A control system is a mechanism — mechanical, biological, or otherwise — that forces a measure towards a reference. One example is a thermostat. You set the desired temperature of your house to 73 degrees Fahrenheit, and the thermostat springs into action, to get its reading to 73 °F or die trying.

The usual assumption is that a control system works like a target, and tries to correct deviations from that target. Take a look at the simplified diagram below. In this case, the control system is set to the target indicated by the big arrow, at about 73 °F. Since control is less than perfect, the temperature isn’t always kept exactly on target, but in general the control system keeps it very close, in the range indicated in blue.

However, there are other ways to design a control system. 

One way is to make a single-headed control system, that has a reference level, and simply keeps the measure either above or below that level. For example, this single-headed control system is designed to keep the temperature above 70 °F:

This is how early thermostats worked, and how many still work in practice. They do nothing at all until the temperature drops below some reference level, at which point they turn on the furnace, driving temperature upwards. Once the temperature returns above the reference level, the furnace is switched off. Barring any serious disturbances, this keeps the temperature in the range indicated in blue. 

This works fine if your house is in Wales or in Scandinavia, where things never get too hot. But what if you want to control the temperature in both directions? 

Easy. You just add a second single-headed control system on top of the first one, controlling the same signal in the opposite direction. This is a double-headed control system, that keeps the signal between two reference values: 

One “head” kicks in if the temperature gets too low, and takes corrective actions like turning on the furnace. The other kicks in if the temperature gets too high, and takes corrective actions like turning on the air conditioning. Together they form a larger control system that, barring any damage or huge disturbances, keeps the temperature in the range indicated in blue.  

(Both “single-headed” and “double-headed” are terms of our own invention. There may be official terms for these concepts in control engineering. If so, we haven’t been able to find them. We would love to hear if there are existing terms, please let us know!)

There is some reason to think that biological control systems in animals are mostly double-headed. This is due to the fact that these control systems are built out of neurons, and neural currents are in units of frequency of firing. Unlike other signals, frequency of firing can’t be negative: the number of impulses that occur in a unit of time must be zero or greater.[1]

Obesity

The current scientific consensus on obesity (link, link, link, link, link) is that it is the result of a problem with the control system(s) in charge of regulating body fat, the set of systems sometimes called the lipostat (lipos = fat). 

We can explore this idea through a few examples. For the purposes of illustration, let’s use BMI for our units. BMI isn’t perfect as a measure — obviously your nervous system doesn’t actually measure its weight by calculating BMI — but it’s a simple and familiar number that will do the trick. In general we should make it clear, all the following examples are greatly simplified. In reality, the body seems to have many control systems to regulate body weight, not just one. 

For starters, we know that the lipostat can’t be single-headed, because with ready access to food, people don’t generally starve to death, nor do they become fatter and fatter until they burst. 

Clearly body weight is controlled in both directions. This means it’s a double-headed system. One part of the lipostat keeps you from getting thinner than a certain threshold. And another, separate part of the lipostat keeps you from getting fatter than a different threshold.

On to the examples. A person with a healthy lipostat would look something like this: 

The two heads are set to different points, leaving a bit of room between the upper and the lower thresholds. This person’s weight can easily wander between BMIs of about 20 and 23, pushed around by normal behavior. But if they go above that upper limit, or below the lower limit, powerful systems kick into play to drive their weight back into the blue range between the two heads of the system.

What about someone whose lipostat is not healthy, someone who has become obese? One way for this to happen is for both heads to be pushed to higher thresholds, like so:

Here you can see that the upper head has been set to a BMI of about 35, and the lower head to a BMI of about 31. As before, their weight is mostly free to wander between those two levels. If they’re trying to lose weight, they can probably push their BMI down to 31. But it will be very hard to push it past that point, since the lipostat will resist them vigorously. After all, the lower limit is designed to keep us from starving to death, so it has a lot of power behind it. 

On the other hand, this person basically doesn’t have to worry about their BMI climbing above 35, since the upper limit is also defended. As long as their lipostat isn’t disrupted any further, they will remain within that range.

However, the heads don’t have to move together. They are at least somewhat independent systems, with separate set points. So another way to become obese is like this: 

This person still has a lower limit of BMI 20, just like the healthy person in the first example. But they have an upper limit of BMI 35, as high as than the obese person in the second example! 

