Your Metabolic Thermostat

In my first book, A Pound of Cure, I introduced the idea of a metabolic thermostat that controls your body weight.  Just like the thermostat in your house that controls the temperature of the room by delivering either cool or warm air in response to the ambient temperature, your metabolic thermostat adjusts your hunger and metabolic rate in response to your body weight.  This concept views your excess body fat as a tightly controlled physiologic parameter, rather than a simple receptacle for extra calories.  Your metabolic thermostat regulates your hunger, metabolic rate, food choices, and willingness to exercise through a complicated interplay between your brain, gut, fat stores, circulating hormones, and the nerves that course throughout your body.  The details of the inner workings of your thermostat are better left to physicians and research scientists. It’s not necessary to understand how it works, just what it does.

Before going into the details of how your body works to regulate your fat stores, let’s discuss the way your body works to control other vital physiological parameters like your heart rate, blood pressure, or blood glucose level.  There is a tight network of sensors throughout your body that work to detect changes in a vital parameter and trigger positive and negative feedback responses to restore the parameter to the normal level.  A good example of this is your blood pressure.  When you stand up quickly, your blood will pool in your legs and cause an immediate drop in your blood pressure.  This will be detected by the millions of sensors that exist in your blood vessels and will trigger an immediate response to increase your heart rate, squeeze down on the blood vessels, and make your heart beat stronger all to counteract this blood pressure drop.  The result will be your blood pressure returning to normal within a fraction of a second after you stand up.  As the blood that has pooled in your legs begins to circulate again, your blood pressure will return to normal and your heart rate will slow down, the blood vessels will squeeze less, and your heart will go back to its normal contractile strength.  

When we look at our body’s regulation of our fat stores, we find a similar system that works to preserve our precious fat stores through periods of famine or starvation. Fortunately, starvation is not a problem we face today in the Western world, but our physiology has not yet gotten this message.  In the modern world, our body’s response to starvation is now triggered predominantly by our attempts at weight loss, rather than it’s intended function, to prevent us from dying of malnutrition.

All of us have a particular body weight that we tend to center around from week to week or even month to month.  One week you may eat healthier, exercise more, and drop a few pounds. The following week, you lose focus and your weight drifts back up, but it will usually center around a predetermined specific weight.  This is because your metabolic thermostat has a set point that regulates your fat stores to keep them relatively stable over time.

When the change in your dieting and exercise habits is more significant, your metabolic thermostat shifts into overdrive.  During periods of starvation (dieting), you will lose a few pounds which causes your body weight to drop below the set point.  This drop will trigger your metabolic thermostat to work to restore the lost weight by increasing your brain’s focus on food (hunger) and by slowing your metabolism down to reduce your body’s need for calories.  

 If you’ve ever thought that your body was set on preventing you from losing weight- you were right.  It’s important to remember that our body’s physiology has evolved over hundreds of thousands of years.  It is only over the last fifty years that diabetes has become a bigger risk than starvation.  Because of this, your metabolic thermostat is designed to minimize your risk of dying of malnutrition.  A caveman who did not experience much hunger had little desire to brave the cold rain to go out to hunt and forage for food.  This did not bode well for his survival.  Furthermore, those individuals that could not down regulate their metabolic rate in response to periods of starvation did not stand a good chance of making it through the long, cold winter without burning up their excess fat stores.  A caveman’s survival was directly linked to his ability to survive on a few hundred calories a day during those periods of famine that inevitably occurred each year.  These same forces that ensured our ancestors’ survival ten thousand years ago are now working directly against us as we try to lose weight by restricting our calorie intake.  

We’ve all been told that weight loss is driven by willpower and the discipline to resist our hunger pangs, but this is difficult if not impossible for most of us.  Resisting a physiologic drive is possible for short periods of time but, uniformly, will fail in the long run.  Let’s use the same “willpower argument” that is used to explain why dieters fail and apply it to a similar exercise in resisting your physiologic drive – breath holding.  All of us can hold our breath for 10 seconds and resist our brain’s drive to breathe.  After 20 seconds, many of us will start to experience significant air hunger and be unable to focus on anything except getting your next breath.  At 30 seconds, one minute, or even two minutes for the most determined, we will eventually succumb to our physiologic drive for oxygen and will give up and take a breath.

Resisting your hunger over time is a similar exercise in futility.  Many of us can ignore our hunger for several weeks, and the more disciplined for several months. But inevitably, as our body weight drops and our metabolic thermostat releases stronger and stronger hunger signals, our constant exposure to food will win out and we will stray from our 1200 calorie a day diet.  As we will discuss later, this does not mean that you cannot lose weight by modifying your food intake, it just means that simply reducing your calorie intake is not the way to do it in the long run.

