Have you ever wondered how and why your body regulates how much you eat and how much you weigh?
It’s a really interesting question and one I think we’ve been looking at backwards for far too long.
We know that our bodies have internal ways to regulate how much we eat, and it seems that the main drive behind how much we eat (beyond taste and social issues) is our bodies attempting to maintain an ideal amount of body fat.
Which begs the question… ideal for what??
Traditionally, we’ve said it’s for ‘energy storage,’ allowing us to have enough body fat to get us through a famine…
Yet, a 170-pound man with 10% body fat has enough body fat to last well over a month without eating at all, which makes this seem like a strange reason to continue to store copious amounts of body fat.
It also doesn’t really explain the large amount of variability when it comes to the different body fat levels.
Why do some people easily maintain a body fatness of around 10% body fat, while others maintain at over 50%?
Instead, what I’m going to argue is that we optimized our body fat for temperature, or more precisely our ability to thermoregulate in a given ambient temperature, and I’ll tell you why I’m making this argument.
To start, when it comes to the ‘energy in’ side of the equation, we can only control how much we eat at a meal, and how often we eat meals.
To accomplish this, our brains receive continuous information about energy stores and energy fluxes in critical organs as well as information about food that is being eaten and absorbed, and about basal and situational energy needs.
Let’s start with the regulations of how much we eat at any given meal…
When we think about how much we eat at any given meal, we know that our meals end long before any physical limit of the stomach is reached.
In other words, we get full, yet if someone forced us, we could keep eating…
This happens because during meals, signaling molecules such as cholecystokinin and PPY are released and cause us to feel full and thus stop eating.
Their secretion tends to be proportional to the caloric content of meals, with larger meals eliciting a significantly larger response and thus a stronger feeling of being full.
Interestingly, you release less cholecystokinin and less PPY when you are in a cold environment. So you eat more in the cold, and you feel ‘less full’ because of the cold.
Conversely, you eat less and get fuller quicker in the heat.
This creates an interesting connection between how much you eat and the temperature you are exposed to.
However, there is another hormone that actually causes you to become hungry and to begin to eat; this hormone is ghrelin.
Fasting increases plasma ghrelin, and injecting people with ghrelin increases food intake.
Ghrelin is lower in the heat, and higher in the cold. So the drive to initiate eating is higher in the cold and lower in the heat.
This means that the current ambient temperature affects how much you eat and how often you eat on an acute level, which is excellent evidence that temperature plays a pivotal role in your overall levels of body fat.
However, your body fat levels also play a role in how much you eat. Your body and your body fat produce signaling hormones that provide your brain with an ongoing signal that is in direct proportion to total body fat.
These signals are insulin and leptin.
Insulin is secreted in basal amounts throughout the day, with large spikes occurring during meals, and both components of total insulin secretion (i.e. basal and meal-stimulated) are directly proportional to body fat levels.
Leptin is secreted in direct proportion to total body fat levels, following a diurnal pattern with less direct connection to meals than insulin.
Intriguingly, both basal insulin and leptin levels are influenced by ambient temperature, effectively muting the signal when you are cold, and turning up the signal when you are hot.
More direct evidence that your body fat and how much you eat is directly related to the ambient temperature of your environment.
The other evidence we have is animals who live in the cold have higher metabolic rates, higher activity levels, yet higher body fat and body mass than those that live in the warmth.
This suggests that fluctuations in body fat may be tied to changes in temperature exposure, and not just to protect us from famine.
The bottom line is temperature matters.