I’m a big fan of the saying
“Knowledge is Power” and since the majority of us are here to change our bodies, and consequently improve our health, understanding how our bodies work makes sense. Like any machine, the body needs fuel (calories) to function, but what does this mean when fuel (calories) is something that we must restrict to reach our goals? I often think it’s like walking a tightrope. In any event, here’s a little information – hopefully it will prove helpful to someone!
[copied from
Weight Loss Information - Metabolism and linked articles]
Metabolism
What is metabolism? In a nutshell, metabolism is the sum of all chemical reactions within organisms that enable them to maintain life. It includes all physical and chemical processes within the body related to body functions. Processes of energy generation and use; including nutrition, digestion, absorption, elimination, respiration, circulation, and temperature regulation.
In very simple terms, your metabolism is the rate at which your individual body engine operates as it performs all it's bodily functions, like the creation/building of various substances (heat, muscle, proteins, enzymes, storage fat, bones) and the breaking down of others (food, storage fat, etc.). Both the building process (anabolic) and the breaking down process (catabolic) occur simultaneously, every moment. The fuel for all the chemical reactions which make up the metabolic process, is food.
How Metabolism/Metabolic Rate Affects Weight
Your metabolism affects weight loss, weight gain and weight maintenance, because it determines how many calories you need. If you have a slower metabolism (you burn calories more slowly) your body needs fewer calories.
What Determines Overall Metabolic Rate or Your Total Daily Energy
Expenditure(TDEE)
In simple terms, three factors contribute to your overall metabolic rate and the amount of energy (calories) you need to sustain your body weight.
1. Your Basal Metabolic Rate (BMR)
Sometimes referred to as Resting Metabolic Rate (RMR), BMR is the number of calories you burn at rest, when sleeping or simply sitting in a chair. This accounts for about 60 percent of all calories used by the body.
2. Your Additional Physical Activity
This accounts for about 30 percent of all calories used by the body.
3. Dietary Thermogenesis
Caused by the Thermic Effect of Food (TEF), dietary thermogenesis is the energy calories required to digest and process the food you eat. It accounts for about 10 percent of energy needs.
Basal Metabolic Rate (BMR)
At 60% of all energy used by the body, BMR is the largest factor in determining overall metabolic rate and how many calories you need to maintain, lose or gain weight.
BMR is determined by a comination of genetic and environmental factors, as follows:
- Genetics. Some people are born with faster metabolisms; some with slower metabolisms.
- Gender. Because men have a greater muscle mass and a lower body fat percentage, they generally have a higher basal metabolic rate.
- Age. BMR is greater in childhood than in adulthood. After 20 years, it drops about 2 per cent, per decade.
- Weight.The more you weigh the higher your BMR will be. For example, the metabolic rate of very overweight women is 25% higher than that of thin women.
- Body Surface Area. This is a reflection of your height and weight. The greater your Body Surface Area factor, the higher your BMR. Tall, thin people have higher BMRs. If you compare a tall person with a short person of equal weight, then if they both follow a diet calorie-controlled to maintain the weight of the taller person, the shorter person may gain up to 15 pounds in a year.
- Body Fat Percentage. People with a higher body fat percentage, have a lower BMR than those with a lower body fat percentage - all other things being equal. Muscle cells contain tiny little power centers called mitochondria, which are where calories get 'burned up.'' The greater percentage of lean muscle tissue in the male body is one reason why men generally have a 10-15% faster BMR than women.
- Diet. Starvation or serious abrupt calorie-reduction can dramatically reduce BMR by up to 30 percent.Restrictive low-calorie weight loss diets may cause your BMR to drop as much as 20%.
- Body Temperature/Health. For every increase of 0.5C in internal temperature of the body, the BMR increases by about 7 percent. The chemical reactions in the body actually occur more quickly at higher temperatures. So a patient with a fever of 42C (about 4C above normal) would have an increase of about 50 percent in BMR.
- External temperature. Temperature outside the body also affects basal metabolic rate. Exposure to cold temperature causes an increase in the BMR, so as to create the extra heat needed to maintain the body's internal temperature. A short exposure to hot temperature has little effect on the body's metabolism as it is compensated mainly by increased heat loss. But prolonged exposure to heat can raise BMR.
- Glands. Thyroxin (produced by the thyroid gland) is a key BMR-regulator which speeds up the metabolic activity of the body. The more thyroxin produced, the higher the BMR. If too much thyroxin is produced (a condition known as thyrotoxicosis) BMR can actually double. If too little thyroxin is produced (myxoedema) BMR may shrink to 30-40 percent of normal. Like thyroxin, adrenaline also increases the BMR but to a lesser extent.
