How to Roast Hen of the Woods Mushrooms
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Home » Nutrition myths put to the test

Meet your metabolism

Submitted by on May 8, 2009 – 8:54 pmNo Comment

dreamstime_71893371If you’re confused by how freely magazines, fitness trainers and marketers of food or supplements throw around the term “metabolism” when trying to convince you that they have the solution to your weight loss problems, then you’re in good company.  When it comes to this mysterious entity called your “metabolism,” it seems that everyone wants to sell you the secret to speeding it up.  Of course, most of these metabolism-boosting claims rest on a basic assumption: that you don’t actually know what your metabolism is and therefore, you don’t really understand how it works.

Meet your metabolism

Your metabolism can be defined as the sum total of all of the biochemical activities that take place in your body: the chemical reactions to break down food into usable energy, the chemical reactions to build new cells and tissues… and all of the chemical reactions in between that you need to facilitate life, like transporting oxygen to cells, removing waste from the body, maintaining adequate body temperature, etc..   You see, every single one of these reactions requires energy.  And the rate of energy expenditure that you need to complete all of these chemical reactions while at rest is called your basal metabolic rate (BMR). It’s this BMR that most people are referring to when they talk broadly about your “metabolism.”

Your metabolism is determined by a number of different variables; for example,  your age, your gender, your height, your body composition, genetics, and your general state of health.  Assuming you were at complete rest, the amount of energy (which we measure in calories) you could eat without gaining or losing any weight would be equal to your metabolic rate.  However, since people are seldom at complete rest, the total number of calories we need to maintain our weights in actuality would equal our BMR + all of the calories we expend in going about our daily activities.  On average, our BMR comprises about 50-70% of the total energy we expend in a given day.  ~10% of the energy we expend comes from the “thermic effect of food”: or, the amount of energy it takes for us to digest, process and store the food we eat.  The remaining 15-30% is from physical activity.

Metabolic myths vs. metabolic facts

So by now it should be clear that there are a few different approaches to losing weight: you can increase your activity level until you burn more calories than you eat (or reduce your calories to achieve the same effect), or you can increase your BMR so that your body automatically burns more calories, even when it’s at rest. Since the BMR comprises the bulk of energy we burn in a day, you can imagine why speeding up the metabolism is the holy grail of diet and fitness gurus alike.

But there are really only a few things that are scientifically proven to SPEED UP your metabolism.  These are:

  1. A higher proportion of lean body mass (muscles): Muscle tissue is more “metabolically-active” than fat tissue, meaning that muscle cells use up more energy when they’re just sitting around than fat cells do.  The higher proportion of your total body mass that is composed of muscle tissue, the more energy your body needs to expend while at rest.  Simple.  If your bathroom or gym scale has one of those body fat analysis functions, give it a whirl.  They’re not the most accurate, but if you use the same one over and over again  under the same conditions, you should still be able to track directional trends in your body fat composition to a reasonable degree over time.
  2. Get a fever: for each degree farenheit increase in body temperature above normal, your BMR increases by 7%!  OK, so perhaps contracting the flu is not the most practical weight loss strategy, but it does account for why people lose weight so quickly when they get sick.
  3. Caffeine (sort of): As far as caffeine goes, the research does seem to show that 300-500mg of caffeine (the equivalent of 2-6 eight-oz cups of normal drip coffee, or just 1 to 1.5 “grande” (16 oz) cups of Starbuck’s Pike Place Roast coffee) will increase BMR by about 1o% for a duration of about 4 hours.  This may explain why so many so-called weight loss pills and beverages contain so much caffeine.  Of course, if you’re getting your caffeine in the form of 300-700 calorie Frappuccinos, you can forget about any net benefit as far as energy-burning goes.  Note also that about 300mg/day of caffeine is an amount that’s considered safe by most health authorities, so while most people can get away with a 2 eight-oz cups a day for a little metabolic ‘boost,’ I wouldn’t push my luck and drink much more than that.  At higher doses, it can cause side effects that range from irritating (sleeplessness, anxiousness, diarrhea, twitching) to, in rare cases, dangerous (heart palpatations or arrythmias, mania/depression, psychosis).

While there are some random, teensy studies (we’re talking along the lines of 10 subjects, in some cases) that suggest certain random foods can have a modest and short-lived impact on increasing your metabolic rate (e.g., hot peppers, green tea), the bottom line is that none of these effects appear to last long enough or are pronounced enough to make a meaningful dent in your overall metabolic rate.  Also, in many studies, even when certain substances (like green tea) were shown to impact metabolic rate test subjects, it didn’t translate into a weight or fat reduction.  So I’d be doing you a disservice to focus on these foods as promising metabolism-boosting solutions.  (Besides, would you *really* want to go on a Jalapeno diet?)  Insofar as green tea does contain caffiene, there may be a subtle effect stemming frm that.

