Weight Loss

The Body’s Fat Thermometer


Author’s Note: As I write this article, the world is in the middle of the COVID-19, or corona virus, pandemic. As are millions of other people around the world, I am living under a “shelter in place” order for at least another few weeks, possibly longer. Literally everything we see, hear, and read these days is about coping with this life-changing event. So rather than give you one more article highlighting the virus and more coping strategies, I’ll instead write on the topic I’d planned to before the virus struck. I hope this will take your mind off it for a little while, at least. Be safe and healthy, everyone – we will get through this.


Although you might not believe it, permanent, long term weight loss isn’t about cutting calories and exercising more. This has been proven in countless studies, and also the countless frustrations of dieters desperately cutting calories and depriving their bodies of nutrition.

So what then is the key to successful weight loss? According to nephrologist Dr. Jason Fung, the solution lies in controlling your body’s ‘thermostat’ — what’s also known as Body Set Weight (BSW).

Think about how a home’s thermostat works to control the desired room temperature: in the summer when it’s hot, the thermostat turns on the air conditioning. In winter, it detects the temperature is too cold, and turns on the heat. The house stays at the perfect temperature despite varying outdoor conditions.

The human body’s BSW, also called an appestat, is essentially a thermostat for body fatness. There are also many powerful satiety mechanisms built into our physiology to make us stop eating. For example, the stomach has ‘stretch receptors’ that signal when it’s too full. The body also has powerful satiety hormones, such as peptide YY and cholecystokinin, that stop us from overeating.

The BSW sets an ideal body fatness that it defends just as the house thermostat maintains the temperature. If we’re too skinny, it can trigger the body to gain weight. If we’re too fat, it triggers a higher metabolic rate (total calories burned at rest) to lose weight. The body tries very hard to maintain its BSW in the original position. This directly contradicts the calories in/ calories out theory that simply eating too many calories causes body fatness, without taking into consideration the BSW, satiety hormones, or other physiological signals. In fact, if you deliberately overeat, your body will try to burn it off.

Think about it:  a ‘calorie’ is not a physiologic notion. The body doesn’t have ‘calorie’ receptors and doesn’t know how many calories we eat, or don’t. A calorie of carbohydrate is metabolized entirely differently from a calorie of fat or protein. The concept of ‘a calorie is a calorie’ is, honestly, pushed heavily by processed food companies. They want to convince you that 100 calories in a sugary drink is the same as 100 calories in an avocado, in terms of weight gain. Or that 100 calories of sugar is as fattening as 100 calories of kale.

Consider artificial sweeteners as well. They have no calories, and so fool our taste buds –  but won’t fool our appestat. If all we had to do to lose weight was eat fake sugar and fake fat and no calories, we’d all lose weight and there’d be almost no obesity crisis, or Type II diabetes crisis. But these are real, despite all the artificial sweeteners.

So what to do?

First, think again about a home thermostat. Suppose it’s set at 70 degrees, but we want the temperature indoors to be 65 degrees. So, we bring in a portable air conditioner. Initially, the temperature will go down – but then the thermostat kicks in and turns on the heat, returning the room temperature to 70. We keep adding more portable air conditioners to cool it down, and the thermostat keeps cranking up the heat to get it back to 70. It’s a futile, no-win battle.

So how about just turning down the thermostat instead?

Decreasing calories to lose weight gets the same result as adding air conditioners to cool off a room – because doing so completely ignores the BSW, or thermostat. Suppose your BSW is set at 150 pounds, but you want to weigh 120 pounds. Conventional advice says cut 500 calories per day to lose 1 pound per week. Initially, your weight may go down to 140 pounds, but then the appestat kicks in to make you gain weight. You become hungrier, and your metabolism slows down in order to regain the weight. So then what do most of us do? Keep restricting calories! But guess what? The body responds again by slowing our metabolism even further.  This is a continual fight against ourselves in an ultimately futile attempt to lose weight.

Turn down the appestat or BSW

Obesity is a disease caused by excessive insulin, not excessive calories. In other words, it’s a hormonal imbalance, not a caloric one. Insulin, which is produced by the pancreas, does several things. First, it transports glucose (sugar) inside of our cells to be used in the chemical reaction that creates energy. When more insulin is produced than is needed, the body is signaled to store the extra food energy as fat. When we fast, insulin goes down, and we burn some of that stored energy. That’s we don’t die in our sleep every night!

Just like a room thermostat, the BSW uses a negative feedback loop. What this means: excess insulin leads an increase in the size of fat cells. Fat cells, in turn, produce more of the hormone leptin which then signals the brain that ‘we’re too fat’. What happens then? Appetite decreases, we stop eating so much, and insulin levels drop. This signals the body to start burning fat instead of eating and storing it, and to return to the original, desired BSW.

Body weight thermostat model

The BSW is, in essence, the balance of insulin effect versus leptin effect. In obese individuals, the insulin effect has prevailed over the leptin effect. This could be for many reasons, but eating foods high in refined (not whole) grains, eating frequently, and eating lots sugar all keep insulin levels high, despite leptin’s best efforts to curb appetite to lower insulin. On the other hand, if insulin is extremely low, such as with Type I diabetes, the body loses weight continuously, no matter how many calories are consumed.

So as fat cells stay over-filled, they produce more and more leptin in an attempt to fight insulin. However, if the root problem hasn’t been addressed – eating too much sugar, too many refined carbohydrates, eating constantly – then insulin levels also continue to rise. When a persistently high level of a hormone exists, resistance to the that hormone will develop.

Leptin resistance is virtually universal in common obesity. With leptin no longer able to keep up the fight, insulin takes over and causes continual weight gain. The insulin vs leptin battle has been lost, and the BSW thermostat is reset upwards.

Body weight thermostat in OBESITY

The key: lower insulin levels

Cutting calories will not reduce insulin’s effect. Instead, the BSW is unaffected and the body desperately tries to regain the lost weight. Eating frequently means constant stimulation of insulin as well.

The key to losing weight then is to help in the Insulin vs Leptin fight. Leptin is already maxed out, so the only thing left is to lower insulin. How to do that?

  1. Eat less sugar.
  2. Eat less refined grains (whole grains are okay – they have many nutritional benefits).
  3. Eat moderate protein and more natural fats.
  4. Don’t eat constantly.
  5. Eat more real, unprocessed foods.
  6. Exercise: proven to reduce insulin resistance; builds more muscle fibers; allows excess body fat to be burned for fuel; increases resting metabolism.

So there you have it. You don’t need some special diet, expensive supplements, or pre-packaged foods to lose weight. You already have the tools within your own body to get that thermostat down to where you’d like it to be.


Laurie Kelly is a Precision Nutrition Certified Level 1 coach, specializing in behavior-based nutrition coaching. She is also a NASM-certified Personal Trainer and Corrective Exercise Specialist, a Road Runners Clubs of America Certified Adult Running Coach, and a Certified Professional Triathlon Coach by the International Triathlon Coaching Association. She’ll help you make lasting positive changes that will keep you healthy for a lifetime. Contact her hereor follow her blog at www.dragonfly-fitness.com.

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