The Biomarkers I Track for Founder Stamina

Every marker below has a "normal" range your GP uses, and a tighter "optimal" range that matters if you're trying to operate at capacity. Normal is a population reference. It includes a lot of tired, average people. Optimal is an operator reference. A result can be technically normal and still be quietly draining your stamina. (I wrote about why normal isn't the same as optimal separately. It's the single most important idea on this whole topic.)

The principle is simple. Treat your body's numbers like a balance sheet. You'd never run your company on "revenue is technically positive." Don't run your body on "labs are technically normal."

I think about the panel in four groups. Each one governs a different part of your ability to sustain output.

Cluster 1. Drive and recovery (the hormone panel)

This is the engine behind motivation, decisiveness, and how fast you bounce back from a hard week.

Free testosterone, total testosterone, and SHBG, tracked together. This is the key point most people miss: total testosterone can look fine while free testosterone (the part your body can actually use) is low, because SHBG (the protein that binds it up) is high. Track all three or you get a false read. Free testosterone is what drives energy, decisiveness, and recovery. If your get-up-and-go has quietly faded, this trio is the first place I look. (It's one of the four markers most GPs miss in founders.)

The full thyroid panel: TSH, T3, and T4. Your thyroid is the body's throttle. The mistake is testing only TSH and calling it done. TSH can sit in "normal" while your free T3 (the active hormone that actually sets your metabolic rate) runs low, leaving you flat, cold, and foggy by mid-afternoon. For stamina you want the full panel, and you care most about free T3.

Cortisol. Your stress hormone, and the clearest signal of whether you're running on adrenaline. The number matters less than the pattern. Cortisol should be high in the morning and taper through the day. A flat or inverted curve means your stress system is depleted: the biological version of an overdrawn account, and a direct hit to afternoon stamina.

Cluster 2. Oxygen and energy delivery

These govern whether your brain and muscles actually get fueled.

Ferritin. Your iron stores, and one of the most under-appreciated stamina markers. Low-normal ferritin is a classic hidden cause of the founder who is "tired but the bloods are fine."

Vitamin B12 and vitamin D. B12 drives red blood cell formation and nerve function. Low levels show up as fatigue and brain fog long before they're flagged as deficient. Vitamin D acts more like a hormone than a vitamin and underpins energy, mood, and immune resilience. In the Nordics, where daylight disappears for months, it's the single most common deficiency I see in founders. Through a Finnish winter, this one is non-negotiable to track.

Omega-3. Less about acute energy, more about the quality of every cell membrane and the inflammation balance underneath your stamina. A low omega-3 index is a slow tax on recovery.

Cluster 3. Stable fuel (the metabolic panel)

This cluster decides whether your energy is steady or a rollercoaster of spikes and crashes, which is what stamina actually is over a long day.

Fasting insulin and fasting glucose, together. This is the early-warning system. Glucose can look normal for years while insulin quietly climbs to keep it there. For steady energy you want fasting insulin low and calm. Optimal DX, a functional-medicine lab service, puts its tighter range at 2 to 5 uIU/mL where the standard lab range is 2 to 19. That is a vendor range, not a guideline, and no clinical body defines an optimal fasting insulin. The less effort your body spends managing blood sugar, the more energy is left for actual work.

HbA1c. Your average blood sugar over the past three months: the trend line behind the daily readings. It tells you whether your fuel system is stable over time, which is exactly the timescale founder stamina is measured on.

Cluster 4. The inflammation and long-game markers

These don't change how you feel tomorrow, but they decide whether you're still operating at full capacity in ten years.

hs-CRP. A sensitive smoke alarm for background inflammation. You want it quiet. Not just under the conventional 3 mg/L heart-risk threshold, but ideally closer to 0.5 or lower. Chronic low-grade inflammation is a constant drain on recovery and stamina you can't feel directly.

ApoB. The most honest single number for cardiovascular risk. It counts the actual particles that build arterial plaque, which standard cholesterol panels under-report. European guidance (ESC and EAS, 2019) sets ApoB goals of under 65, 80 or 100 mg/dL depending on how much cardiovascular risk you carry. Under 70 is a longevity-clinic target, not a guideline number. This is the marker the Outlive / Medicine 3.0 approach made famous, and it's worth tracking even though it doesn't affect how you feel this week.

Homocysteine. An amino acid that climbs when your B-vitamin metabolism is off, and a marker linked to both cardiovascular and cognitive risk. The fix is often simple (B12, folate, B6), which is exactly why it's worth knowing your number.

Magnesium (RBC). Track the red-blood-cell version, not serum. Serum magnesium can look normal while your cells are depleted. Magnesium sits behind sleep quality, muscle function, and stress resilience: three things that quietly set your stamina ceiling.

Blood is half the picture. For an operator, three physical metrics matter just as much:

• VO₂ max. The single best predictor of how much physical capacity you have in reserve. High VO₂ max means the long day costs you less. It's the closest thing there is to a stamina score. And the strongest lever in the longevity vs wellness framework for founders.
• Body composition (InBody). Muscle mass versus fat, tracked over time. Muscle is metabolic armor; losing it silently is one of the clearest signs you're under-recovering.
• Muscle strength. Tracked directly, because strength holds up function and resilience as you age in a way that weight alone never shows.

I don't test all of these every month. A full panel once or twice a year sets the baseline. The few markers that were off get re-checked more often until they're in the optimal range. The wearable data (HRV, sleep) fills the gap between blood draws as the daily signal.

Most founders I meet have at least one of these sitting in "technically normal, actually draining" territory (usually ferritin, vitamin D, free T3, or fasting insulin) and have been blaming their willpower for something their biology was causing.

If you've never had a panel beyond what your GP runs, start there. Get free testosterone (with SHBG), a full thyroid panel, ferritin, vitamin D, fasting insulin, and hs-CRP. That handful catches the most common hidden stamina drains for a founder. Read them against optimal ranges, not just normal ones. If something's off, that's not a character flaw to push through. It's a setting to fix, and fixing it gives you back energy you didn't know you were missing.