Objective metrics that change the most as we age | Empirical Health

Track 100+ biomarkers as you age server-island-start<br>Objective metrics that change the most as we age<br>Brandon Ballinger<br>May 23, 2026

Yeats wrote of an aged man as “a tattered coat upon a stick.” But for most of us, aging comes as small diminishments: the morning that takes a little longer to shake off, the flight of stairs you notice, the cold that lingers a week instead of three days, the run that used to feel easy. We register these shifts as fatigue or a busy season before we register them as time. The body is keeping score long before we are.

Each of these feelings has a number behind it.

The morning that takes longer to shake off lines up with kidney and liver filtration slowing down. Two glasses of wine hit differently at forty than at twenty, and as eGFR drops a few points per decade, the cleanup takes longer.

The flight of stairs you notice shows up as red blood cells slowly enlarging, each one a little less efficient at carrying oxygen.

The cold that lingers a week instead of three days is your lymphocyte counts thinning, the frontline cells that recognize and clear viruses.

The run that used to feel easy tracks with hemoglobin A1c creeping upward, insulin sensitivity stiffening and muscles slower to pull glucose from the blood.

None of these metrics move more than a few percent in a year, but stacked across decades, they describe an aging body.

Which biomarkers rise and fall fastest with age? Which of these can be stopped, slowed, or reversed? In this post we’ll go through them.

The biomarkers that rise and fall the most with age.

How kidney function (eGFR) declines with age

eGFR is the standard measure of how fast your kidneys clear waste from the blood, and it’s the runaway leader on this list. In our data it falls roughly 6 to 7 points per decade after age 20, from a typical 110 mL/min/1.73m² in the twenties to around 75 by the eighties. Its correlation with age (r = -0.51) is nearly twice as strong as the next-best signal on the panel. The drop matches the consensus from the Baltimore Longitudinal Study of Aging, which has tracked the same loss for decades.

The interesting part isn’t the average slope, though. It’s the spread. The Baltimore cohort found that roughly a third of healthy older adults had essentially no measurable decline in kidney function over decades of follow-up. We see the same pattern in our data: a real subset of people in their seventies hold eGFR values that would be normal in a thirty-year-old. What separates them from the steep decliners is unromantic and familiar. Blood pressure under good control. Fasting glucose and A1c in the healthy range. No protein leaking into the urine. Avoidance of chronic NSAID use. These are the factors that show up over and over in the longitudinal data on “preserved” kidney aging.

One small technical note: the eGFR formula uses age as an input, which slightly exaggerates the apparent age effect. Cystatin C is an alternative that doesn’t bake age into the equation. It shows a gentler but still pronounced decline, so the kidney aging signal is real even when you back out the math.

Why hemoglobin A1c rises with age

HbA1c measures average blood sugar over the past three months. In our data it ticks up about 0.1 percent per decade. That sounds tiny, but it adds up. A 25-year-old with an A1c of 5.0 will, on average, sit closer to 5.5 by 70 even if they never develop diabetes.

Part of this is real glucose intolerance. Insulin sensitivity drops with age, beta cells respond more slowly, and visceral fat tends to accumulate. Part of it is mechanical. Red blood cells live longer in older adults, giving glucose more time to glycate the hemoglobin inside them.

The rise is not inevitable, though, and this is one area where the gap between trajectories is wide. Endurance athletes, people who keep their waist circumference stable through middle age, and people who do regular resistance training largely flatten the slope. Continuous glucose monitor studies suggest that postprandial spikes (not fasting glucose) are the earliest sign of age-related glucose intolerance, and they respond quickly to changes in meal composition and walking after meals.

Why red blood cells get bigger with age (MCV and MCH)

Two of the top five risers, mean corpuscular volume (MCV) and mean corpuscular hemoglobin (MCH), are about the size of red blood cells. MCV climbs about 1 femtoliter per decade. Most people will move from the low 80s in their twenties into the low 90s by their seventies, all within the normal range.

This is one where the underlying driver matters more than the average. The most common reasons MCV creeps up are subclinical B12 or folate insufficiency (both important for DNA synthesis in maturing red cells), modest hypothyroidism, and regular alcohol intake. MCV is one of the most sensitive markers of long-term alcohol use, more sensitive than the liver enzymes most people...