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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!

Reference:

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.

When we talk about healthy metabolism, how do we know that it is healthy or messed-up? Doctors and scientists do certain tests to look at metabolic markers that are discussed at the bottom of the page. But you do not have to visit a doctor to know if your metabolism is working right. There are simple ways to tell if things are good at home using minimal equipment.

Measure your aerobic fitness level

Aerobic fitness level, or aerobic power, is the single best description of your health, because it is the most direct measure of what your metabolism is capable of. It is your body’s ability to output energy (mechanical force and heat) by burning chemical energy sources using oxygen. Your aerobic power depends on many things:

• ability of your heart to effectively move blood through your body,
• ability of your lungs to provide oxygen,
• ability of vascular system and blood to deliver oxygen to cells,
• your muscle mass,
• blood flow through your muscles,
• ability of your muscles to burn food into energy using oxygen.

Scientifically, aerobic power is measured as your maximum rate of oxygen consumption under maximum exercise load on a treadmill or similar machine and when you reach your maximum heart rate. It is called VO2max and is expressed in liters per minute (L/min) or milliliter pres minute per kg of body weight (mL/(min*kg).

But you can get and quick estimate of your aerobic power without all that equipment. Aerobic respiration fully kicks in after about 2 minutes of high intensity exercise. The time you can maintain aerobic energy production after 2 minutes is a pretty good estimate of your aerobic metabolism.

So the test is: determine how much time you can run. Not sprinting, but not jogging either – run at about half maximum pace. This will get your heart rate close to its maximum.

• under 2 minutes. You have almost no aerobic power left in you. You metabolism is in very bad shape. Expect big problems.
• 2 to 10 minutes. You are just one step above very unfit. You need to take serious steps to become fitter, otherwise things will get worse pretty soon.
• 10 to 30 minutes. You are at average fitness level.
• 30 minute to one hour. You are very fit.
• More than one hour. You belong to the elite club of very healthy people. Just keep living the way you always did, because whatever you have been doing was right.

I see people objecting to this measure because it does not take into account age and sex. As far as gender – yes, females get a little slack. Lower every number by 15%. As far as age, common sense tells us that these numbers should be lower for older people since older people are known to be less healthy. But think about it: the fact that most people do not take necessary steps to maintain their body healthy is exactly the reason for such an expectation. After all, we know many examples of people who did take those steps and could run marathons in their 60s. So you cannot be “healthy for your age”.

You are either healthy or unhealthy. If you are old and have little aerobic power left, face the truth: you are not “normally unfit” for your age, you are simply unfit.

You level of aerobic fitness is the most reliable predictor of your health. Fact number 1: lean and unfit men have double the risk to die of any cause than lean, fit men. Fact number 2: lean and unfit men were higher mortality risk than obese (high BMI, see below), fit men.

Measure your resting heart rate

Another quick and easy measure of your metabolism is your resting heart rate. The more efficient your cardiovascular and respiratory systems at bringing oxygen to cells, the less often your heart has to pump blood through your body. Sit comfortably for five minutes, then count your pulse.

• Under 50 beats per minute Congratulations! You are as healthy as a human being can be. Your aerobic metabolism is at the athlete’s level. Continue living the way you always did.
• 50 to 60 bpm You are very healthy. Keep maintaining your healthy lifestyle.
• 60 to 70 bpm. You are normally healthy.
• 70 to 80 bpm. You are in average health category, but the trend is bad. Time to start thinking about including more exercise and general activity in your lifestyle, or things will get worse over next few years.
• 80 to 85 bpm. You are in poor cardiovascular health category. Start taking necessary steps now or expect big problems pretty soon.
• Above 85 bpm. You are extremely unhealthy. Time to go write a will, because every day could be your last one.

Here are a few facts about heart rate: 1) for every 10 bpm increase, your risk of death from any cause is up 25% (45% if you have a history of heart disease); 2) If your bmp is above 84, your risk of death is 80% up relative to normal range (159% if you have a history of heart disease).

Are you overweight ?

Being overweight or obese is a manifestation of improperly functioning metabolism.

Most often, you weight is assessed using Body Mass Index, or BMI. For adults, BMI is your body mass in kg divided by your height in meters squared:

BMI = mass(kg)/heigh(m)2

The following are BMI ranges:

<18.5: low

18.5–24.9: normal

25–29.9: overweight

>30: obese

BMI can be somewhat misleading, however, because it does not take into account what part of your body weight is muscle and what part of it is fat. A lot of athletes would actually be overweight by this measure.

So a better quick measure of your obesity level will be to look at specifically the amount of fat your body is storing. There are ways to do it precisely, but for a quick assessment just look at the mirror.

Can you grip a handful of belly fat at about your bellybutton level? Most of extra fat your is deposited at the abdominal area of your body. If yes, then you are most likely in the overweight category.

Look at the trend of your weight change. Has your weight been pretty stable in the last 10-20 years? If yes, that is good news, because it means that your energy metabolism is properly balanced and you are in the stable weight zone.

If your weight has been slowly increasing over last few years, even if it is by a couple of pounds per year, your metabolism is out of balance. Your energy intake (food) is way above your energy expenditures and you are in the weight gain zone.

See my article here about energy balance. Fat gain is actually the last thing your body is going to do to balance out its energy intake and output, after all other options are exhausted. So being in the weight gain zone means that your metabolism is seriously messed up. Here is a fact: those who gained 15 kg (about 30 pounds) in adulthood have 5.5 higher risk of developing diabetes compared to those who kept their weight stable. Those who lost 10 kg (20 pounds) had about twice lower risk of diabetes. It is always better to be closer to a weight loss zone. Here is a graph of this research (from Haffner, 2006):

Being obese is bad, but being obese and physically unfit (see my aerobic fitness test above) is a lot worse. Obese/unfit is absolutely the worst category. Expect 3 times higher risk of death from any cause and elevated risks for most chronic diseases and cancer.

