Metabolism overall is about conversion. What is metabolism in relation to weight control is how our bodies convert food, liquid and other substances such as oxygen into energy and other usable things. If we are unable to convert all the food and substances into things we need right now, what we can’t eliminate has to be stored.

Taking in more energy than we can convert is like storing too much fuel around our house. It sits there going toxic and is potentially dangerous.

We call the entire process of breaking down food/substances into small pieces for energy (catabolic) and constructing the things our bodies need like proteins (anabolic), metabolism. It’s obviously a complicated process, not an organ or a thing. Yet we talk about it as if it’s something tangible that we can consciously control.

Metabolism and Weight

Our Metabolic Rate is the speed of the chemical reactions converting our food intake into useful components. Bill can have exactly the same calorie intake and activity level as Ben, but Bill puts on weight while Ben loses it. The difference is the rate of conversion.

Eating 1900 calories, if Bill has a 10% slower metabolism he uses 190 less calories than Ben. If Bill needs 2000 calories to maintain his body’s functions and activity levels, then Ben will only need 1800. Bill has a deficiency of 100 and Ben has an excess of 100.

100 calories a day doesn’t sound like much (less than 20grams of milk chocolate or a couple of squares), but using our rule of thumb of 3500 calories to a pound, after 35 days, Bill loses a pound and Ben gains one if metabolism stays the same.

Key parts of the metabolic system

Some key parts of the system are the endocrine system, particularly the thyroid and the pancreas, our parasympathetic and sympathetic nervous systems, and at a cellular level the mitochondria.


The thyroid controls how quickly the body burns energy and makes proteins.


Involved in break down and digestion of food into amino acids, glucose and lipids as well as creating insulin, a vital hormone for energy and fat storage.


These are our cellular energy factories. Organs that need more energy such as muscle cells, the heart and liver have more mitochondria. Metabolism is a function of the number, size and efficiency of mitochondria.

More mitochondria and faster metabolism comes from more muscle mass. An extra 1lb/.45kg muscle burns an extra 50 calories per day. Using our rule of thumb of 3,500 calories to the 1lb/.45kg, 50 calories a day equates to 1lb/.45kg body weight in 10 weeks (50 x 7 x 10)

The efficiency of our mitochondria relates to aerobic exercise our HGH (human growth hormone) levels and the amount of free radical damage. When we are young, our mitochondria are relatively undamaged and so function better. This is one reason metabolism can decrease with age.

Metabolism and Stress

Efficient digestion needs our parasympathetic nervous system to be active. When stressed, our sympathetic nervous system is more active. Chronic stress keeps cortisol levels high resulting in fat gain particularly abdominal fat.

Eating while stressed therefore will cause you to gain weight. Your appetite naturally decreases in a stressful situation because energy for digestion goes to fight/flight processes. Afterwards however, appetite increases to make up for lost time.