Looking the part did not mean they could switch fueling systems

Longevity · Fasting insulin

Most Masters cyclists assume they have good metabolic health because they look like they do. Lean build, decent VO2max, no obvious signs of trouble. A 2018 study in the American Journal of Physiology made that assumption a lot harder to defend.

Twenty healthy young men. Split into two groups at the median of HOMA-IR, the index built from fasting insulin and fasting glucose. The groups were matched so tightly that any difference had to come from insulin sensitivity itself: both averaged roughly a 23.5 BMI, around 16.5% body fat, and similar aerobic fitness. On paper, they looked like the same athlete.

Then the researchers changed fuel availability during moderate-intensity exercise and watched what happened.

Same fitness. Same body fat. Completely different fuel-switching capacity. Fasting insulin was the split, not VO2max.

The number that tells you everything

The way to measure metabolic flexibility is not total fat burned. It is whether the body responds when the fuel mix changes. Push more fat into the bloodstream and see if the system notices. That is what this study did.

In the high insulin sensitivity group, the correlation between fat availability and fat oxidation was r = -0.83 (p < 0.01). Strong, statistically significant. When lipid availability went up, those men burned more of it. Their metabolism tracked the fuel environment in real time.

In the low insulin sensitivity group, the correlation was r = -0.25 (p = 0.48). Essentially flat. No meaningful response. The slope was about five times steeper in the sensitive group (-0.139 against -0.025). Not a subtle gap. Two completely different physiologies in the same exercise bout.

What metabolic flexibility actually means

The term gets used loosely. People think it means burning fat during a long ride. Burning some fat on a long, easy ride is what everyone does. It is the default when glucose demand is low enough and nothing else is going wrong.

Real metabolic flexibility is the ability to shift fuel use toward what is actually available. When fat supply increases, do you use it? When carbohydrate comes in, do you shift toward it? When you go from rest to hard effort, can you make that transition quickly? A flexible metabolism responds to its environment. An inflexible one stays stuck regardless of what is happening around it.

The men in the low sensitivity group were not inflexible because they were unfit or overfat. They were inflexible at the level of fuel-switching regulation, probably because chronically elevated insulin had blunted the signaling that responds to fat availability. The machinery for detecting and using lipid was there. It just was not listening.

Why fasting insulin is the signal worth watching

The Rollfast Longevity Audit uses fasting insulin, and targets an optimal value under 5 uIU/mL, because the standard lab range is built to detect disease, not to describe a metabolic system at its best. Most labs flag insulin as high somewhere around 25 uIU/mL. By that threshold, you can have fasting insulin at 12, 14, or 18 and get a completely normal result. You are nowhere near diabetic. You are also, according to this data, unlikely to have the fuel-switching capacity of a truly insulin-sensitive athlete.

HOMA-IR tells a similar story. It multiplies fasting insulin (in uIU/mL) by fasting glucose (in mmol/L) and divides by 22.5. High values reflect the combination of elevated insulin and elevated glucose that defines progressive insulin resistance. The researchers used the median HOMA-IR of their sample to define the split, and the two groups that emerged had essentially identical external markers of health. The difference was internal.

That is the point. External markers, including VO2max and body composition, do not tell you how well your fuel regulation is working at a cellular level. Fasting insulin gets closer to that question.

The lean, fit Masters rider who should pay attention

A typical high-performing Masters rider trains consistently, keeps body fat in the 14-18% range, and gets regular blood work. If that work comes back with glucose in the 90s and insulin in the low double digits, everything looks fine by standard interpretation. This study is a reason to look harder.

Not every lean, fit athlete has impaired metabolic flexibility. Some do, some do not. You cannot assume you have it based on fitness or appearance. The men in the low sensitivity group would have had every reason to assume they were fine. They were matched to the other group on the markers most athletes actually measure.

Get fasting insulin measured specifically. Note the actual number, not just whether it is flagged. Compare it to an optimal target rather than a disease-detection cutoff. If it is 11 and climbing year over year, that is a trend worth addressing before it shows up in any other metric.

The caveats to state plainly

This study used twenty young men, not Masters cyclists, not women, and not a population with the training history of a serious age-group athlete. The correlation between insulin sensitivity and fuel-switching flexibility is an association, not a proof of cause. These findings should sharpen your interest in your own fasting insulin number. They should not lead you to diagnose yourself or make major training decisions based on one small study.

Training itself improves insulin sensitivity, which is one reason endurance athletes as a group tend to have better metabolic health than age-matched non-athletes. Training is not irrelevant. Training alone did not separate the two groups in this study. Both groups were exercising. Both groups were lean. The insulin sensitivity signal persisted anyway.

What the number is telling you

The case this study makes is narrow and clean. Metabolic flexibility, defined as the ability to adjust fuel use when fat availability changes during exercise, was predicted by fasting insulin and glucose, not by VO2max or body composition. The correlation in the sensitive group was strong enough to be meaningful. The absence of any correlation in the other group was not a small difference. It was a physiologically different system.

If you are a Masters rider who has never had fasting insulin measured, or who has seen it on a lab result and only noticed that it was not flagged, this is the study that makes that number worth caring about. Not because you are sick, but because the whole point of longevity-oriented training is to keep the systems working well before the first sign of trouble appears.

Fasting insulin under 5 is the target because the range where fuel regulation is genuinely responsive sits well below what standard labs treat as normal. My 2.5 is one data point. Yours is worth finding out.

Looking the part did not predict who could switch fuels.