17 February 2006

More than combustion engines

Protein, carbohydrates and fat are handled by the body in different ways and certain conditions need to apply for food to be turned into fat or for fat to be removed.

Lately there have been a lot of questions from Health24 readers that indicate that people believe the human body works exactly like a combustion engine.

The assumption is made that food ingested will be transformed directly into energy (like petrol) and if a person has eaten a fatty meal or ‘sinned over the weekend by gorging on fatty foods and drinking large quantities of alcohol’ the ‘sin-fat’ can be removed by doing twice as much exercise the day after.

This is a simplistic approach and not how the human body works. I will, therefore, attempt to explain how various macronutrients, such as protein, carbohydrate and fat are handled by the body and what conditions need to apply for food to be turned into fat or for fat to be removed from the fat depots.

Basic principle
Always keep in mind that your body is a highly complex organism with thousands of interlocking systems that govern its metabolism.

What happens to food when we eat it?
a) Proteins
Protein foods like meat, fish, eggs, milk, yoghurt, cheese and legumes, are digested in the gastrointestinal (GI) tract and broken down by enzymes into their basic units, which are called amino acids. These amino acids are transported through the walls of the GI tract into the blood, which carries them to the liver for distribution to the general circulation system.

Absorbed amino acids are primarily used by the body to make new proteins which are used to build new muscle tissue when growth occurs (infants, children, teenagers, pregnant and lactating women, and body building) or when used-up or damaged tissue has to be repaired (a constant process in the body where cells are being replaced or after wasting illnesses).

Protein is only used as a source of energy when the diet contains practically no carbohydrates. Using protein for energy purposes is basically a wasteful process that depletes lean muscle tissue, something that is not desirable even when an individual is trying to lose weight. The idea is to lose fat mass, not muscle mass.

b) Carbohydrates
Carbohydrates, such as the sugars and starches which are found in cereals, grains, fruit and vegetables, are also digested to their most basic component, namely glucose. Glucose is transported into the blood and used as the primary and most readily available source of energy for all physical activity. This physical activity does not just include your workouts at the gym, but every movement of the body, the energy supply required by the human brain, the energy used in our cells to fuel metabolic processes, and the energy we use even when we are sleeping or resting.

Carbohydrates as such are only stored in small quantities in the form of glycogen in the liver and muscles. This supply is rapidly used up when we exercise and athletes know that they have to replenish their energy sources before, during and after exercise by using high-energy, high-carbohydrate drinks or carbohydrates with a high glycaemic index.

Carbohydrate is not easily turned into fat for storage in the human body, unless your total energy intake exceeds your energy requirement by a considerable margin.

c) Fat
Fat digestion and absorption differs considerably from that of proteins and carbohydrates. Fat in foods such as cream, butter, oils, margarine and fat that occurs in foods derived from animals (meat, chicken skin, fatty fish, egg yolk, full-cream milk and yoghurt, cheeses), is broken down in the GI tract to free fatty acids and monoglycerides.

These free fatty acids and monoglycerides combine with bile salts to form complexes that move the fats into the cells in the walls of the intestine. Here the free fatty acids and monoglycerides are reassembled into fats, called triglycerides, which together with other forms of fat such as cholesterol and phospholipids, are first transported via the lymphatic system (not the blood) and then emptied into the blood stream and taken to the liver. In the liver the fats are ‘repackaged’ and primarily transported to the adipose tissue where they are metabolised and stored.

If you eat a fatty meal today, the fat will be stored in your fat tissue and doing excessive exercise will not necessarily shift this stored fat.

To release fat from the fat depots you need to reduce the total amount of energy you ingest so that you are in a negative energy balance by eating an energy-reduced diet and increasing your energy expenditure for more than just one day. You still need freely available energy to be able to do the exercise, so this does not mean that you should reduce your carbohydrate intake.

Weight loss recommendations
To lose weight, you need to also eat less fat all round so that the fat in your diet is not constantly being stored in your fat depots. This is why nutritionists recommend the following for sustained weight loss:

  • Eat less fat in total to prevent fat storage in the fat depots;
  • Continue eating sufficient unprocessed carbohydrates to fuel your exercise;
  • Continue eating a moderate amount of protein to prevent loss of lean muscle mass;
  • Reduce your total energy intake moderately to induce a negative energy balance which will make your body mobilise its fat stores;
  • Do regular, aerobic exercise to stimulate your metabolism and mobilise your fat stores.

This translates into the following:

A low-fat, high-fibre, energy-reduced diet, which contains plenty of unrefined, unprocessed carbohydrates and a moderate amount of protein, combined with a regular exercise routine.

The best examples of individuals who use high-carbohydrate, low-fat diets are the ultra-athletes, like the top cyclists participating in the Tour de France. Their diets consist of nearly 70% carbohydrate, about 15% protein and 15% fat, and they are among the leanest, healthy people on the planet.

If you have ‘pigged out’ for a day, but this lapse has not increased your total energy intake over the week or month, it will not change your body weight. Conversely, if such dietary excesses do push up your total energy intake for the week or month, then exercising frantically for one day will not alter the negative weight gain effect.

Remember our bodies are much more complex than simple combustion engines. Be consistent, stick to your diet and do your exercise and you will achieve sustained weight loss over time. – (Dr I.V. van Heerden, DietDoc)

Any questions? Ask DietDoc


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