Like all the other systems in our body, there is never just one way that our physiology works to preserve our fat stores.  In addition to modifying your hunger in response to periods of decreased calorie intake, your body will also adjust the rate that it burns calories.  When you reduce your calorie intake, your metabolic thermostat works to decrease the rate that you burn calories.  This explains why you lose 4-5 pounds the first week (on a 1200 calorie a day diet), 1-2 pounds the new few weeks, and eventually “plateau” and stop losing weight without changing your diet.  Eventually, your metabolic thermostat changes your metabolic rate to ensure that you are only burning 1200 calories a day so that you don’t lose any more of your precious fat stores.  If you are motivated and get to the gym every day, get on the treadmill, set it to calorie mode and walk or run until the machine tells you that you’ve burned 300 calories, your metabolic thermostat will adjust to this increased calorie expenditure by down regulating your metabolic rate even further that night as you sleep, directly sabotaging your weight loss efforts.

Furthermore, your metabolism is controlled through many different avenues.  It is not just an oven that burns inside you, burning calories and giving off heat.  Rather, it is controlled in a primal center of your brain that adjusts your desire and motivation to move subconsciously.  If your metabolic thermostat is working to down regulate your calorie expenditure, you will fidget fewer times per hour, be less likely to go workout at the gym, and more likely to take the elevator over the stairs.  Your subconscious will trigger you to preserve your precious fat stores and your conscious mind will not be able to overcome it.

The increase in our hunger and slowing of our metabolic rate in response to starvation is a response that all of us understand since we’ve experienced it first hand.  However, there is another set of responses that occur if we’ve eaten too much that are opposite of those we see in response to starvation.  If we gain a few extra pounds, our metabolic thermostat will work to normalize our body weight back to our set point.  We will decrease our hunger and find we have little interest in food.  Additionally, we will be triggered to move more, take the stairs, and walk further and faster in an effort to increase our metabolic rate.  We see this each New Year after the winter holiday feasts result in a few (or more) pounds of weight gain.  Come January 1st, most of us have a body weight a few pounds above our set point.  Our metabolic thermostat responds by decreasing our hunger so that compliance with that 800 calorie fad diet seems like a reasonable undertaking.  Our metabolism is also increased, so waking up early to go to the gym seems like a good idea.  We’re all convinced that this year is going to be the year we finally get rid of those extra pounds. That is until we lose a few pounds and drift below our set point.  The hunger returns with a vengeance and our metabolism downshifts making us more likely to press the snooze button than wake up for the early morning workout.

The overfed functionality of our metabolic thermostat is as important as the starvation side for two reasons.  First, bariatric surgery works by harnessing this side of the thermostat so that our physiology is working to trigger weight loss- not prevent it.  Second, it’s a malfunction of this side of the thermostat that results in weight gain in the first place.   Whether we choose to lose weight by having weight loss surgery, changing our diet, exercising, or even taking medications, we have to ensure that we are working to lower our metabolic thermostat’s set point, rather than trying to fight against it.  If we are able to lower our set point below our current body weight, the overfed functionality will drive our physiology to decrease our hunger and increase our metabolism.  This is the holy grail of weight loss. It’s where our body is actually helping us to lose the weight rather than fighting against our attempts.

One last important point about our metabolic thermostat concerns its “width.”  Most people are able to lose around 10% of their total body weight before the increase in hunger and decrease in their metabolic rate becomes insurmountable.  I’ve found this fact to be uncannily accurate.  A 250 pound woman is usually able to lose 25 pounds before she loses focus and starts to regain weight.  While this is true for most of my patients, there are several exceptions.

First, young people are often able to lose 20% or more of their body weight before they begin to hit the dreaded plateaus that ultimately sabotage even the most determined dieter.  Men seem to be more resistant to the impact of age on the width of their thermostat.  Most women begin to lose their ability to lose more than 10% of their body weight in their 30’s while men are often able to extend this into their 40’s.

There is also a subset of people (around 10% by my estimate) that preserve their ability to lose and maintain more than 25% of their body weight through calorie restriction.  This is likely the small group of people who do find success living by the “eat less, exercise more” theory of weight loss.  There is little understanding of the physiologic factors that make these people resistant to the hunger and metabolism changes that occur as a result of starvation that plague the rest of us.  For this unique group, obesity may in fact be a lifestyle choice.  When they clean up their diet and exercise calorie restriction and portion control, they lose weight effortlessly, without plateaus.  When they consume large amounts of calorie dense foods, they develop significant obesity.  Unfortunately, our society views the success and failure of these folks and extrapolates it to the rest of us.  I am completely convinced that most people are unable to lose and maintain more than 10% of their total body weight through traditional dieting methods.  If you are among this majority, your focus cannot be on developing better methods of starving yourself. Instead you should focus on how to lower your body’s set point.

Before we start to explore the ways that we can lower our metabolic thermostat’s set point, we should first examine the reasons that our set point becomes elevated in the first place.  Any attempts to lose weight must start by eliminating all factors that trigger set point elevation.  Weight loss is hard enough without having to fight against medications, food addictions, or other factors that are triggering your body to increase your fat stores.