Biochemistry of Metabolism Explained

Chapter summary

The main metabolic pathways have been known for over 40 years. Current research is about the controls: genes, gut bacteria, hormones and psychology. Those controls are still being revised.
Our food comes from three groups of compounds: lipids (fat), protein and saccharides (carbohydrates). We break them into small parts, absorb them, rebuild them and break them down again for energy.
Homeostasis: The body holds a steady state, such as 37°C and blood sugar between about 70 and 170 mg/dl. About 80% of daily energy goes to this upkeep, including brain function and renewing dying cells. The idea goes back to Claude Bernard’s “harmonious ensemble”.
The basic chemistry is shared by bacteria, worms, rats and people.

Blood glucose: about 40 kcal, enough for a few minutes.
Glycogen in liver and muscle: a short-term reserve that runs out in about a day of fasting.
Fat: about 10kg, holding around 100,000 kcal. This could sustain the body for 30 to 40 days with water.
Carbohydrate supplies quick energy. Fat is the long-term reserve.

Fat is part of every cell, including muscle, brain and nerves.
It cushions and supports organs such as the heart, liver, kidneys and spleen.
It insulates against heat loss.
Myth: A fatty piece of bacon does not simply turn into hip fat. It is more likely that a carbohydrate food such as a muffin is converted into fat.

Protein and amino acids are not stored. They are in constant turnover.
Amino acids broken down for energy produce ammonia, which the body converts to urea and excretes.
Nitrogen balance means intake matches excretion. Negative balance signals tissue breakdown.

Genes affect appetite by switching on a hormone in fat cells that signals the hypothalamus in the brain.
A “sluggish metabolism” is not one thing. It is the result of tens of thousands of control steps across billions of cells.

Calories in, calories out: This view treats metabolism as a simple energy ledger.
Hormonal control: The author’s view is that regulation matters more than the ledger, so what you eat changes how much you want to eat.