Does fast food make you fat? Well, it’s complicated. According to a recent video released by the Brit Lab, it's not exactly the food but rather the fat in it that can lead to weight gain. Unfortunately, pound for pound, fast food usually has a higher fat content than what you might whip up for yourself at home.

The British National Health Services recommends that the average man consume around 2,500 calories a day to maintain his current weight. For women, this number is closer to 2,000. But while counting calories has never been easier thanks to new apps and careful labeling, at the end of the day all this counting is moot without a solid understanding of what a calorie actually is.

Calories are used to measure the amount of energy in food but it's not until you hold these foods to the fire that you realize just how much stored energy they have. For example, in the video below, Brit Lab’s Greg Foot lit a piece of lettuce and a potato chip on fire. While the lettuce flame quickly went out, the chip burned on. Foot explained that most of this extra energy, or calories, was due to the chip’s high fat content.

In order to fully understand the correlation between fast food and weight gain, Foot decided to push his body to the limit and consume 4,500 calories over his daily limit by eating fast food meals. Though the number may sound monumental, Foot explained that it’s far easier to do than you may realize, seeing as our bodies have evolved to crave the salt, sugar, and fat these types of foods are jam-packed with.

Unfortunately, Foot learned the hard way just how much more difficult it is to burn off calories than it is to consume them. For example, although Foot easily consumed nearly 7,000 calories in his day, after just over an hour of solid non-stop exercise, he had only managed to burn around 1,200 calories. So, in the end it seems that fast food isn’t inherently bad, but its high fat content plus our low physical activity that can lead to weight gain.