Open Future HealthBasic Human Body Metabolism

Metabolism is the system that converts food into the energy your body needs for life.

Your metabolism as two primary goals, (1) to supply continuous energy, (2) to keep the glucose level in the blood constant.

No matter what food you choose to eat, the body deals with it all in a similar way. Digestion begins in the mouth, continues in the stomach, and as food enters the small intestine and large intestine the process continues.

General Notes on Metabolism

Oxidation of Food

The food you eat is never oxidized directly. Food is broken down into many chemical parts in a step by step process. This allows the body to control the energy release.

Food mostly becomes either Local FileMono-saccharide's, Local Filelong chain fatty acids, or Local Fileamino acids.

The energy for your body has TWO primary sources: glucose, and free fatty acids. In extreme situations protein can be also be broken down to produce glucose.

Glycolysis (Breaking down glucose)

Dietary sugars enter the blood stream from the gut quite readily, other carbohydrates likewise are easily digested. Starchy carbohydrates eventually also become glucose. They pass via the liver to body cells.

Glucose is generally considered to be the energy source for human metabolism. Glycolysis (splitting sugar) of glucose creates pyruvic acid, which becomes Acetyl-CoA, which is the main energy input for aerobic metabolism.

Insulin, the master hormone, helps glucose enter the body cells where it's either turned into Acetyl-CoA for energy or stored as glycogen. Be aware though that this is a small and limited energy store.

Lipolysis (Breaking down fats)

Lipolysis is the breakdown of lipids and involves hydrolysis of triglycerides (fats) into glycerol and free fatty acids.

Dietary fat has to be disassembled in the small intestine into free fatty acids, which are then small enough to pass through the intestine wall. Fatty acids in the blood are readily accepted by all the bodies cells except the red blood cells, the central nervous system and the brain. Inside your cells, free fatty acids are accepted by the mitochondria, where β-oxidation creates Acetyl-CoA for energy output.

Glycolysis or Lipolysis: one or the other.

At any one time in a cell only one of those processes can function. The switch is the hormone insulin. If glucose is high, insulin is "on," and the priority is to reduce blood sugar and burn Acetyl-CoA from glucose. If insulin is low, blood sugars are normal, so insulin is "off" and fatty acid sources of Acetyl-CoA can be used.

Protein broken down into amino acids can become a fuel, but that's irregular, in the case of illness or starvation. Amino acids are used for building hormones and body tissues.

It is possible via gluconeogenesis for amino acids from protein to be converted into glucose. If you body really needs glucose, it can always make some, even if there is none in the diet.

If you need to learn more search for "Krebs cycle" or " TCA cycle" which are alternative names. The energy is produced according to "Chemiosmotic Theory" which is the work of Peter Mitchell.

Basic Human Body Metabolism

The energy for your body has TWO prime sources glucose, and free fatty acids. It's possible in starvation for body protein to be broken down to provide energy, but that's not normally the case.

There are two metabolic states: where glucose is abundant, and where glucose is scarce.

Glucose Abundant State

Glucose can readily be transformed into acetyl-CoA, in body cells, but the reverse is not true. That's a very important fact, because the fat around your waist can only come from monosaccharides. Once glucose moves into body cells and becomes acetyl-CoA, it can't reverse itself.

When you eat fat, it's turned into free fatty acids. Using the Standard American Diet model, in the presence of insulin, free fatty acids compete with glucose to fuel the body. Glucose is supplied to the brain because free fatty acids cannot cross the blood brain barrier. The heart and muscles prefer to burn free fatty acids if they are abundant.

When glucose is high, insulin is "on" because the body is trying to restore the glucose level in the blood to normal. Insulin helps glucose enter the muscle cells of the body, and promotes the creation of glycogen. Insulin also turns on fat storage.

Fat storage involves collecting 3 x free fatty acids and connecting them with a bond of glycerol making triglycerides. That removes both some glucose and some free fatty acids into storage.

Glucose Scarce State

When glucose is low, insulin is "off" because the body is trying to keep the glucose level in the blood to normal. Free fatty acids in the blood are not converted into triglycerides, they are directly used for energy production, being converted into acetyl-CoA both in muscle cells and in the liver. Excess acetyl-CoA, in the liver is converted into ketone bodies, β-hydroxybutyrate and acetoacetate.

The fat you eat can't (there are some tiny exceptions) become glucose.

It is possible via gluconeogenesis for amino acids from protein to be converted into glucose. If you body really needs glucose, it can always make some, even if there is none in the diet.

If you need to learn more search for "Krebs cycle" or " TCA cycle" which are alternative names. The energy is produced according to "Chemiosmotic Theory" which is the work of Peter Mitchell.

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