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Metabolism: Truths and Myths

Metabolism. It’s one of the most popular and least understood topics of discussion between us women. It’s talked about as though it’s a car, easy to steer, manipulate, speed up, or slow down as you please. It’s frustratingly blamed for a lack of weight loss, or on the other side of the coin, for weight gain. In truth, metabolism is a complex bodily process that has a much greater responsibility than your weight. ​


This week, I want to debunk several common myths about metabolism so that with better understanding, we can stick to a path of stable weight maintenance, without all of the latest fads tossed in.

Metabolism isn't just about calories and weight

Before debunking any myths, I want everyone to understand what the word metabolism means, on a basic level. “Metabolism” refers to the complex processes of either breaking down or building up compounds and occurs within every single cell of your body.

When we discuss metabolism, we are typically referring to the “breaking down” portion of the process. From this breakdown, each cell is generating the energy it needs to live, giving your body, as a whole, the energy it needs to function and survive. When you eat food, it is first digested into its teeniest, tiniest components. Then, it is absorbed into your body’s cells. Once these small components are in the cells, they will undergo different chemical processes to turn into energy. All of your body’s cells use this energy in a different way. Your muscles need energy to move. Your lungs need energy to breathe. Your brain needs energy to send signals throughout the nerves in your body. When you eat, that food goes to keeping your body alive, so it is much more than just calories and weight.

Most of the energy you use is burned at rest

Simply keeping your body alive and breathing uses more calories than anything else during the day. This is referred to as the “Basal Metabolic Rate” (BMR), the amount of calories you need to consume in a day for all your cells to function if you do nothing but lie in bed, breathing. Your BMR accounts for 50 to 80 percent of your daily caloric needs, and is only as low as 50 percent for someone like a professional athlete, who’s entire life revolves around physical activity.

For a moderately active person, physical activity only accounts for 20 percent of your daily caloric needs. So, in a 2000 calorie diet, just 400 of those calories go towards your physical activity.

The remaining 5 to 10 percent of calories are used in the actual digestion of food. This is referred to as the “Thermic Effect of Food”.

Muscles account for the largest energy usage at rest

Alone, muscles use up about 20% of your basal energy needs, or the amount of calories needed if you were doing nothing but living. Muscles require a large amount of calories and oxygen to keep alive and functioning, regardless of physical activity.

Your major organs also require a large amount of energy. Your liver requires about 19% of your calories, just behind the muscles. Your brain takes 17% of your energy needs to function. Last, your heart and kidneys use up 8% and 7% respectively. Together, these major organs account for half of your energy usage, but nothing alone uses more energy than your muscles.

This is one reason why, typically, people with more muscle have less fat. Simply the existence of those muscles requires a significant chunk of your energy needs. This is the reason why men have a higher metabolic rate: men’s bodies are composed of a larger percentage of muscle than women’s bodies. It is commonly recommended that people trying to maintain a healthy weight or lose weight work on strength training exercises that help increase your lean muscle mass.

You can't boost your metabolism

It’s not like jumpstarting a car or switching on the turbo. In fact, there’s very little to nothing you can do to change your body’s natural metabolic rate. You’ll see certain foods and compounds advertised at boosting your metabolism and aiding weight loss. While it’s true that some foods such as coffee or spicy peppers may cause a slight increase in calorie usage, this number is miniscule and will not actually cause weight loss. Sadly, this knowledge is often manipulated as a marketing tool to reach those who are trying to lose weight.

We mentioned the Thermic Effect of Food just above. Some people also believe that certain foods burn many more calories because they are difficult to digest, and will actually result in a net loss of calories to the diet. The most common one that I’ve heard is regarding celery. Because it has obvious fibers and is composed largely of water, some believe that in consuming it, you are actually burning more calories than it contains. What they are referring to is essentially an increase in the Thermic Effect of Food, specifically due to celery. Even if celery did increase the thermic effect of food compared to other foods, it would still not be enough to result in any weight change.

You CAN decrease your metabolism

It just doesn’t seem fair does it? You can’t increase your metabolism, yet you can decrease it. And I’ll tell you why. Your body isn’t built to shed weight. Humans evolved in an environment where you had to hunt for or gather food. It was an environment where you weren’t sure when or where your next meal would come from, so if your body wasn’t designed to store energy, you would starve. Today, we live in a food rich environment where you can find food on street corners, pharmacies, and even places like the Home Depot. What’s more is that this environment is saturated with energy dense foods.

