The Thermodynamics of Losing Weight

Thermodynamic is the study of the flow of mass and energy.  Because mass and energy cannot be created or destroyed, by balancing the mass and energy flows in a system, you can determine useful things.  For example, if you know how much energy is contained in fuel going into a car and how much energy is contained in the exhaust gases leaving the tail pipe plus how much energy is lost to friction, you can determine how far the car can go on a tank of fuel.

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Thermodynamics can be applied to weight loss as well since the body is a system.  This is a good way to determine if the newest fad diet might work or it is all full of false promises.Because everyone would love to eat whatever they want and not exercise yet stay the weight they want, there are all sorts of scams out there.  Let’s look at the thermodynamics involved to see what will work and what won’t.

The basic equation

You may not be a big fan of math and equations, but don’t worry, the equation here is pretty simple.  This is the first law of thermodynamics, which balances the energy going into a system with that coming out:

Prouduction of energy + Energy In – Energy Out = Change in storage 

So, the difference between the amount of energy produced in a system plus the energy going into a system and the energy coming out is equal to the change in the amount of energy in the system.  This relationship works for a jet engine, an air conditioning system, and the human body.

Apply this to the body

OK, so how can we use this equation to understand weight loss?  Well, energy is not produced in the body, unless you’ve swallowed some plutonium or some something, so that term is gone.  Energy in is the food we eat, which is measured in the calories we consume.  Energy out is a combination of the energy we use and that which is lost by, um, other means.  Change in storage is how much fat our body creates to store excess energy that we have not used for later.  Muscle and brain cells are also storage of energy, so it’s not all bad.

So, put all of this together and you get:

Change in weight = Calories in through food – calories out through waste – calories burned through exercise – calories burned by metabolism

If you want to lose weight, you need to make the calories you take in through food be less than the calories you expel plus the calories you burn exercising plus the calories you burn just keeping your body working and warm.  It is really that simple.

Calories taken in through food

Obviously, one way to lose weight would be to take in fewer calories.  There are many fad diets that advocate changing the kinds of food you eat, saying that you can eat all the green beans you want if you cut out the dinner rolls and cake.  This may seem like a magical way to lose weight while still eating all you want, but the truth is that things like rolls, pasta, and cake have a huge number of calories compared to things like vegetables.   It’s still really about the calories.  

It isn’t the carbohydrates and sugars that are making you gain weight, but the huge number of calories those foods contain.  For example, if you go to Applebees and get their triple chocolate meltdown, which is just a modest piece of chocolate cake with a scoop if ice cream, you’ll take in 1070 calories!  That’s about half of the average calorie content most people should have in a day.  A whole cup of green beans is only 31 calories.  You could eat a whole gallon of green beans and still not take in as many calories as you had in that simple dessert.  (There may be other reasons to avoid sugar and carbohydrates, such as control of your blood sugar levels.)

Losing weight by reducing the number of calories you take in is fairly simple.  You just need to start keeping track of how many calories you take in and figure out how many calories you can consume while maintaining the same weight.  Then, to lose weight, you just eat fewer calories. 

Tracking calories also has the benefit of allowing you to find out which foods have a lot of calories so that you can avoid them or at least eat smaller portions.  (Just split that triple chocolate meltdown and you’ve cut 535 calories!)  There are a lot of aps that make keeping track of calories fairly simple. 

An alternative plan to cutting calories would just be to leave some things out.  Since most people eat enough calories each day to maintain their weight, leaving off a side item or skipping dessert will result in you taking in less than enough calories than needed for weight maintenance, meaning that you will start losing weight.  Instead of getting a burger and fries, just get the burger.  Instead of getting the large coke, get a small one.  Instead of a steak with a baked potato and a salad, just eat the steak and salad.

Calories lost through waste

Some diet foods or supplements claim that you can lose weight by not absorbing calories.  For example, a few years back there was a fat substitute on the market, Olestra, that your body would not absorb, meaning that it just passed through.  The issue with this approach is that things can get a little messy.  Some diet supplements claim that they block absorption of calories.  While this may sound great, many of these claims are questionable.  Our bodies normally do what they were designed to do with food – digest it.

Calories burned through exercise

Energy is energy.  If you move yourself from your desk to the mall ten miles away, it takes the same base amount of energy whether you walk, jog, or drive your car to move you from your desk to the mall.  Beyond the base amount of energy needed to move yourself, there is an additional amount which is lost as heat to the air.  Because you’re moving the whole car when you drive, you’re using more energy to drive to the mall than you will if you walk.  Taking a scooter will burn less energy than taking a heavy car.

Note that energy is different from power.  Power is the rate at which you are using energy.  Running takes a lot more power than walking, so you may burn as many calories running for 15 minutes as you do walking for 30 minutes.  If you run a mile in 10 minutes, however, you’ll use about the same amount of energy as you will walking the same mile in 20 minutes.  You’re using less power to walk but you’re doing it for twice times as long.  Intense exercise, like running and swimming laps, burns more calories than mild exercise like walking, so choose more extreme exercises, if you can handle it, if you don’t have a lot of time to spare.  Alternatively, just find ways to work exercise into your day, like walking from the back of the parking lot or between buildings, going up and down the stairs each time you go to the bathroom, or doing a few squats in your cube between tasks.

