Ketones and fasting get marketed like shortcuts. This conversation is the opposite: a grounded look at how your body actually produces energy—and what changes as you age.

Dr. Brianna Stubbs is a researcher at the Buck Institute for Research on Aging and a world-class endurance athlete. We talk about metabolic flexibility (your ability to switch fuels), what ketones can and can’t do, and when fasting helps—or backfires—depending on training load, recovery, and goals.

In this episode:

•  What “metabolic flexibility” really means (in plain English) 
•  Exogenous vs. endogenous ketones—and why the difference matters 
•  Fasting, performance, and muscle: where people get it wrong 
•  What endurance athletes can reveal about aging physiology 
•  Practical fueling principles that support longevity—not fads 

The goal isn’t extreme restriction or constant optimization. It’s building a metabolism that stays resilient, adaptable, and recoverable over decades.

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Topics: longevity, fitness over 40, endurance training, aging athletes, recovery, injury prevention