Many factors effect the speed of metabolism,including:

1. Environment:
for example, if you live in a friendly and happy family or country, you are more relaxed, metabolism will slow down, it is easy for you to gain weight.
If you are born in a troubled family or country,your metabolism will be fast.

2. Genetics/personality:
for example, some people are calm and laid-back naturally, this makes them more relaxed, thus slow down metabolism,they gain weight easily;
Some people are born to be aggressive,active,sensitive,perfectionism,ambitious and alert, they change their moods easily,this makes their metabolism fast, it is not easy for them to gain weight,no matter how hard they eat.

3.Intelligent:
some people hardly think, they are “indifferent” to things, some may call them “stupid people”, their metabolism is extremely slow,they get fat easily and more likely than not,heath is compromised.
While some people think a lot,a small matter will cause him to think a lot,they can detect any nuances and small changes,they never stop thinking, how to do this,how to do that,why this,why that….their metabolism is very fast.
Lots of cases can prove this: for example,one skinny man suddenly became very large and fat after a head injury which caused him to become a retarded person.when your brain is dead and insensitive to any social matters,you are literally an animal, your metabolism is slow to minimal, you will get very fat if fed regularly.

Sexuality:
People with strong libido (high Testosterone)have very fast metabolism,those with “relaxed” libido (that is,low libido) have very slow metabolism.

Pressure:
People who live under constant pressure have fast metabolism,those without pressure have slow metabolism.
Of course pressure can be also linked with environment,genetics, personality and libido. For example,same problem,some may think it a big pressure, some don’t feel pressure at all.

Now let’s get to some myth that some people still believe:

1. Myth one: Eat more you will gain weight.
This is not necessarily so, if your metabolism is fast, no matter how hard you eat, you will never gain that much.
If you have slow metabolism, even you eat a little,you still gain weight;Actually many fat people eat much less than skinny ones. The key is metabolism.
Several experiments have proved that this myth is wrong.

2. Myth two: some people overweight and some people skinny is because different digestion and absorption of your food.
overweight people have good digestion and absorption,while skinnies don’t.
At a glance, you may think it makes sense, but after a second thought, you will realize,this is incorrect.

Many skinny people eat a lot, if they have bad digestion and absorption, how come they have so good appetite?
Actually lots of people with stomach and intestine problem (poor digestion) still get overweight.

To sum up, Why are some people overweight and some people skinny? the big factor and the invisible hand is: fast metabolism and slow metabolism!

In fast metabolism mode, you burn your energy faster than intake, in slow metabolism mode,it is opposite.

There is one article in BBC website mentioning that one experiment showed it is not due to metabolism, it is a hoax.