Nutrition's Energetic Cycle

As far as our bodies are concerned, energy equals calories. When we say proteins and carbohydrates supply 4 kcal per gram, and fat supplies 9 kcal per gram, that’s another way of quantifying the energy that each macronutrient contributes to the metabolic functions that keep us running—literally and figuratively.

How those macronutrients turn their kilocalories into usable energy is a story familiar (if not exactly fresh) to any veteran of college biochem. As for carbohydrates, our bodies convert both complex starches and simple sugars into six-carbon glucose units, which split into two, three-carbon pyruvates in a process called glycolysis. These pyruvates then enter the cellular mitochondria where they undergo a decarboxylation reaction that yields a molecule each of acetyl-coenzyme A (two acetyl-CoAs per glucose). Each acetyl-CoA then joins what’s called the tricarboxylic acid (TCA; or Krebs, or citric acid) cycle, whose many enzyme-catalyzed steps ultimately produce carbon dioxide, water and energy.

In the case of proteins, proteases liberate the individual amino acids, whose carbon backbones then undergo conversion to acetyl-CoA and introduction into the cycle. Fat catabolism is more complicated, with fatty acids first splitting from their glycerol backbone, the backbone getting converted to glucose in the liver, and the resulting glucose then following the steps for carbohydrate metabolism. Meanwhile, the fatty acids undergo a process called beta oxidation, which turns chains with an even number of carbons into acetyl-CoA, and those with an odd number of carbons into succinyl-CoA, a TCA intermediate that slips into the cycle somewhere around step seven.

In all cases, the process pays out its energy in the form of adenosine triphosphate (ATP). The complete oxidation of one molecule of glucose—from glycolysis through the TCA cycle and including a subsequent process called oxidative phosphorylation—yields 30 to 38 molecules of ATP. These ATPs go on to fuel our cells’ day-to-day energy economy. So while macronutrients are the metabolic equivalents of long-term savings accounts, ATP is the body’s liquid cash.