Why two people can eat the same way and end up in different places

When I started out, I’ll admit I thought of insulin resistance as mostly a carbohydrate problem. Eat fewer carbs, bring the insulin down, fix the issue. Clean and simple.

It’s not wrong, exactly—carbohydrate restriction is still the single biggest lever most people have, and it’s where I’d start with almost anyone. I wrote a whole post about insulin resistance being the root that connects almost everything, and the dietary piece does most of the heavy lifting there.

What I’ve seen over time, though—across private clients and clinic patients—complicates the simple version. Two people can eat the exact same way, hit the same macros, stay just as strict, and still end up with very different insulin sensitivity.

The gap usually isn’t on the plate. It’s in everything around it.

Diet gets you most of the way—then the rest of your life shows up

Insulin sensitivity is really just how well your cells listen to insulin when it knocks. I unpacked the full mechanism in the root-cause post, so I won’t rebuild it here. Diet is the loudest input into that conversation; it sets how much insulin your body has to make in the first place.

Your cells don’t only respond to what you ate, though. They respond to whether you slept, whether your body thinks it’s safe, whether your muscles have been used, and whether your internal clock has any idea what time it is.

Those are lifestyle levers, and in my experience—so far—they’re the difference between someone whose numbers fall into place once the diet is clean and someone who does everything right on paper and stays stuck.

Let me walk through the ones I see matter most.

Sleep: the fastest way to fake insulin resistance

This is the one that surprises people, because the effect is so fast.

In one study, a single short night—about four hours—was enough to measurably reduce insulin sensitivity in healthy people. Not a month of bad sleep. One night. Stretch it to a week of five-hour nights and the drop holds, somewhere around twenty percent in healthy young men.

Read that again: healthy people, eating a controlled diet, made temporarily insulin resistant by sleep alone.

I see this play out constantly. Someone cleans up their diet, feels great for a stretch, then hits a season of bad sleep—a new baby, a brutal quarter at work, a few weeks of doomscrolling past midnight—and assumes the diet quit on them. The diet didn’t quit. The sleep pulled the rug out from under it.

If your metabolic numbers aren’t moving the way you’d expect, sleep is one of the first places I’d look, long before blaming the food.

Stress: cortisol works against insulin on purpose

Cortisol’s job, in part, is to raise blood sugar. That’s not a malfunction; it’s design. When your body senses a threat, cortisol pushes glucose into the bloodstream so you’ve got fuel to fight or run—and it actively opposes insulin to keep that glucose available.

That system is brilliant for a short emergency. It’s corrosive when the emergency never ends.

I’ve watched people with a genuinely dialed-in diet stall out because the stress loop never let up—undereating, overcommitting, running on adrenaline, never giving the nervous system a window to register that the threat has passed. Chronically elevated cortisol keeps blood sugar and insulin demand higher than the diet alone would predict.

This is why “just cut the sugar” misses the people who barely eat sugar and still can’t move the needle. Their stress physiology is writing checks the pancreas has to cover.

Muscle: the glucose sink most people are letting shrink

This lever is almost absurdly underused.

Your muscles are the largest place your body parks blood sugar—on the order of eighty percent of the glucose you absorb after a meal gets pulled into skeletal muscle. When a muscle contracts, it opens a door for glucose that doesn’t even need insulin to unlock it. Movement itself clears blood sugar.

That’s a huge lever.

The flip side is the quiet problem: when muscle goes unused, that glucose sink shrinks, and the body has to lean harder on insulin to handle the same meal. Sedentary living doesn’t just fail to help—it actively narrows one of your biggest avenues for staying insulin sensitive.

I’m not talking about punishing yourself in a gym. A walk after meals, some resistance work a couple times a week, generally using the body you’ve got—the people I’ve worked with who add this in tend to see their other efforts start working better, almost like the diet finally has somewhere to put the fuel.

Why identical diets diverge—light and timing

This one’s subtler, and it took me a while to take seriously.

Your body runs on a roughly twenty-four-hour clock, and that clock has a say in how your cells handle glucose. When researchers deliberately knocked people out of sync with their internal clock—the pattern shift workers live in, eating and sleeping at the “wrong” biological times—blood sugar climbed even though the body pumped out more insulin to compensate. A few people slipped into a prediabetic-range response within days, purely from the timing.

Most of us aren’t shift workers, but plenty of us live a watered-down version: dim, artificially lit mornings, bright screens at midnight, the biggest meal of the day landing right before bed. The signal the clock is trying to read gets muddy.

The fix is refreshingly low-tech. Morning sunlight on your eyes and skin, fewer screens after dark, and trying not to eat heavy late at night—I wrote more about why light is a nutrient if you want to go deeper. Anchoring your clock is one of the cheaper insulin levers available, and almost nobody’s pulling it.

The background load

I’ll keep this one brief, because it’s more “worth knowing” than “fix it tomorrow.”

Some of what we’re swimming in daily—plasticizers, certain pesticide residues, the endocrine disruptors I’ve written about as part of the modern environment—can interfere with insulin signaling at the cellular level. No single exposure is the villain; it’s the cumulative load over years. You can’t control all of it, and I’d never want anyone losing sleep over it (which, given the section above, would be self-defeating). Reasonable reduction where it’s easy—better water, less plastic around hot food—is plenty.

So the opening question answers itself

Two people can eat the same way and diverge because insulin sensitivity isn’t set by food alone—it’s food plus sleep plus stress plus movement plus timing plus accumulated exposures, stacked together. The plate is the biggest single input; it’s still one input among several.

This is genuinely encouraging, even if it sounds like more to manage. It means that when your diet is solid and the numbers still won’t budge, you’re not broken and the food isn’t failing you. It means there’s a lever you haven’t pulled yet.

All that to say—

If you’ve cleaned up your diet and your fasting insulin or other markers still aren’t where you’d hoped, resist the urge to just eat even less or restrict even harder. Look around the plate first.

Pick the lever that’s most obviously broken for you. If you’re sleeping five hours, start there. If you haven’t taken a real break in years, start there. If you sit all day, a daily walk might do more than another dietary tweak. You don’t have to perfect all of these at once—you just have to find the one that’s holding you back and give it some attention.

Diet is a lifestyle. So is insulin sensitivity. The food matters enormously; it just was never built to carry the whole thing alone.

I hope this fills in a piece the carbohydrate conversation usually leaves out, and gives you somewhere new to look when the obvious answer isn’t moving the needle.

Rance Edwards is a National Board Certified Health and Wellness Coach (NBC-HWC) with over 2,000 clinical hours of experience, specializing in chronic disease management and lifestyle medicine.

If your diet is dialed in and something still feels off—or you want a partner who looks at the whole metabolic picture, not just your plate—I’d love to talk. Book a free discovery call—no pressure, just a conversation about where you are and what the next step might look like.

Sources

  • Donga, E., et al. (2010). A single night of partial sleep deprivation induces insulin resistance in multiple metabolic pathways in healthy subjects. The Journal of Clinical Endocrinology & Metabolism, 95(6), 2963–2968. DOI
  • Buxton, O.M., et al. (2010). Sleep restriction for 1 week reduces insulin sensitivity in healthy men. Diabetes, 59(9), 2126–2133. DOI
  • Scheer, F.A.J.L., et al. (2009). Adverse metabolic and cardiovascular consequences of circadian misalignment. Proceedings of the National Academy of Sciences, 106(11), 4453–4458. DOI
  • Merz, K.E., & Thurmond, D.C. (2020). Role of skeletal muscle in insulin resistance and glucose uptake. Comprehensive Physiology, 10(3), 785–809. DOI

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