This article is about how human body balances its energy intake and energy expenditures, and its relevance for weight loss and exercise.
First, we need to realize that human body does not like change — it has multiple compensatory mechanisms to oppose change and keep things at balance: weight, temperature, blood glucose level and so on. This is called homeostasis. But at the same time, our organism needs to adapt to changes to increase its chances of survival. When we want to lose fat, we need to push metabolism out of its comfortable equilibrium state and make it undergo change.
Energy balance like any balance is made of input, or intake, and output, or expenditures.
Energy input is food that you eat. All macronutrients can be used to produce energy. Carbohydrate (glucose) is the default energy source, then fat, then protein. When energy input changes, human body will attempts to balance out by doing the following in this order:
- Adjust you appetite to make you eat more or less. It you try to diet, first thing that happens it that your appetite will increase, so that you return to a normal level of food consumption. But let us assume you have iron will, then the next thing your body is going to try is:
- Adjust energy production, or metabolic rate. For example, if you eat less, your body will slow down its energy production so you can run longer on smaller amount of calories. You will tend to be more tired, less active — those are manifestations of lower metabolic rate. If you eat more, on the other hand, your body will attempt to burn that extra food by increasing metabolic rate.
- Tap into or make more reserves in a form of fat. If it not possible to balance out energy with metabolic rate and appetite, your body will deposit or use fat for energy. Fat is our long-term energy storage.
When energy output changes — for example, if you force you body to produce more energy through exercise, it will also attempt the same things but in slightly different order:
- Adjust energy production, or metabolic rate, to meet energy needs. If you exercise, your metabolic rate will increase and this increase will even persist after exercise, because now your muscles have increased oxygen supply and are metabolically more active.
- Adjust appetite. It you do not take precautions at this point, you will also start feeling hungrier — your appetite will increase and you will eat more to meet higher energy needs. But let us assume you have good will power, and are not eating more, then your body will have to:
- Tap into or make more reserves in a form of fat. If it not possible to balance energy output by adjusting energy intake and metabolic rate, your body will use or make reserves in a form of fat.
Note that in relation to metabolic rate, food intake and exercise work in the same direction: increasing either one will increase metabolic rate. But in relation to fat storage they work in opposite directions. Increasing food intake will facilitate more fat storage, while increasing exercise will facilitate fat loss.
If we put both energy intake and energy expenditures on a graph, the equilibrium area is where intake and output are about the same. This is the stable weight zone. Within this zone, human body can adapt to changes in energy intake and / or energy output by adjusting its metabolic rate and appetite without having to change the amount of stored fat:
Stable weight zone does not mean any particular amount of stored fat; it means a zone where amount of fat is not changing. Where you are exactly in the zone depends on your genetic makeup and environmental factors. On this graph, points A and B illustrate two different people, or the same person in two different environments. Person A runs at low metabolic rate — he eats little but also spends little. Person B runs at high metabolic rate — he eats a lot, but also spends a lot through physical activity. Both are at stable weight, but could be at any weight — normal, overweight or underweight.
For weight loss, you need to push your body away from stable weight zone into the weight loss zone — where energy expenditure is way more than energy intake. In this area (upper left), your body is at its limit of adjusting metabolism and will have to start using its fat reserves.
Weight gain area is where output is less than input (bottom right). Significant part of US adults does slowly gain weight and become overweight or obese, so those folks belong to this bottom right corner.
This model describes the middle part of the human population, and takes into account its spread in genetic makeup. Some people never get fat no matter what they eat. They are in a stable weight zone, but close to the border with a weight loss zone. Other folks gain weight very easily even with moderate diet — they are borderline weight gain zone.
Lets see how this model relate to real world. In an experiment below, various diet combinations with exercise were tested on their effect on different parameters, including body weight, body composition and metabolic rate. This was a 14 week experiment on obese women. Below, I highlighted numbers that I used:
HED is for high-energy diet (2600 kcal) high carbohydrate + exercise;
ND is no diet (normal diet) + exercise;
LCMP is a low calorie (1200 kcal) low carbohydrate diet + exercise;
CON is a control — normal diet and no exercise.
REE is metabolic rate, or resting energy expenditures.
Numbers in the table show change over 14 weeks: decrease is negative, increase is positive number.
Lets put numbers on the graph where they belong. Metabolic rate is in red (kilocalories per day); weight change is in blue (kg):
In the middle, with normal diet and exercise or normal diet /no exercise, there is very little weight loss or gain, the body is able to adjust to this relatively small change in activity by adjusting its metabolic rate, and possibly appetite and food intake. (The kind of exercise used in this particular experiment was relatively modest.) Metabolic rate is a bit higher with exercise as compared to no exercise, as expected.
On the right, subjects given high calorie diet and exercise still did not show much weight change, but look at the metabolic rate increase! Actually, it went up by about 1/6th of the total energy intake. Here, human body shows that it would increase its metabolic rate and energy intake rather than start burning its fat.
In the top left corner, however, when low calorie (1200 kcal) diet was given in combination with exercise, significant weight loss did occur, 6.5 kg. There was little change in metabolic rate. What happens here is that increasing energy output and decreasing energy intake push metabolic rate in opposite directions (see above), so the net change is very small. Or think this way: exercise has compensated for a drop in metabolic rate due to reduced diet. But decreasing energy intake and increasing output both push fat reserves toward using them — that is why significant weight loss was observed.
I hope I explained everything clearly. The take home messages are:
For weight loss, you must push your metabolism out of its comfort stable weight zone.
This is best achieved by a combination of calorie-reduced diet and exercise!
Effects of a popular exercise and weight loss program on weight loss, body composition, energy expenditure and health in obese women.
Kerksick C and others. Nutr Metab (Lond). 2009 May 14;6:23.