How much does metabolism actually vary?

This has been bugging me for some time now. I know some people say they have "fast" or "slow" metabolisms, but by how much does it actually vary between individuals? Is it like only a 100-200 cal difference, or can the "fast" metabolism people truly each a billion calories and not gain weight? Or is it that they just eat less/are more active without paying attention to their eating/activity levels, so it seems like they eat "whatever they want" but they really are within a normal persons calorie limits.

Also, how much does cardio, weight training, eating, etc. affect your metabolism. I'm more or less looking for a numerical value, as I already know it speeds it up.

Anyone know anything about this? Anyone?

The thing about metabolism is that it's still pretty much a mystery to everyone. It depends on your genes, really.

I have a friend who's still skinnier than me even though I eat 1200 calories a day and exercise 4 days a week, and she eats at least double that with barely ANY exercise. I've read magazine articles about people who can't seem to get a healthy BMI reading no matter how healthily they eat or how much they exercise.

Of course, there are things you can do to improve your metabolism (ie. exercise, eat fibre, break meals into smaller, more frequent meals). But other than that, the bottom line is that you're basically stuck with what you've got.

Thanks for asking this question, because this issue just drives me bananas.  I agree with Umi, I think that there is a great deal that science just doesn't know about metabolism.  Anyway here are some of my thoughts.

1. Conservation of energy: This is the things that trips me up more than anything else.  Iit takes a certain amount of energy to perform a certain amount of work.  Moving 150 pounds 1 foot requires a certain amount of energy.  Period.  No exceptions.  (Not including friction etc.) So how is it that metabolism can vary so much?  My understanding based on my reading is that fit people tend to burn otherwise unused calories as heat, while unfit people are more likely to store unused calories as fat. 

But I had a friend who ate 1200 calories a day, ran 3-7 miles a day, and was overweight.  It just doesn't make sense.  Sometimes I think the human metabolism may be the proverbial perpetual motion machine.  Like Umi wrote, I saw a TV program about this really fat guy, who had completed like 20 Iron Man length triatholons.  Cardiovascular ability off the charts.  And his body looked really toned, like a walrus or something, but still fat.  It just doesn't make sense.  I mean it has to take a certain amount of energy to move a guy that big 26 miles.

2. Things that increase metabolism: Logically if you did everything that every article and fitness guru said would increase your metabolism, your metabolism would be so fast you would spontaneously combust.  I think this must be a case of diminishing marginal returns, i.e. you can only get your metabolism so fast.  Each beneficial thing will increase it some, but at a certain point you stop seeing a benefit. 

I've read that each pound of muscle burns 10 calories a day, hence the idea that increasing your lean muscle mass increases your metabolism.  But that's not very much.  10 additional pounds of muscle means you burn 1 banana more worth of calories a day.  Of course that will add up over time.

3. A calorie is a calorie is not a calorie:  OMG.  Where to begin.  Again the conservation of energy.  At a certain point all calories must be the same, unless there is some flaw in the way we measure, which I think is done by actually burning the food in question, which isn't exactly what your metabolism does, so maybe that's a problem.

My understanding is that fat calories are preferentially stored as fat, but again if you're consuming less than you expend then it shouldn't matter, right?  Dean Ornish wrote in "Eat More While Losing Weight" that it takes 25% of the energy of a carb or protein calorie to convert it to fat.  So maybe a calorie is not a calorie.

Above all try and reach your own misinformed conclusions.  There is so much disagreement on these issues (another thing that drives me crazy) between fitness experts, doctors, and scientists, they can't possibly all be right.  I think part of the problem is that the demand for information on this topic far exceeds the actual knowledge and information available. 

it varies from person to person really, theres some people on here who weight the same as me- around 90kg, but eating 1500cals for maintainence, even with exercise, and im eating around 2800-3100 for maintainence with slight exercise, so it can vary, its mostly genes though. other factors include height, muscle mass, thyroid conditions, etc.

Metabolism varies, between the VAST MAJORITY of people, about +/- 5%.  Anyone telling you they eat half as much, excercise just as much, and still gain weight, are not telling you the whole truth.  There are too many excuses in this "industry" and people need to stop lying to themselves.

Excuses are human nature, and those telling you they can't lose weight have mental barrier, not a physical one.  There are very few exceptions, and people latch onto the exceptions as the rule.  There is no way that 70% of the US population (overweight or obese) are exceptions to the rule.  And the rule is, diet and excercise.  And I can tell just by looking around, they aren't dieting and excercising !!!