical, and population research. Aging cell, 13(3), 391-400. https://doi.org/10.1111/acel.12207