By the time your fasting glucose goes above 100, you've likely been insulin resistant for 5-15 years. Insulin resistance is the quiet driver of cardiovascular disease, type 2 diabetes, obesity, Alzheimer's, PCOS, fatty liver, and accelerated aging. And yet almost no primary care doctor tests it.
This article is about why that's a massive gap and what to do about it.
What insulin does
Insulin is the hormone your pancreas releases when you eat, especially when you eat carbs. Its job is to shuttle glucose out of the bloodstream into muscle, liver, and fat cells. When your cells respond well to insulin, a small amount of insulin can move a lot of glucose. This is insulin sensitivity.
When cells stop responding, because of chronic over-eating, excess body fat, lack of exercise, or genetic factors, your pancreas has to release more and more insulin to do the same job. This is insulin resistance. For years, your fasting glucose may still look normal because your pancreas is compensating. It just requires dramatically more insulin to keep it there.
Why you can't just test glucose
Standard blood tests only show glucose and HbA1c, which don't reveal insulin resistance until it's fairly advanced. A 40-year-old with fasting glucose of 92 and HbA1c of 5.4% might look "normal." But if their fasting insulin is 18, they're already deeply insulin resistant, they're just successfully forcing a normal glucose reading through elevated insulin.
By the time glucose rises, they'll have another 5-10 years of accelerated metabolic damage behind them.
What to actually test
| Marker | Target | What it means |
|---|---|---|
| Fasting insulin | < 6 μIU/mL | Optimal insulin sensitivity |
| Fasting glucose | 70-85 mg/dL | Lower end of "normal" is best |
| HbA1c | < 5.3% | 3-month glucose average |
| HOMA-IR | < 1.5 | Calculated: (insulin × glucose) / 405 |
| Triglycerides:HDL ratio | < 2.0 | Strong proxy for insulin resistance |
The single most useful number: fasting insulin. Anything above 10 μIU/mL suggests meaningful insulin resistance. Anything above 15 is significant. Most primary care visits never order it. It should be on every adult's baseline labs.
Why insulin sensitivity matters so much
Body composition
Insulin-resistant people store fat more readily (especially visceral fat), struggle to burn it, and face more hunger/cravings. Improving insulin sensitivity is often the unlock for stalled fat loss.
Testosterone and hormones
Insulin resistance suppresses testosterone in men and drives androgen excess in women (PCOS). It also worsens thyroid function and raises cortisol.
Cardiovascular disease
Insulin resistance damages blood vessels directly through inflammation, dyslipidemia, and endothelial dysfunction, long before diabetes develops.
Brain health
Alzheimer's is increasingly called "type 3 diabetes." Brain insulin resistance is implicated in cognitive decline and dementia.
Longevity
Insulin resistance accelerates biological aging on nearly every measurable axis. Long-lived populations consistently have low fasting insulin.
How to improve insulin sensitivity
1. Build muscle
Muscle is the single largest site of glucose disposal. More muscle = more insulin-sensitive tissue. Resistance training is the highest-leverage single intervention. See building muscle after 40.
2. Walk daily, especially after meals
A 10-20 minute walk after meals cuts post-meal glucose spikes by 20-30%. 10,000 steps per day over time dramatically improves insulin sensitivity. See our 10,000 steps guide.
3. Zone 2 cardio
2-3 sessions per week of 30-45 min. Improves mitochondrial function and insulin signaling without cortisol cost. See zone 2 guide.
4. Reduce processed carbs and ultra-processed food
Not low-carb, just lower-processed. Whole grains, fruits, potatoes, oats, rice are fine for insulin-sensitive or recovering individuals. Sugar-sweetened beverages, refined carbs, and ultra-processed foods drive insulin spikes and inflammation.
5. Prioritize protein and fiber
Both dramatically blunt glucose and insulin responses. 1g of protein per lb of goal bodyweight + 30g+ of fiber daily is a strong foundation.
6. Lose visceral fat
Visceral fat is an endocrine organ that drives insulin resistance. Even 5-10% weight loss in overweight individuals can dramatically improve insulin sensitivity.
7. Sleep 7-9 hours
One night of 4-hour sleep can transiently reduce insulin sensitivity by 25%. Chronic sleep deprivation is a major driver of metabolic dysfunction.
8. Manage stress
Cortisol is insulin's metabolic rival. Chronic stress drives insulin resistance.
9. Consider specific supplements
- Berberine, 500mg 2-3x/day. Often compared to metformin in effect.
- Magnesium, deficiency worsens insulin resistance
- Omega-3, reduces inflammation that drives IR
- Alpha-lipoic acid, evidence base is modest but real
10. Consider pharmaceuticals if indicated
- Metformin, decades of safety and efficacy data; often used for metabolic optimization and longevity
- GLP-1 medications, dramatically improve insulin sensitivity as a secondary effect of weight loss. See our.
- TRT, improves insulin sensitivity in men with low testosterone. See TRT guide.
The leverage play: resistance training + daily walking + high protein + adequate sleep. Four interventions that cost nothing and that most people don't do consistently. Adding them back in often reverses insulin resistance within months, before you need medication.
Continuous glucose monitoring
CGMs, once used only by diabetics, are increasingly used by healthy adults to understand their personal responses to foods. Wearing one for 2-4 weeks teaches you which meals spike your glucose and which don't. Powerful educational tool, especially for people with metabolic concerns.
Get fasting insulin tested
OPTML's comprehensive panel includes fasting insulin, glucose, HbA1c, and the other markers that reveal real metabolic health, not just the ones that say "normal" until it's too late.
Order your panelThe bottom line
Insulin sensitivity is the upstream driver of most modern chronic disease. It can be measured cheaply and improved dramatically with lifestyle changes. Get fasting insulin tested. Don't settle for "your glucose is normal." Most people are quietly insulin-resistant for a decade before their labs catch it. Be the one who catches it early.