- Exercise. Physical exercise not only influences body weight by burning calories, it also helps raise your BMR by building extra muscle. So you burn more calories even when sleeping.
How to Calculate Basal Metabolic Rate (BMR)
There are several methods of determining BMR.
Harris Benedict Formula
The Harris Benedict equation is a calorie formula using the variables of height, weight, age, and gender to calculate basal metabolic rate (BMR). This is more accurate than calculating calorie needs based on total body weight alone. The only factor it omits is lean body mass and thus the ratio of muscle-to-fat a body has. Remember, leaner bodies need more calories than less leaner ones. Therefore, this equation will be very accurate in all but the very muscular (Harris-Benedict will under-estimate calorie needs) and the very fat (Harris-Benedict will over-estimate calorie needs).
Katch-McArdle Formula
If you know your body fat and lean body mass (body muscle) percentage, you can obtain a fairly precise estimate of your BMR. For example, the formula from Katch & McArdle takes into account lean mass and therefore is more accurate than a formula based on total body weight.
Both forumals are available
online here
Dieting, Calories and RMR
Calories determine fat loss. If you eat more calories than you burn, you will not lose fat, no matter what foods or food combinations you consume. It's not possible to circumvent the laws of thermodynamics and energy balance. To burn fat you must create a calorie deficit between calories-in and calories-out. This calorie deficit forces your body to use stored body fat to find the required energy and make up the deficit.
There are approximately 3500 calories in a pound of stored body fat. So, if you create a 3500-calorie deficit through diet, exercise or a combination of both, you will lose one pound of body weight. If you create a 7000 calorie deficit you will lose two pounds and so on. The calorie deficit can be achieved either by calorie-restriction alone, or by a combination of fewer calories-in (diet) and more calories-out (exercise). This combination of diet and exercise is best for lasting weight loss.
[the following material copied from Dave Greenwald's
Power Store News
How low is too low?
When one restricts calories the RMR slows. It's believed this is an adaptive process to prevent starvation as the body is perceiving the calorie deficit as a form of famine. Researchers have been surprised to learn, however, that what effect they believed calorie restriction would have on the RMR was not so. It appears the body reduces the RMR greater than what would be expected for "X" level of calorie reduction. This creates a perfect environment for an eating disorder and the "yo-yo syndrome."
If one restricts calories too severely and also exercises the opposite net effect may occur with respect to weight loss and increasing lean mass. Even If one attempts to restrict calories too severely and add muscle through weight lifting the dieter may become weaker and not stronger. The dieter may also lose muscle rather than gain it. It makes sense if you think about it:
Muscles require adequate energy (calories) to grow. Muscles are a nutrient dense source the body can use for fuel if calorie restriction is too severe. When one tries to exercise and is overly restrictive with calorie consumption muscle is used as a fuel and nutrient source. If that same person is exercising, the muscle wasting will be enhanced because the body needs even greater fuel than if the person wasn't exercising. Since this person isn't consuming the calories necessary to sustain the RMR+Food +Digestion+Activity the body will not only NOT grow more muscle, but will use the very muscle they're trying to add to.
For women the problems of severe calorie restriction continue. Restrained eating is also associated with ovulatory disturbances that are related to bone health. It is now well-accepted that low energy intake is a correlate to amenorrhea and oligomenorrhea and that, in this reduced estrogen environment, women are at increased risk for the development of early osteoporosis. The amenorrhea
and associated problem of poor bone development or maintenance may increase the risk of stress fractures (ACSM's Health & Fitness Journal, July/August 1999).
Lyle McDonald, author (The Ketogenic Diet) and noted body building trainer gives some sound advice for would-be dieters. He states that any restriction over 1000 calories per day below maintenance is asking for muscle loss and the inability to add lean tissue regardless of the weight training activity endured. For many, 1000 calories is excessive and Lyle further states that reducing calories below 1200-1500 per day is likely counterproductive for maintaining lean muscle or growing any new tissue.
This applies to LC as well as to 'traditional' dieting. You've heard of the Metabolic Advantage, no doubt, but were you aware that it translates to ensuring you don't eat too little? When you eat a high fat diet and burn fat as your primary fuel source fat grams are actually closer in calorie number to 5 per gram than 9.
-Nat
Your body, the machine.
.
Hybrid Mode