Conversely, these are a few things that are scientifically proven to SLOW your metabolism.  These are:

  1. Aging. Starting in your 20′s, your BMR slows by a few percentage points each decade.  According to a 2005 review of the literature on aging and energy expenditure by researchers at the USDA Human Nutrition Research Center on Aging at Tufts University, this decline equates to about 150 daily calories per day LESS burned at rest, per decade. You do the math: by the time you reach age 50, that’s 450 calories PER DAY that you could eat with impunity as a teenager that you will no longer automatically burn. So if you’ve ever wondered why you were able to eat whatever you wanted in college and not gain a pound and now you don’t eat any more than you did then but can’t seem to stop gaining weight, this is likely one of the culprits.  Note that this decline is not inevitable, and can be counteracted by increasing your proportion of total lean body mass (muscle) to help compensate for this age-related travesty.
  2. A higher proportion of body fat:  Fat tissue is less “metabolically-active” than muscle tissue, meaning that fat cells use up less energy when they’re just sitting around than muscle cells do.  The higher proportion of your total body mass that is composed of fat tissue, the less energy your body needs to expend while at rest.
  3. Starvation/Skipping meals: When you don’t eat, your body uses up stored carbohydrate (called glycogen) from your liver and muscles to keep it running.  But after about a day or two of not eating, these reserves will be used up and your body goes into a ‘starvation’ mode to help preserve itself.  This includes breaking down muscle to use the proteins to build essential new cells, enzymes, blood proteins or hormones; breaking down stored fat to use for energy; and slowing down the rate of all non-essential biochemical processes to help conserve energy.  This means slowing down the metabolism.  Research has shown that the BMR will slow down to the tune of 30-40% in cases of actual starvation— as in cases of anorexia, famine or hunger strikes.  That means your body adapts to use at least 30% less energy to keep itself functioning during times of severe stress.  But you don’t need to be fasting for days to have this effect: even going long stretches without eating during the day, like when you skip breakfast or skip lunch, can have a negative impact on your metabolic rate.  (Granted, it won’t be nearly so dramatic, but…) Ideally, spacing your daily intake into smaller, more frequent meals throughout the day will help prevent your metabolism from slowing down unnecessarily.  I tend to prefer eating 4-5 smaller ‘meals’ per day rather than 3 big ones for this reason.  (And also because I get hungry every 3-4 hours, of course).
  4. A lower total body weight: When you lose weight, your total body mass is decreased, which means it takes less energy to sustain its basic functions.  This helps account for that plateau effect when you’re dieting.  If you’re successfully losing weight on a specific calorie intake and all of a sudden you get stuck and can’t lose any more, chances are your metabolic rate has caught up (caught down?) with your lower calorie level.  This means you’d have to cut out even more calories (or burn more through increased activity) in order to keep losing.  *Sigh*  No good deed goes unpunished.

Which takes us to our last category.  These are common things I’ve seen people CLAIM will speed your metabolism, but in fact, DO NOT.

  1. Eating protein: Eating protein in and of itself will not speed up your total daily energy expenditure in any appreciable way. The claim is likely based on a theoretical extrapolation from the observation that breaking down protein requires a bit more energy than breaking down other nutrients (it is said to have a higher ‘thermic effect’).  In theory then, it is suggested, that people with diets higher in protein should burn a higher amount of total energy.   But I haven’t come across any substantial research that demonstrates that this theory translates into actual practice, since a) the thermic effect of protein is not that much greater than it is for other foods, and because the combined total thermic effect of all our food still only accounts for a small percentage (only about 10%) of our total energy expenditure.  However, if you eat a little bit of protein after doing enough strength-training exercise such that you end up building new muscle tissue, then at the end of the day you will end up with an increased BMR. So all is not lost.
  2. Drinking water: I have no idea where this claim even comes from, so I can’t comment any further than to say that guzzling water will not speed up the rate at which your body expends energy.
  3. B-vitamins:  Several B-vitamins are required as co-factors to break down food and turn it into usable energy, which is indeed one aspect of your metabolism.  However, having sufficient (or extra) B-vitamins does not increase the amount of energy your body actually needs to comple its metabolic tasks.  Therefore, B-vitamins cannot be said to speed up your metabolism.  And on a related note, I have not come across ANY nutritional supplements–vitamins, herbs or minerals– that can speed up your metabolic rate.  Though there is no lack of products that claim they will.

Measuring your metabolism

So now that you know what it is, how do you measure it?

In fact, there are several ways.

One of the most accurate assessments would be to use a machine that measures the exchange of gasses when you breathe– how much carbon dioxide you expire and how much oxygen you inspire.  This process is called indirect calorimetry. As it turns out, this is mathematically related to your metabolic rate and therefore will provide the most accurate estimate of how much energy you consume.  Alas, your best chance to have access to a machine that does this is to be hospitalized in the ICU.  Not worth it.

The next best way, then, would be to use one of many mathematical formulas developed to give general estimates, using variables that you plug in to the equation.  My favorite such equation is called the Mifflin-St. Jeor equation, and it goes a little something like this:

For males: resting energy expenditure= 10 x (weight in kg) + 6.25 x (height in cm) – 5 x (age in yrs) + 5

For females: resting energy expenditure= 10 x (weight in kg) + 6.25 x (height in cm) – 5 x (age in years) – 161

(To get your weight in kg, divide your weight in pounds by 2.2.  To get your height in cm, multiply your height in inches by 2.54.)

The answer will give you a pretty good approximation of how much energy your body uses at complete rest.  To figure an estimate of your TOTAL energy expenditure that includes activity, you can multiply the resting expenditure by an “activity factor” as follows:

Multiply by 1.2 if you’re sedentary (little or no exercise)

Multiply by 1.375 if you are lightly active (you do light exercise or sports 1-3x per week)

Multiply by 1.55 if you are moderately active (you do moderate exercise/sports 3-5x per week)

Multiply by 1.725 if you are very active (you do hard exercise/sports 6-7x per week)

Multiply by 1.9 if you’re ‘extra active’ (you do very hard exercise/sports AND you have a physical job)

The answer is a pretty good estimate of how many calories you should budget per day to maintain your weight.  To lose weight at a rate of 1 pound per week, you would subtract 500 calories per day from your total weight maintenance figure.

The mystery of metabolism, exposed!

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