The Serious Science Behind Weight Loss
Exercise, BMR and Food Intake. What are the Facts?
Nov 14, 2009
The energy equation for food intake re-states a basic law of physics: Energy Output from a machine equals Energy Input, which means we will gain weight if we eat too much and exercise too little. The body operates in a different way from a machine, but it needs fuel in the form of food, and it expends energy when we walk, talk and sleep. Surplus food is stored as glycogen and fat, but these energy stores can be mobilised. Glycogen in our muscles and liver is short-term storage, whereas fat represents a long-term investment.
We even need some energy to process our food, particularly first thing in the morning. Dietary Induced Thermogenesis (or DIT) refers to the energy needed to digest our food, particularly starchy complex carbohydrates, proteins, and to a lesser extent, fats. The latter can pass directly into the blood stream, partly digested, so it's a lot easier to absorb fat than get rid of it. Up to 10% of the energy available within our diet is used for DIT, which is not taken into account when labelling foods with “energy content.” This figure is calculated experimentally by burning a small sample of each food in a calorimeter using pure oxygen, which oxidises the chemicals in the food, releasing all their chemical energy as heat energy in one quick burst.
The body releases the chemical energy of food in a different way -- slowly, at body temperature, in a series of steps using enzymes. This is a valuable clue to successful dieting. If we choose foods which need substantial digestion, such as starchy foods rather than sweet, sugary foods, and include adequate protein while restricting excess fat intake, we can take advantage of dietary induced thermogenesis.
What is Basal Metabolic Rate (BMR)?
Like a car "ticking over," our body still uses up energy when it is resting and sleeping. Depending on our age, gender and weight, this figure can be calculated as our personal BMR and used to predict how much more we need to eat when we are awake and working. Our BMR changes throughout life and is affected by age, illness, hormones, diet and exercise.
Consider what happens when we choose to go on a sensible weight-reduction diet:
- When calories are first restricted, the weight loss is slow and should not approach the “starvation mode” of crash diets characterised by relentless hunger and the painful switch to fat and protein burning metabolism normally reserved for endurance athletes - or those who are genuinely starving to death. Proteins are not a good choice as an energy source if they happen to be sitting inside essential tissues such as muscles. Excess protein from the diet is a different matter, yielding as much energy as carbohydrates (2).
- Once weight is lost, there will be a small fall in Basal Metabolic Rate (or BMR), as you would expect from any reduction in body weight. Smaller machines require less maintenance. As weight loss continues, BMR continues to fall, so the myth that overweight people have a slower metabolism is not true. They have a slightly raised BMR to cope with the maintenance of a larger body mass.
- If weight loss is too fast, this drop in BMR is more marked (1) as the body switches to starvation mode and attempts to conserve energy under these conditions. This makes further weight loss more difficult and is one hazard of crash diets, the other being serious malnutrition resulting from low intake of essential vitamins and minerals.
A return to a normal diet which meets the daily energy needs of the body will return the BMR to its higher level within a few days, but for crash dieters this may lead to binge eating and substantial weight gain - the “yo –yo” diet syndrome. This is why weight loss must be gradual and comfortably within the psychological horizons of the subject.
Where does Exercise Fit In?
Exercise increases our BMR on a short term basis, and this persists for a while during recovery, so it makes sense not to over-eat or drink immediately after exercise if we want to cash in on EPOC, or Excess Post-exercise Oxygen Consumption. As we recover from exercise, our BMR returns to normal. A combination of sensible dietary restriction and sustainable exercise is a winner for weight loss so long as we do not immediately replace all the calories burned off by consuming extra food or drink.
Controlling our Appetite
The key to appetite control seems to be through our glycogen stores, in our liver and muscles, rather than our fat stores, which are metabolically inert in comparison. When these glycogen reserves start to fall after a few hours of fasting, we feel hungry. We have enough glycogen to last about 24 hours, so this is not too difficult to bear, assuming we keep fully hydrated in the meantime. After this, more protein and fat is mobilised, which is why low carbohydrate diets such as the Atkin's diet cause weight loss. However, do you really want to push your metabolism needlessly into starvation mode and deprive your body of its natural appetite triggers?
The Problem with Late-night Snacks
Eating complex starchy carbohydrates shore up the glycogen levels long term – compared to sweet snacks which are easily absorbed and digested and do not stimulate DIT, which is more easily mobilised in the morning. Late night meals do not trigger DIT to the same extent, so that midnight take away is destined for a one way trip via your stomach, intestines, liver and those fat depots. Bon voyage!
References:
- Bean A. (2006) The Complete Guide to Sports Nutrition. A&C Black.
- Department of Health (2001) DRVs for Food Energy and Nutrients for the UK. TSO London.
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