What a CGM Shows About Exercise After 45

Because of what happens next. Exercise makes muscles more sensitive to insulin for hours afterwards, and it does so through a pathway that does not need insulin at all — the contraction itself moves glucose transporters to the muscle surface. The net effect of a hard session, measured over the following day rather than the following hour, is lower average glucose, smaller post-meal rises and better insulin sensitivity. The sensor shows the cost; the benefit is spread over the twenty-four hours that follow and is harder to see.

A trace that rises during a hard workout and settles within an hour or two is the trace of a healthy response. The pattern that would matter — and that a clinician looks for — is different: glucose that does not come down after exercise, or that rises with easy exercise, or that is high before the session begins. Those are questions for the panel, not for a training change.

A walk after a meal. The post-meal rise in glucose is the part of the day most people are most interested in, and the single most reproducible way to flatten it is ten to fifteen minutes of easy walking beginning shortly after eating. Studies in people with and without diabetes show a meaningful reduction in the post-meal peak, from a mechanism that is the opposite of the hard-training one: gentle contraction draws glucose into muscle without provoking a stress response. The effect is visible on a CGM the first time it is tried, which is the reason a two-week sensor run can change a habit that a lecture cannot.

The same applies, less dramatically, to standing rather than sitting after meals, and to timing a workout after a meal rather than before it, for people who find their glucose runs high after eating. None of this requires a CGM to be true. The CGM is what makes it believable.

Whether you are fit. Glucose is one fuel and the sensor sees only its concentration in the fluid under the skin, with a lag of several minutes behind the blood. It does not measure the fat the body is also burning, the glycogen in the muscle, or the fitness adaptations that matter. Endurance athletes have used CGMs to try to time fueling during long events, and the honest state of that evidence is that the sensor lags too far behind the blood, and the blood lags too far behind the muscle, for the trace to guide fueling in real time. The athletes pages cover what does measure training.

Nor does a CGM tell you whether you should train hard. A person over forty-five who has been told by an app that their workouts cause glucose spikes, and who responds by training less hard, has been talked out of the single most effective thing they could do for their long-term glucose by a misreading of a normal trace. That is the risk the main page names when it says the interpretation is the product.