We cannot just have carbohydrates to burn for calories. We need protein and fat. Which presumably means we do not just burn them for calories but need some part of them intact.
So do we not burn them for calories? Do we burn part of them for calories and use other parts intact? Does the calorie count include just the part we burn or do we count the other parts we use for calories?
In: Biology
We can have just carbohydrates to burn for energy, and in a healthy body, carbohydrates are burned first. Brain and other nerve cells can only process glucose; if you don’t ingest it; your liver has to break down muscle tissues to extract glycogen and supply glucose to the brain.
Fats are both used as energy store and as building blocks; most cell membranes are based on triglycerides and many metabolic processes require fats as solvents for other things.
Protein does not need to be burned for energy; proteins are used as energy source only if there are no others; among other things because cells can’t process them as easily as carbohydrate, because there’s some nasty nitrogen residue left overs etc. Normally proteins get broken down in amino acids and cellular mechanisms use these to repair and build cells, build enzymes, build transport molecules for key chemical reactions etc.
Burning calories of any of the carb/protein/fat for energy is just creating a chemical reaction with excess energy that your body uses as energy. Nothing is ‘burned away’ as all components of the original are left in tact (conservation of mass) but are now in different forms. Carbs and fats go to water and carbon dioxide that leave your body in various orifices. Proteins are broken down into amino acids that your body then uses for various things.
When you burn protein into calories you are destroying the amino acids. Each molecule of amino acid can be used to either build cells or to burn for calories. There are mechanisms in your body to regulate how much is burned. The body prefers to use glucose or fat but if you have too much of a type of amino acid or too little glucose then the body will not hesitate to use these amino acids as a source of energy. The body may even destroy cells to get enough energy to survive. But if you have enough energy the body will even make new amino acids, although the human body can not generate enough of every amino acid.
There are essential fatty acids (fats) and essential amino acids (proteins), but there are no essential carbohydrates. Why? Because your body can convert fat or protein into carbohydrates for the body to use. So, when you eat protein, your body can either use it for energy, or use it to build muscle – but not both. This is why weightlifters often eat carbohydrates AND protein, so the body isn’t wasting protein to use as an energy source.
The body can generate some amino acids *de-novo,* generally from certain carbohydrates from the *Citric Acid Cycle* and the *Urea Cycle.*
The citric acid cycle is the process whereby carbohydrates are oxidized to produce ATP, water, and CO2.
Glutamine is generated from Alpha-ketoglutarate and urea. Asparatic acid is generated from Oxaloacetate and certain amine compounds. Arginine is synthesized from fumarate and urea, just as a few examples. Both of these precursors are generated from the citric acid cycle use to oxidize sugars for energy. When proteins are metabolized for energy this process is essen
Humans cannot synthesize eight amino acids. These are called Essential Amino acids and these need to be obtained from food. As nonspecialist omnivores this isn’t usually a problem. In plants the biosynthesis pathways for these 8 are significantly longer and more complex than the others. But plants can get free energy from the sun. Access to nitrogen tends to be a bigger limiting factor for them.
Generally the body tends to retain EAA’s and tends to avoid metabolizing them for energy in favor of the nonessential ones. Although this isn’t a subject that’s well studied. Poor regulation of EAA breakdown may play a role in the pathology of diabetes. This isn’t a problem if you eat a relatively diverse and balanced diet though, unless you have certain rare genetic conditions.
However since children’s bodies are growing they’re more likely to suffer from protein deficiency if they have inadequate food or only have certain specific foods available. Cereal grains, for example, tend to be deficient in 5 or 6 of the 8.
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