Your question doesn’t have a simple answer, John. One of the most informative studies about the correlation between biomarkers and the chances of becoming a centenarian is the Swedish AMORIS study. AMORIS followed people until they either died or reached 100. While glucose levels in the normal range or below were more frequently found in the centenarians, the association was less strong than the association with (now you’ll be surprised) elevated cholesterol.

But back to glucose. When we talk about “elevated glucose,” we need to differentiate between 3 “flavors” of elevation: impaired fasting glucose (IFG, the one that is most often measured in clinical practice), impaired glucose tolerance (IGT, elevated glucose levels post meal consumption), and elevated HbA1c. IFG and IGT can manifest either individually or together. Once glucose excursions occur long enough and to a large enough extent, HbA1c becomes elevated. Thus, contrary to the IFG and IGT, which are snapshots of glucose control, HbA1c covers a window into the past, says, 3 months of glucose control.

IFG and IGT are mainly the results of insulin resistance (IR). In the case of IFG, it is the liver that is IR, in the case of IGT, it is chiefly muscle tissue. Both are unfavorable for disease-free survival, and the AMORIS data suggest that lower glucose levels, which are indicative of good insulin sensitivity, coincide with longevity. Though, the data also suggest that the causality might be not that strong. The big question mark that I see is that metformin, though it lowers blood glucose, doesn’t show up as a strong lifespan expander. The lifespan extension achievable in rodents is in the single-digit percentages, which will inevitably become smaller in more complex animals (like us). That observation also indicates that factors other than glucose may have a stronger effect on longevity.

But you are right to suspect that keeping blood sugar within the (arbitrarily defined) normal ranges is principally a good idea.