Basis Human Energetics and Fuel Partitioning

My journey in learning about myself has meant endless hours of reading everything and then, reading some more. It has given me many “aha” moments and helped me understand so much more about what a low carb life really means.

Here is an excerpt from book THE ART AND SCIENCE OF LOW CARBOHYDRATE LIVING, co-authored by Jeff S. Volek, PhD, RD, and Stephen D. Phinney, MD, PhD.

Basic Human Energetics and Fuel Partitioning

Adults can vary tremendously in body composition, ranging from 5% body fat at one extreme to 50% at the other. Lean body mass (protein containing tissue) also varies with height, gender, and physical training, but not across as wide a spectrum as body fat.

Compared to fat and protein, an adult’s carbohydrate reserves are minor (ranging from a few hundred to 2000 kcal at most) and don’t change that much except after extremes of exercise or food intake. When dietary carbohydrate is restricted, tissue glycogen levels decline but do not go extremely low due to compensatory shifts in whole body fuel use away from glucose towards fatty acids and ketones. Fat, on the other hand, has reserves of over 40,000kcal but these 40,000kcal of energy reserves from fat are only accessible when you are keto-adapted.

This adaptation to carbohydrate restriction is facilitated by sharply reduced insulin levels and takes a few weeks to be fully implemented. Once keto-adapted, the body can maintain its lean tissue composition on a moderate protein intake and sustain prolonged physical exertion using fat as its predominant energy supply.


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