Diet and eating advices are so abundant in our day-to-day life that I sincerely hope somebody comes up with a device that makes me disappear at will, just so I can quietly escape such conversations when I see them coming. And this is coming from somebody who loves to cook.

I sometimes hear that this or that other food is good for you because it “burns fat”. Pineapple is the most common one in my experience. To be honest I never paid too much attention to this, thinking it was just a figure of speech, a way of saying “this food is low in calories” or “this food has many elements that are good for you”. Generic stuff people say all the time. It didn’t occur to me that people actually market pineapple, or whatever else, as magic food that burns the fat inside your body and therefore helps you lose weight in the long run. And other people believe them and even force themselves to eat stuff they don’t like in the hope that it will do something useful. Losing weight by eating? Isn’t that upside down?

This was until one of my favourite Youtube scientist and general debunker decided to make a video about it. Note: the video is in Italian, but English captions are available (shameless self-ad: I added them), and I highly recommend you watch it.

This video explains in very simple terms why “fat burning” cannot exist. I am going to summarize the reasoning in a few bullet points:

  • The amount of “calories” in food describes how much energy we can produce by burning that product. The energy is defined by computing how much the temperature of a given body of water is raised during this process.
  • 1 g of fat contains 9 calories. Yes this includes the fat stored in your body.
  • 9 calories raise the temperature of 9 grams of water by 1 degree.
  • So in ideal conditions, burning 100 grams of fat produces 900 calories. Either you use all of them in a relatively short amount of time (as a comparison, moderate swimming activity for 25-30 minutes burns about 200 calories in an average adult), or it is dissipated as heat. Such heat raises the temperature of 90 kilos of water by 9 degrees.

Now even assuming that we all have 90 kilos of water in our body (most people don’t even weight 90 kilos all together, so they definitely have less), it is going to gain 9 degrees Celsius. Starting from a normal body temperature of 36 degrees, this means your body reaches 45 degrees. Regular “medical” thermometers (those you use to measure your fever) don’t even get that high, for the simple reason that you are going to be dead quite soon. Hyperthermia is a real thing: it’s even got its own Wikipedia article, which does not report how quick your demise is going to be (I checked). I am going to assume it is relatively speedy, and not pleasant either.

So your juicy pineapple might be tasty but that’s the only reason to eat it. Fair enough. This more or less confirms what I already suspected on my own. The really interesting stuff comes now.

It is actually possible to burn fat inside your body! If you don’t mind dying in the process, of course, because if you thought fat were a bitch thermodynamics is even worse. Anyway there are substances able to interfere with your metabolism, causing the fat stored in your body to start burning. The simplest such substance is called 2,4-Dinitrophenol. Apparently somebody had the idea of putting it in diet-loss pills, before people started dying and they learned exactly how much you cannot fool physics like this.

Funny interlude: between watching the video and writing this article I of course completely forgot dinitrophenol’s name. I am not a chemist, sorry. I did not feel like skimming through the video again to find its name, so I decided to be clever about it and Google several variations of “fat burning substances”. You can imagine how that went. At the third try I finally spotted my substance in the Wikipedia page listing anti-obesity medications. In the process I found several websites that are very eager to tell me all about what I should be eating to lose weight more quickly. Yes green tea is there, since if you have to believe these websites green tea will cure you of everything this side of the black plague.

I am forced to conclude that Science: 0, Internet medicine: 1. I am going to eat some pineapple to cheer myself up. Just kidding, I have got some quite tasty dry cookies with white wine around, I am going to dig into those today.