Interesting read on using fat sources for fuel rather than carbs.

Metabolic efficiency is a broad concept that refers to the body’s ability to use fat as its preferred fuel source (since each of us has roughly 40,000 calories of stored fat, we have a virtually limitless fuel reserve if we can effectively tap into it). Our bodies are almost always using a mixture of carbohydrates and fat for fuel, but the more fat we burn the more efficient we are. From a performance perspective, the longer and more intensely we can exercise while using more than 50% fat as fuel, the more efficient we are. At some point, as intensity increases, all of us shift into a higher carbohydrate burning state, and this shift is known as your crossover point or Metabolic Efficiency Point (a term that I believe was coined by uber dietician and triathlon coach Bob Seebohar). The good news is that by manipulating our training and nutrition we can become more efficient and push that crossover point back.
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