Because the energy needed to do exercise must come from somewhere, exercising more is a good way to reduce the number of calories you store and lose weight.  If you take a hike, play a game of voleyball, or go for a long bike ride, your body has no choice but to supply you with the energy needed to accomplish these tasks.  (OK, you could end up passed out on the side of the road, but this is not too likely.)  If you continue to eat the same amount but add some form of exercise, the thermodynamics say you’ll lose weight.  Depending on the exercise, you might also start building up muscle, which will increase the amount of energy your body burns when you’re just sitting there.  Just be sure you don’t increase your calorie intake because of all of that exercise.  

Burning calories just by sitting there

If you have a friend who seems to be able to eat all that he wants but never gain any weight, he probably has a high  metabolism.  While you may envy him, realize that he’ll be the first to go should society collapse while us low-metabolism folks will be able to survive for months on our fat reserves.  But I digress….

Your body naturally burns calories to keep you warm, supply all of your cells with the nutrients they need, and keep your lungs inflating and deflating.  When you’re young and growing your metabolism is working overtime, which is why you can eat whatever you want as a teenager and never gain an ounce, but just looking at a doughnut causes you to gain a pound when you’re 46.  Unfortunately, there isn’t a lot you can do about your metabolism since it is mainly a product of your heredity and age.

That said, there are some things you can do to increase how many calories your burn by just sitting there.  For example, muscle burns more energy than fat, so lifting weights to build up muscle will cause you to increase your resting energy usage.  Choosing exercises like cycling, swimming, and jogging, which build up muscle, will also help.  Keeping yourself a little cool by keeping the heat down a little in winter and not layering up quite enough will also help burn calories since your body must then work harder to maintain your core temperature, but this is not very pleasant.

Some weight loss plans and supplements claim to be able to cause you to lose weight by increasing your metabolism.  There are also some foods that claim to boost metabolism.  It is good to be skeptical since no one has really found a no-work way to lose weight yet, but you can try some different things out and see if they have an effect.  If you do find something, please let us know in the comments for this post.

Note, one issue with weight loss through reducing calories is that you lose muscle along with fat, which causes your metabolism to decrease.  There are many stories of people who lost 50 pounds or more through dieting the first time.  Often it was fairly easy to do.  They then go back to their old eating habits, however, and gain the weight back and then some.  The next time they try to use the same diet to lose the weight, it doesn’t come off like it did the first time.  What has happened is that they have lost muscle so that their resting energy usage has decreased.  Their body may have also gone into starvation mode, reducing their metabolism to conserve calories. 

Unfortunately, once the damage is done this can be difficult to reverse.  It is therefore not a good idea to go on extreme diets where you drastically cut calories.  Instead, cut calories morderately, perhaps increasing them again briefly if you find that you are no longer losing weight before dropping back down again.  Make changes that you can live with for the rest of your life rather than going on eating regimends that you can only do for a month or two.  Replacing empty calories like sugars with proteins, which help build muscle, along with adding mild strength training to your plan can also help to combat muscle loss and metabolism reduction.

Putting it all together

Looking at the basic equation again,

Change in weight = Calories in through food – calories out through waste – calories burned through exercise – calories burned by metabolism

here’s what thermodynamics says is the best way to lose weight:

  1. Start a weight loss plan is to take some data.  Write down everything that you eat, figure out the energy you burn through exercise, and adjust your exercise and your calorie intake until you maintain the same weight for a week.  You can then figure out how many calories you burn just through your metabolism and waste products, which is:


Calories burned through metabolism and waste products = Calories eaten – calories burned through exercise  

This is how many calories you can take in and not do anything other than your normal daily routines and maintain your weight. 

2.  At that point, start to look for minor reductions in calorie intake and additions in exercise that you can do to start to use more calories than you take in each day.  Start to track your weight weekly.  Shoot for weight loss of a few pounds per week, or maybe 10 pounds per month. 

3.  If you hit a plateau that lasts more than a a few weeks but you have been hitting your net calorie targets, your metabolism may have adjusted.  Look for ways to boost your metabolism such as muscle gain.  Increasing your calorie intake a little while you also burn those extra calories through strength training/additional exercise may also help.

4.  Once you get within your target range, gradularly increase your calorie intake until you are maintaining the weight but not gaining.  Continuing to do exercise will help keep you healthy overall, so it is good to continue to do some exercise even if you don’t need it to lose weight.  (Not all of those who are skinny are healthy.) 

Evaluate that diet

If you see a new fad diet, take a look at it from the thermodynamic standpoint.  If it doesn’t somehow change your energy balance so that you reduce the number of calories that end up being stored, it will not work.  It also might hurt you, depending on what’s involved.

(Note, I am a rocket scientist, not a dietician or a physician.  Before you start any exercise regimine or diet, consult with a doctor to be sure it is healthy for you and that there are no physical issues that could result in medical issues should you start an exercise or dietary program.)

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Disclaimer: This blog is not meant to give financial planning or tax advice.  It gives general information on investment strategy, picking stocks, and generally managing money to build wealth. It is not a solicitation to buy or sell stocks or any security. Financial planning advice should be sought from a certified financial planner, which the author is not. Tax advice should be sought from a CPA.  All investments involve risk and the reader as urged to consider risks carefully and seek the advice of experts if needed before investing.


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