Metabolism
Metabolism
[Cueball, on the left, and White Hat are sitting on chairs on either side of a table, facing each other. They each have plates of food and glasses of some beverage set in front of them. Each has picked up a portion of food on a fork to eat it.]
Cueball: I have one of those metabolisms where I can eat whatever I want and my body converts it to energy and stores the excess as fat.

Eating is fundamentally a process where energy from food gets absorbed into the body in order to drive every cellular process in the body.  Energy that is absorbed but not needed in the short term gets converted and stored as body fat. This is called metabolism . Consuming too much food and not exercising enough are major factors for obesity , which is a problem in many first world countries today, especially in the United States .
For obese people, losing weight is often an enormously difficult task.  Standing in stark contrast, there are also lean people who do not seem to ever gain any weight even though they appear to eat whatever and however much they want.  This leads some people (including the lean people themselves) to believe that one can have a special metabolism where excess food energy somehow does not affect the body.  This belief is common, though not supported by scientific evidence.  The comic makes fun of that kind of notion.  While Cueball describes to White Hat how his metabolism is "special" (the phrase "one of those" implicitly meaning unusual), he is in fact only describing the normal case: no matter what he eats, his body converts the food to energy and stores any excess food as fat which stays in his body for future use.
The title text stretches this further, telling about the normal habit of drinking water (and the consequences of not drinking it) as something odd.  Starting to feel bad at first and eventually dying if refraining from drinking for too long a time are perfectly normal consequences of dehydration. This was also touched upon in 1708: Dehydration , in which Megan spent all day researching whether low-grade dehydration is really a thing -- ironically forgetting to eat or drink at all, to predictable results.
Obesity has only fairly recently become a public health issue due to lifestyle changes brought on by technologies such as industrialization and trade.  Human bodies evolved under conditions where it was hard to ever find enough to eat, so to store as much excess energy as possible as fat was a beneficial adaptation.  Historically, stored fat would be consumed during hard times that was sure to come.  The act of collecting food through farming or hunting/gathering also demanded physical labor which limited the amount of excess energy that would remain.  In comparison, people nowadays hardly need to expend any energy to buy their food from a nearby market.  They also have much more sedentary lifestyles and rarely ever go hungry.  Without an active commitment to exercise more or eat less, there would almost never be a shortage of energy and no chance for body fat to be used. Randall has previously shown how bad his health becomes when he starts eating lots of fat (or sweet) food in 418: Stove Ownership .
There are many rational explanations for why some people might not gain weight despite eating a lot.  For example, it's possible that they only eat a lot during special occasions and social gatherings, where they are easily seen eating.  On more private occasions when no one is watching, they could just as well eat much less or even skip entire meals.  They might also lead a much more active lifestyle and thus require more energy than an average person despite their thin appearance.  Other less pleasant reasons might include chronic diseases, parasite infections, or eating disorders.
This is the second comic in a row about food, the previous being 1743: Coffee .
[Cueball, on the left, and White Hat are sitting on chairs on either side of a table, facing each other. They each have plates of food and glasses of some beverage set in front of them. Each has picked up a portion of food on a fork to eat it.]
Cueball: I have one of those metabolisms where I can eat whatever I want and my body converts it to energy and stores the excess as fat.
