With overweight and obesity skyrocketing around the world, researchers are working hard to identify the causes so we can help those people whose weight is increasing and those whose attempts to lose weight fail.
The standard answer - calories in exceed calories out - sounds reasonable, but in practice the conscious restriction of calories does not seem to work very well for controlling weight and overcoming obesity.
A possible reason, for some people at least, may be that they aren't metabolically flexible.
Mitochondria are granular organelles found in the cytoplasm of most cells. They have an outer membrane, and a multiply-folded inner membrane. Inside the second membrane is a viscous matrix containing a large number of proteins used to produce energy for the cell.
When a meal of fats and carbohydrates is eaten, both substances are taken into cells. Although both macronutrients are available to be converted into energy, typically the mitochondria will use the carbohydrate first. The insulin that is secreted in response to eating carbohydrate inhibits fat oxidation and encourages it to be stored away. Insulin also enhances glucose oxidation by up-regulating another enzyme involved in carbohydrate metabolism.
In normal cells after an hour or two, insulin will decline and less glucose will be available to the mitochondria. Free fatty acids will still be present in the cell and will then be allowed to get into the mitochondria where they will be used to produce energy. This is called metabolic flexibility. When carbohydrate is present, the mitochondria will preferentially use carbohydrate. When free fatty acids are present but carbohydrates are in short supply, the mitochondria will normally switch over to using fatty acids for fuel.
There is some evidence that obese individuals have a decreased ability to oxidise free fatty acids in skeletal muscle. It is not yet known whether this is due to them having less mitochondria or there are defects in the mitochondria of skeletal muscle.
It is possible to measure the relative use of carbohydrate or fat for fuel by the mitochondria by a technique called Indirect Calorimetry. This is a simple test where a person sits comfortably after a short fast and breathes through mouthpiece into a gas analyser.
The mitochondria use different amounts of oxygen and produce different amounts of carbon dioxide when they metabolise carbohydrates and fats. This is expressed as the Respiratory Quotient (RQ) or the Respiratory Exchange Ratio (RER). When carbohydrate is used as fuel, more CO2 is produced for a particular amount of oxygen consumed and the RQ is higher. The RQ for pure carbohydrate is approximately 1.0. When fat is used for energy, less CO2 will be produced for a particular amount of oxygen and the RQ will be lower. The RQ for pure fat is about 0.7.
By measuring your RQ we can determine what proportions of fat and carbohydrate you are using. We can also further determine your resting metabolic rate, which is how much energy (calories) you need each day. Knowing this allows us to gauge if you are metabolically flexible. If you aren't changes to diet and exercise can help to increase your ability to burn both carbohydrates and fats.
How flexible do you think you are? Metabolically speaking!
Article Author: David Beard, Calico Exercise Physiologist and Healthy Ageing expert