This person is sometimes obese. On the one hand, unlike a person with a healthy lipostat, there’s nothing to keep this person’s weight from drifting up to a BMI as high as 35. So if they’re not “careful”, if they eat freely and without particular attention, sometimes it will.

But on the other hand, there’s nothing keeping this person from driving their BMI as low as 20, by doing nothing but eating less and exercising more. They don’t risk hitting a starvation response until they are well into the healthy BMI range, so they have little difficulty losing weight when they want to.

Lots of people find it really hard to lose weight. But you also encounter a lot of people who say things like, “when I was overweight I just decided to lose some weight, counted calories for a while, and made it happen, and it wasn’t that hard.” The double-headed model may explain the difference. Calorie-counters who sometimes drift upwards but can easily lower their weight on a whim have an altered upper threshold but a healthy lower threshold, while everyone else has had both their upper and lower thresholds pushed to obese new set points, and they face massive biological resistance when they try to return to a lower BMI.

Slightly Complicated

Our friend and colleague ExFatLoss likes to describe obesity as a slightly complicated problem. No one has solved obesity yet, but it doesn’t seem totally chaotic, so maybe there are just a few weird things that we’re missing. We agree that this seems likely, and one way that obesity could be slightly complicated is if different things are causing changes to the thresholds of the upper and lower heads of our lipostats.

To take a traditional example, perhaps eating lots of sugar raises your upper threshold, and eating lots of fat raises your lower threshold. In this model, if you eat lots of sugar but not lots of fat, your weight might drift up, but you can still control it. If you eat lots of fat, your weight is pushed up and can’t be pushed back down.

To take an example that seems more plausible to us, maybe one contaminant raises the upper threshold of your lipostat, and a different contaminant raises the lower threshold. Perhaps phthalates raise your upper threshold. This wouldn’t be very noticeable by itself, because you could still control your weight with diet and exercise. But maybe on top of that, exposure to lithium raises your lower threshold. This would keep you from pushing your weight back down. In combination, exposure to both contaminants would force you into obesity. (We should stress that this is a hypothetical, we have no idea whether these particular contaminants affect one head, or both, or neither.) 

So much for things being slightly complicated. One way that obesity could be very complicated is if there are not just two heads, but lots of them, maybe dozens. This is almost certainly the case. Biology tends to be massively redundant, so the most likely scenario is that the body has several different ways of measuring your body fat, and each of these measures probably has its own control systems. So you probably have many “upper” and “lower” thresholds, all interacting. It might look something like this:   

In this case, there are five heads making for five thresholds. The black thresholds have been forced wide open, defending a healthy lower BMI but a pretty high upper BMI. The red threshold is an additional lower defense, trying to keep BMI above 21. And the white thresholds are fixed to defending a range that’s solidly overweight to obese. This person is most likely to end up somewhere in the range that’s darkest blue, but could see movement all over the place. They won’t face serious resistance unless they try to push their BMI above 35 or below 20. But anything that raised the set point for that red threshold or the bottom black threshold would seriously limit their ability to stay lean.

Again, even this more complicated example is probably an oversimplification. While these models are good for illustration, real biology almost certainly involves more than 5 heads, defending lots of different thresholds in many different ways. 

There is at least one other way in which a person could become obese. As before, you could set the lower limit quite high, say to keep a person’s BMI above 31. Then you could set the upper limit below the lower limit, like so: 

The behavior of such a system is left as an exercise for the reader.

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[1]: The systems engineer and control theorist William T. Powers explains this idea in Chapter 5 of his book Behavior: The Control of Perception:

The “reference signal” is a neural current having some magnitude. It is assumed to be generated elsewhere in the nervous system. It is a reference signal not because of anything special about it, but because it enters a “comparator” that also receives the perceptual signal. … 

The comparator is a subtractor. The perceptual signal enters in the inhibitory sense (minus sign), and the reference signal enters in the excitatory sense (positive sign). The resulting “error signal” has a magnitude proportional to the algebraic sum of these two neural currents — which means that when perceptual and reference signals are equal, the error signal will be zero. If both signs are reversed at the inputs of the comparator, the result will be the same. The reader may wish to remind himself here of how a neural-current subtractor works by designing a comparator that will generate one output signal for positive errors, and another for negative errors. (This is necessary because neural currents cannot change sign.)