The word metabolism is thrown around a lot these days. Some say that if yours is too slow you might gain weight. You hear that some people have a "good" metabolism. Others try to sell you programs that boost it. But what exactly does this all mean? In many cases, not a whole lot.

Technically, metabolism is the word to describe all of the biochemical reactions in your body.  It's how you take in nutrients and oxygen and use them to fuel everything you do. Your body has an incredible ability to grow, heal, and generally stay alive. And without this amazing biochemistry, you would not be possible.

Metabolism includes how the cells in your body:

• Allow activities you can control (e.g. physical activity etc.).

• Allow activities you can't control (e.g. heart beat, wound healing, processing of nutrients & toxins, etc.).

• Allow storage of excess energy for later.

So when you put all of these processes together into your metabolism you can imagine that these processes can work too quickly, too slowly, or just right.

Which brings us to the metabolic rate. This is how fast your metabolism works and is measured in calories (yep, those calories).

The calories you eat can go to one of three places:

• Work - exercise and other activity

• Heat - from all those biochemical reactions

• Storage - extra leftover unburned calories stored as fat

As you can imagine, the more calories you burn as work or by creating heat, the easier it is to lose weight and keep it off because there will be fewer leftover calories to store for later. It seems like simple math, but there's a slight hitch in that thinking.

There are a couple of different ways to measure metabolic rate.  One is your resting metabolic rate (RMR), which is how much energy your body uses when you're not being physically active. The other is your total daily energy expenditure (TDEE), which measures both the resting metabolic rate as well as the energy used for work (e.g., exercise) throughout a 24-hour period.

So, what affects your metabolic rate? In a nutshell: a lot!

The first thing you may think of is your thyroid. Your doctor can check your thyroid with a simple lab test. This gland at the front of your throat releases hormones to tell your body to speed up your metabolism.  Of course, the more thyroid hormone there is, the faster things will work and the more calories you'll burn. An underactive thyroid has the opposite effect. If you have regular physicals and lab work, this is unlikely to be an unknown issue for you.

But that's not the only thing that affects your metabolic rate. How big you are counts too. Larger people have higher metabolic rates; but your body composition is crucial! 

As you can imagine, muscles that actively move and do work need more energy than fat does. So, the more lean muscle mass you have, the more energy your body will burn and the higher your metabolic rate will be. Even when you're not working out.This is exactly why weight training is often recommended as a part of a weight loss program. Who wouldn't want muscles to be burning those calories for them? 

The thing is, when people lose weight, their metabolic rate often slows down. Naturally, as you get smaller, you will require fewer calories to make your body work throughout the day; so, your RMR decreases. You can offset that effect a bit with more muscle mass.

Aerobic exercise also temporarily increases your metabolic rate. Your muscles are burning fuel to move so they're doing work. However, the key word there is "temporarily."

The type of food you eat also affects your metabolic rate! Your body actually burns calories to absorb, digest, and metabolize your food. This is called the “thermic effect of food” (TEF). You can use it to your advantage when you understand how your body metabolizes foods differently. Fats, for example increase your TEF by 0-3%; carbs increase it by 5-10%, and protein increases it by 15-30%.  By trading some of your fat or carbs for lean protein you can slightly increase your metabolic rate.

Another bonus of protein is that your muscles need it to grow. By working them out and feeding them what they need they will help you to lose weight and keep it off.

This is just the tip of the iceberg when it comes to metabolism and how so many different things can work to increase (or decrease) your metabolic rate. Fad diets and pseudo-science-based products and programs will try to sell you on the idea of raising your metabolism. Researchers still don't know exactly what makes people gain or lose weight, so eating a balanced diet and changing your body composition to have more lean muscle are still your best bet.