We all know that your body runs solely on glucose. Right? As a thought experiment would it change your diet choices if you learned that wasn't true? Would you care if you learned that excess carbohydrates, like dietary glucose, were challenging for your body and often lead to disease? Suppose we look at that specifically.
Just as a hint, we now know that glucose is not the predominant fuel the body uses. At rest (and even during moderate exercise) fat accounts for at least 50-60% of fuel burned by tissue and much of the remainder is protein. This also means that according to current dietary guidelines from the government (which imply more than 50% carbohydrates consumption) our liver must turn carbohydrates into fat, as various fatty acids, to give our tissues the fuel they require.
So how do we know that?
The basic measure is called resting energy requirement. Science has measured this fairly carefully so we know bio-chemically how the various tissues process chemicals for their energy needs. Interested? Let's take a look at some detail.
As a measure of resting energy requirement, the brain uses roughly 22% of the body’s total energy. Because it is fastidiously protected against pathogens, only small molecules can cross the blood-brain barrier. This means that the brain mostly burns glucose and it also can run happily on ketone bodies (broken down from fat in the liver). Indeed, ketones may be an essential fuel. Breast milk provides babies with a considerable proportion of ketones for the brain. Of the other major organs, the liver, which is 21% of resting energy, and the heart, which uses 9%, both predominantly burn fatty acids. The heart burns only fat, but the liver is metabolically flexible, often utilizing excess protein.
Of the minor organs, the kidneys require mostly glucose, as do the eyes and red blood cells. However, the intestines burn a mix of fuel. The small intestine prefers protein, specifically glutamine, which provides the nitrogen necessary for rapid cell turnover. The large intestine prefers short chain fatty acids, which are a byproduct of the bacterial digestion of fiber. Muscle utilizes a mix of fuels. At rest, muscle cells oxidize almost 100% fatty acids, which account for 22% of the body’s resting energy. When exercising, muscle requires faster access to energy and begins to burn carbohydrates, stored as glycogen, the fatty acid storage molecule, within the tissue. It also burns some proteins and in very short bursts phosphocreatine, a molecule that serves as a rapidly mobilizable reserve of high-energy phosphates in skeletal muscle and the brain.
As the intensity of activity rises, so does the need for glucose and amino acid. For example, at a slow jog, muscles still burn 60% fat, but in a 100-meter sprint, carbohydrate and protein will make almost all the fuel. Counter intuitively, athletic training increases the cellular machinery to oxidize fatty acids, thus increasing the fat/glucose ratio in exercise. When adding up the composite fuel requirements, it is clear that glucose is not the predominant fuel the body uses. At rest (and even during moderate exercise) fat accounts for at least 50-60% of fuel burned by tissue and much of the remainder is protein. This also means that according to current dietary guidelines (which imply more than 50% carbohydrates consumption) our liver must turn carbohydrates into fat to give our tissues the fuel they require.
Something to ponder when you consider your diet or that given to your kids at school as the result of government edicts. If you wonder why the nation is so fat, in simple terms, we eat the wrong kind of food most of the time. For a bit more detail look try this link.