Why do people with fast metabolisms don’t win weight as easy as people with normal metabolisms?


Edit: I meant “gain weight” and not “win weight”.

In: Biology

I think you mean gain in your question. I think the difference is seen with fast and slow. Metabolism is the process of turning food into energy. If you have a fast metabolism you turn food into energy quickly, meaning that the body isn’t holding onto the food in the form of fat to be later used as energy. It is just releasing it now. If you have a slow metabolism, just the opposite is happening. Your body is storing the food as fat for later use. This just means your body is hoarding fat.

The notion of a ‘fast’ or ‘slow’ metabolism isn’t really accurate. There are some processes that go beyond simple calories-in/calories-out though:

1. Gut bacteria. In your body, the food is converted to energy by bacteria. These bacteria tend to specialized. To simplify a bit, you have ‘Twinkie bacteria’ and ‘celery bacteria’, each excelling at converting a certain kind of food into energy. The more you eat of a given type of food, the more the affiliated bacteria thrive and become efficient at converting it into energy. If all you ever eat are Twinkies, you have a gut full of Twinkie bacteria that are experts in extracting every last iota of energy from a Twinkie. If all you ever eat is celery, you have a gut full of celery bacteria that do their level-headed best to find any sort of energy in what is (essentially) indigestible fiber.
Needless to say, the person who develops their Twinkie bacteria at the expense of their celery bacteria is going to gain a lot of weight from eating Twinkies and almost none at all from eating celery. The celery bacteria guy is going to be the reverse – and since Twinkies are enormously more calorie-dense than celery, they’re going to be a lot thinner.
2. Resting calorie consumption. Your body needs a certain amount of energy simply to exist, largely based on weight and musculature. The heavier you are and the more muscle you have, the more calories you consume just being you. That’s why weight training tends to be more effective than aerobic exercise for losing weight: it raises the base consumption level so even if you take a day off, you’re still burning calories while lounging around. It’s also why people’s weight tends to ‘plateau’ after a certain point – even with a bad diet, it’s tough to eat your way past a certain weight.
3. Normal activity varies. If you take two people with seemingly identical lifestyles and attach a step monitor to them, you’ll often find that one will have 3 – 4 times the number of daily steps as the other. Or, as my mother used to put it: “it’s easier to keep off the weight with a son than a daughter”. These variations in activity are simply part of who you are rather than a conscious effort and can shift your calorie consumption a few hundred on a daily basis.
This is about as close to it gets as ‘metabolism matters’. People with legitimate metabolic issues – such as low thyroid function – tend to engage in less activity because they have less energy to do so. However, it’s not the *metabolism* that’s at fault – it’s the fact that they only got up to get a drink of water 2 times during the TV marathon rather than 5 times.