Determine your body composition: fat content

Relatively cheap devices are out there to measure fat content in your body precisely. They work by measuring electrical resistance of your body. Fat is better insulator than muscle, so if you know resistance, you can calculate fat content (height, weight, age are also taken into account) Some of them measure total body resistance through feet, some through hands, both do a good job.

Fat content is a much better measure than BMI, because it really is a measure of your body composition, not just total mass.

Below are guidelines for fat content (normal fat content is highlighted):

Note that these “recommended ranges” are simply based on the spread of population for fat content, they just show how BMI is related to fat content at different ages. It does not mean that these numbers are your targets or indicators if good health. Given what we know now about weight gain in adulthood (see above), the recommendation I can make is: stay lean or even get leaner as you age. You can maintain and build up muscle and burn fat successfully at any age using a combination of aerobic and resistance exercise, and limiting your calories in food.

Scientific (medical) metabolism assessment

Glucose tolerance

This is a very common metabolic measure when your body’s ability to metabolize glucose is measured. You are given 75 g of glucose, about the same amount as two cans of Coca-Cola, and then your blood level of glucose and insulin is monitored over 2 hours. Insulin is your body’s response to glucose – this hormone tells muscle and liver cells to absorb and metabolize it.

The faster your body can bring glucose level back to normal and the less insulin it needs to accomplish the goal, the better is your glucose control mechanisms. Higher insulin response means that your cells are more resistant to insulin, which is a signature of type 2 diabetes. Your response will look something like this (from Hamburg, 2007):

On this picture, black squares are people who spent 5 days totally inactive resting in bed (But not sleeping! Sufficient sleep time improves glucose response).

Lipid profile

Lipid profile, or lipid panel, or simply “cholesterol” is determined primarily to assess the risk of cardiovascular disease. Lipid profile can also be used for assessment of conditions such as atherosclerosis and diabetes.

Lipid profile includes determination of several markers, some of which are signs of good, healthy metabolism, and others are signs of problems. The table below summarizes markers of lipid profile:

	Abbreviation	Good health	Bad health
Fat, triglyceride, or triacylglycerol	TG, TAG	low	high
Apolipoprotein B	ApoB	low	high
High density lipoprotein, “good cholesterol”	HDL	high	low
High density lipoprotein to total cholesterol ratio	HDL to total	high	low
Low density lipoprotein; small dense particles; “bad cholesterol”	Small LDL, or sdLDL-C	low	high
Low density lipoprotein; medium particles	Medium LDL	low	high
Low density lipoprotein; large particles	Large LDL	high	low

There is still an argument going on about what markers on this list are most reliable predictors of problems. Most agree on using HDL to total cholesterol ratio as a marker of good health (low risk of CVD) and small LDL-C as a marker of bad health (high risk of CVD).

Most people are not familiar with all these subtleties about “good” and “bad” cholesterol and think that cholesterol and fat are the culprits. This story has gotten a lot of publicity, as can be inferred from a variety of products labeled “zero cholesterol” and “zero saturated fat”. Consuming more cholesterol with food will not necessarily mess up your blood lipid profile, just like consuming more fat will not necessarily make you fatter.

Our body has about 30 grams of total cholesterol and is able to make cholesterol at a rate of about 1 g per day. If you eat more cholesterol with diet, your body will shut down production of its own, provided that your metabolism is working properly. 1 gram of cholesterol is the amount contained in about six eggs.

What will indeed mess up your lipid profile most are these four things in this order:

• Insufficient physical activity
• Over-consumption of calories in any form
• Insufficient or irregular sleep time
• Large amount of refined carbohydrate in diet (sweet sugars and starch)
• and influences that will improve your lipid profile are the ones opposite to listed above.

Blood pressure

High blood pressure may either be a sign of poor salt and water metabolism, poor kidney function, or poor cardiovascular system function. I will not go into more detail on this here, because there are a lot of resources available. I will just mention that using combination aerobic-resistance training, it is possible to significantly reduce your blood pressure.

References:

Effect of changing heart rate during treatment of hypertension on incidence of heart failure.
Okin PM and others. Am J Cardiol. 2012 Mar 1;109(5):699-704.

All-cause and cardiovascular mortality in relation to changing heart rate during treatment of hypertensive patients with electrocardiographic left ventricular hypertrophy.
Okin PM and others. 2010 Sep;31(18):2271-9.

Healthy percentage body fat ranges: an approach for developing guidelines based on body mass index.
Dympna Gallagher and others. Am J Clin Nutr September 2000 72: 694-701

Relationship of metabolic risk factors and development of cardiovascular disease and diabetes.
Haffner SM. Obesity (Silver Spring). 2006 Jun;14 Suppl 3:121S-127S.

Physical inactivity rapidly induces insulin resistance and microvascular dysfunction in healthy volunteers.
Hamburg NM and others. Arterioscler Thromb Vasc Biol. 2007 Dec;27(12):2650-6.

The role of reducing intakes of saturated fat in the prevention of cardiovascular disease: where does the evidence stand in 2010?
Astrup A and others. Am J Clin Nutr. 2011 Apr;93(4):684-8

Cardiorespiratory fitness, body composition, and all-cause and cardiovascular disease mortality in men.
Lee CD, Blair SN, Jackson AS. Am J Clin Nutr. 1999 Mar;69(3):373-80.

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