The magic of a table or two is we can fit extra biomarkers people want even if there isn’t space in the main text. Also, where strengths or weaknesses are shared, it will cut down on words.
I’d define a biomarker as a measurable phenotype that correlates positively or negatively with a specific biological function or failure thereof. So both terms of risk factors and health factors. I would define a “good” biomarkers as having the following characteristics: 1) Robust dynamic range of predictive power, 2) Strong predictive power, 3) Strong correlation with the biological function, 4) Easy to measure in humans and 5) Low cost to measure.
A robust dynamic range means there is a big difference with minimal overlap between a positive result and a negative result. Consider the difference between two populations, on a scale of 1-200. In the first scale, the populations have an average of 38 and 40, respectively. With a giant population size, there could be a correlation with longevity there. But it sucks, as there’s likely tons of overlap. Better to have the average be 30 vs 100. A second aspect of dynamic range is the age range over which the biomarker is useful.
A strong predictive power means the biomarker works well. To work well, the biomarker is hard to manipulate without also altering the phenotype (ie longevity). For example, grip strength. If grip strength is a proxy for general sarcopenia, training only grip strength might improve the biomarker without improving lifespan. Also, predictive power includes the odds ratio. For example, if people with the biomarker are 1.2x more likely to die sooner, that’s not very strong. If they’re 10x more likely, way more useful. Instead of odds-ratios, some might use age-adjusted years, which I think Savva calls QALYs.
However, a strong correlation is also needed. If that 10x difference has a p value (likelihood of being false on 0-1 scale) of 0.06 vs the 1.2x having a p value of 0.00001, that means the 10x might be less reliable. Similarly, if the correlation coefficient is lower, it may not be as useful.
Easy to measure means biomarkers should be assays that are quantifiable, have minimal operator-bias, and can be performed with minimal training. Things like blood draws and standard assays, VO2max, and grip strength vs MRIs, cognition tests, spinal taps.
Cost includes dollar amount per test, specialized equipment needed for the test, time to perform, and time to analyze. If a blood test performs equally to an MRI, the blood test is superior.
I think for this review, discussion of interventions should be avoided when possible. Instead, the focus is on ‘how well and under what circumstances do these biomarkers correlate with longevity, and what are the weaknesses of these biomarkers?’