It's Not Your Metabolism
Now I'm going to stick my neck out here and say it's not usually because of a slow metabolism. It's usually due to an underestimation of calories being consumed from food and drink; or an overestimation of how much they're doing in the gym... they're not doing as much as they think!?
Our resting metabolic rate (RMR) is the number of calories your body burns at rest, just carrying out it's bodily functions, regulating your body temperature, hydration, breathing etc...
This won't vary greatly from person to person, maybe only a couple of hundred calories or so. This is because our organs require a lot energy to function; the brain, heart, lungs, kidneys, liver make up most of our RMR.
After that it's muscle tissue. At rest, muscle tissue uses around 10-15kcals per kilogram of muscle, per day. So, for example, if your friend has an extra 10kg of muscle compared to you, they only burn an extra 100-150kcals per day... about the equivalent 20-30g of milk chocolate... not a huge amount.
On top of RMR, our body also uses energy digesting food. This called the thermic effect of feeding (TEF). This raises metabolism slightly, but not a significant amount and also needs to be balanced against the amount of calories you're eating, so it doesn't contribute much to your overall metabolism.
Next comes activity. This includes the energy you use while training and from non-exercise activity, like walking around, shivering, fidgeting etc... So these 2 together are where you can add a significant chunk to your metabolic rate. Non-Exercise Activity Thermogenesis (N.E.A.T) is the amount of calories you burn outside of training sessions. People who have more labour intensive jobs, such as builders, or gardeners tend to have a higher energy expenditure due to N.E.A.T. People who have more sedentary jobs, tend to have a lower expenditure here. This is the first place where you can significantly and positively alter your metabolism. By making more conscious decisions to be more active, like walking from the car park, or train station to the office; taking the stairs instead of the lift; going for a walk on your lunch break; or getting up every hour, then you can start to increase the number of calories you burn from N.E.A.T.
The second place you can start to increase your metabolism is by training with weights. This is because you not only burn calories during the session, but you also damage the muscles, which increases the breakdown of protein in the muscles and promotes the growth of muscle protein. Add to this a higher protein diet (to further increase muscle protein growth), then it's this increase in muscle protein synthesis that increases your metabolism.
So the take home message:
You can't really control your metabolic rate, or how many calories your body burns.
What you can do is move as much as possible on a daily basis. Train with weights to promote muscle growth.
Get a decent portion of protein with meals to help with muscle protein growth.
This is the stuff that YOU CAN CONTROL
Hopefully this gives you a few ideas on how you can rev your metabolism up as much as possible, however if you do have any questions, drop me an email and I'd be more than happy to answer them