When you drastically lose weight, your body undergoes what is called “metabolic adaptation”. This means that your body adjusts its metabolism through a variety of methods in order to help your body return to your previous weight. When you remain at one weight for a long time, your body set this weight as it’s normal weight. When your weight changes from this weight, be it higher or lower, your body makes adjustments in order to return you to what it has set as your normal weight.

Unfortunately, several studies have shown that weight loss causes a decrease in your metabolic rate, even years after the weight has been lost. More shocking, is that the rate is lower than would be expected or calculated for a person of this size. These both support “metabolic adaptation” in that the body adjusts itself to return to what it has set as its normal weight. This is likely a reason why many people find great difficulty in keeping of weight once it’s lost and why many more end up gaining more weight back than they had lost in the first place.

Everyone metabolizes differently

In the past, scientists and medical professionals alike believed that all foods affected people similarly, however recent researches showed that people react very differently to the exact same foods. Basically, an apple isn't the same apple to everyone. 

You and your best friend could have the same weight and general body composition, yet completely different metabolisms. You may have to try very hard to not put on any pounds, while your friend may be able to eat whatever he or she likes without any changes in weight.

This can be massively frustrating for many of us, and it is no consolation that professionals are still unsure about why these drastic differences exist. There are, however, several hypotheses and known facts that affect the rate of your metabolism: age, genetics, muscle mass, sex, and hormones. All of these affect your metabolic rate.

The main takeaway from this is that what works for your friend or neighbor, may not work for you in terms of weight loss.

Age decreases metabolism

Even if you weren’t completely sure about this fact, you have probably guessed it simply by observing yourself and those around you as they age. We aren’t exactly sure why this decrease in metabolism occurs with age, however it is possibly due to a decrease in the amount of lean-muscle mass, increase in sedentary lifestyle, decrease in your body’s ability to feel full after a meal, and changes in hormone levels. There are likely a large number of factors that contribute to this metabolic decrease with age.

Now what?

So how can everyone get ahold of this whole metabolism thing?

It’s undoubtedly a complex process, but now that we all accept and understand this fact, we can move forward.

Calculate your personal caloric needs. There's no exact way to measure exactly how many calories you need unless you go inside a special chamber that measures your breathing and does certain calculations, but certain online tools can help you gain the best estimate by using an equation that considers your age, weight, height, sex, and level of physical activity. You can also get an idea of your weight to height ratio by calculating your BMI and understanding if you are at a healthy weight for your height. This site is a useful tool that may help you calculate both.

Skip the crash dieting; the fads that tell you to just consume grapefruit juice or the ones that tell you to eat three bananas every 6 hours and two pieces of cheese every 4. They are silly. You can’t trick your body into losing weight. It’s just not possible.

Understand that sustaining weight loss has a high failure rate of greater than 80%, but rather than letting this discourage you, let this fact empower you. Now you know how to beat it.

Maintain or increase your lean muscle mass. This is key. Often with weight loss, there is a reduction in both lean muscle mass and fat. Since muscle is a large (and controllable) player in your basal energy expenditure, keeping muscles strong and present will help avoid gaining back the weight you’ve lost and may help in weight loss in general.

Find healthy changes that work for you and that you can realistically stick to permanently. Yes, that’s right. Permanently. Forever. I know it sounds shocking, but it’s true. The key to losing weight and keeping it off is to find a way that you can make those changes permanent. Pretty soon, those changes hardly feel like strange changes at all and rather just become a normal part of life. Sometimes these changes start small and then grow and evolve. Try cutting out soft drinks, juice, and other sugary drinks for starters. This may lead to another smaller change after the first is successful. Perhaps after that, you’ll limit added sugars in foods in general.

General weight loss strategies still apply. People who have lost weight and kept it off have several things in common, including monitoring their weight, calorie counting and restricting caloric intake, and regular exercise. There is no real secret to weight loss, just real effort.

Last but not least, a few nutrition tips:

​Rosenbaum, Hirsch, Gallagher, Leibel. Long-term persistence of adaptive thermogenesis in subjects who have maintained a reduced body weight. AM J Clin Nutr. 2008; 88(4):906-912.

Weck, Bronstein, Barthel, Bluher. [Strategies for successful weight reduction - focus on energy balanc]. Dtsch Med Wochenschr. 2012. 137(43):2223-8.

Rosenbaum & Leibel. Adaptive thermogenesis in humans. Int J Obes (Lond). 2010. 34(01):S47-S55

Zeevi & Korem et al. Personalized Nutrition by Prediction of Glycemic Responses. Cell. 2015;163(5):1079-94.




Renee Cotter, MD

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