The Slow Metabolism Myth- Understanding The Role Of Metabolism in Weight Gain
It is a disarmingly simple concept- some people are cursed with a naturally slow metabolism and gain weight in spite of how carefully they watch what they eat. On the other side of the spectrum, a fortunate few are blessed with fast metabolisms which allow them to eat any and everything without gaining weight. As a central aspect of the idea of genetic predispositions towards weight gain and obesity, the slow metabolism hypothesis has long been hailed as a common explanation as to why some individuals appear to gain weight no matter what they do. As popular an argument as this may be there is ample evidence to the contrary (read my article on genetics and weight gain) and the rationale of genetically predetermined disorders can’t account for the prevalence of obesity throughout Westernized populations. Such notions nevertheless do increase the sales of many fitness related products while adding to the cacophony of misinformation regarding the role of our metabolism in losing and gaining weight. Unfortunately, an individual lacking in-depth knowledge of nutrition and physiology can’t necessarily see any distinctions between rational deductions from observable evidence and baseless speculation. Most of what passes for fitness related information today is either a form of entertainment or ideologically driven pseudoscience, propagated by those who stand to profit the most. The complex nature of how our body works is often dumbed down to simple concepts that people feel they can relate to easily. The idea of slow metabolisms being a widespread problem for millions of people and the cause of your own seemingly inexplicable weight gain is one that is delightful empowering as it lays the ‘blame’ for your extra pounds on your somewhat underperforming metabolism. This argument makes you feel better about yourself and more inclined to purchase products that purport to help you lose weight by speeding up your metabolism in one way or the other but this is hardly the case. The reality is that weight gain isn’t mysterious and slow metabolisms are hardly to blame.
What Does Metabolism Really Mean?
Metabolism is defined as the total sum of all the chemical reactions taking place in your body. Our metabolic processes fall into two categories, they are either anabolic- which refers to the action of building up, or catabolic- which refers to the mechanism of breaking down. A classic example of an anabolic process in our metabolism is muscle building, where as a response to the stress of exercise our body increases the amount of skeletal proteins. An increase that we externally observe as our muscles getting bigger and stronger (see my article on how muscles get bigger and stronger for a more detailed description). Catabolic metabolic processes are the exact opposite and a common example would be the breaking down of our foods to provide energy for the cells in our body and thus keep us alive. When we are healthy these processes occur in a state of equilibrium, a balance that is referred to as homeostasis.
There is one other important but somewhat complex concept to contend with in understanding metabolism and that is the first law of thermodynamics. The first law of thermodynamics states that energy cannot be created or destroyed and so conceptually in a fixed physiological system such as a living organism, total energy coming in must be the same as the total energy out, unless some of it is lost or stored. Energy coming into our body refers to the calories we obtain from food and drink. Energy going out refers not only to energy used in physical activity but also for the work performed by the cells in our body. Work in this context also applies to the biological work of repairing cells, and our body’s struggle to maintain homeostasis such as the constant regulation of body temperature. Even reading this article requires some element of biological work as your muscles contract to keep your eyes focused on the screen.
How We Measure Metabolism
The primary way that our body produces energy is through the combustion of carbohydrates and fats in the presence of oxygen. This function is called aerobic metabolism, and it leaves behind measurable byproducts in the form of carbon dioxide and water. That being the case we can roughly estimate an individuals’ metabolism by measuring the amount of oxygen consumed and the volume of carbon dioxide produced. The process to measure metabolism is called indirect calorimetry. Calorimetry means the measuring of heat energy given out or taken in and the term ‘indirect’ is used since we can’t exactly measure the amount of carbon dioxide coming out of each cell in your body at any given time. We can however get an overall view of what’s going on by observing the exchange of carbon dioxide and oxygen from your lungs.
A device called a metabolic cart is used to measure these gases and it requires you to wear a mask very similar to what a scuba diver would wear, except it doesn’t provide air but rather records air coming from your lungs. From indirect calorimetry we can calculate your basal metabolic rate- which is the minimum metabolic rate your body requires to maintain your body weight and functions. Many factors can affect your basal metabolic rate such as genetics, age, gender, hormones and most importantly for our purposes- diet and activity level. When the amount of food energy (calories) ingested is more than what your body uses then there is a concurrent gain in body mass. This gain in body mass is our body’s way of storing energy for a time when it isn’t readily available and the way it does so is in the form of fat tissue. Our ability to store considerable reserves of energy in the form of fat is one of the hold offs from our past, serving to help our ancestors of survive countless famines and the scarcity of food that has defined human life for the past several thousand years.
The Slow Metabolism Myth
Getting back to the idea of a slow metabolism, average metabolic rates for men are estimated to be about 1.0 kilocalories per hour per kilogram of bodyweight. Women usually have slightly slower metabolisms than men (and we will explain why) and their metabolic rates are usually about 0.9 kilocalories per hour per kilogram of bodyweight. It seems natural to think that someone who is overweight and has difficulty losing weight might have a slower pthan normal metabolism but contrary to popular opinion, someone who is obese would have a GREATER energy expenditure than someone who is thin.(3) The mechanics of it makes total sense as it takes more energy to maintain a larger body mass. The bigger the organism the more energy is needed for homeostasis as someone weighing over 400lbs has far more biological and mechanical work to do than someone weighing 150lbs. So why do some people struggle with weight gain while others don’t? It must be emphasized that the causes of obesity are complex however differences in rates of metabolism are not valid explanations as to why it happens.
One key issue is our inherent difficulty in estimating exactly how much food we consume during the course of a day- a challenge that studies have shown can be even harder for those who are overweight. As much as we may think we know how much we eat on a regular basis, indirect calorimeter testing has shown time and time again that even the most conscientious among us tend consistently under-report our food intake. Compounding this issue is the unfortunate reality that it doesn’t take much for our body to gain weight. Our bodies have no way of knowing that food is available to many of us in developed countries to the point where we throw away billions of pounds of it every year. All our bodies know is that storing excess energy is critical for our survival as it is nevertheless designed to endure continued periods of low food supply. That being said we are all very much efficient at gaining weight and keeping it on and we do so with an ease that is hard for most to fathom. To put the average human’s weight gaining ability in perspective, a weight gain of 44 pounds over the course of ten years would only require you to eat 30-40 kilocalories more than your body needed everyday. That’s all it would take. Not a ton of junk food nor hefty servings of high calorie food and drink. Just half of an apple, a plum, a cup of asparagus, 1/7 of a cup of rice, 1/3 of an ounce of cheese, or 2 fluid ounces of soda over your daily energy expenditure requirements would add over forty pounds in a decade if there was no change in energy requirements. A sobering thought, but one of the many reasons so many people gain weight. It really doesn’t take much at all for it to happen.
Slow Metabolism Versus Fast Metabolism Explored
A study presented at the 2008 annual Endocrine Society Conference in San Francisco further highlights the problems with the slow metabolism concept as it was demonstrated that obese people process food at exactly the same rates way thin people do when they are in a rigidly controlled environment. In a study conducted at the University of Colorado Health Sciences Center in Denver, 26 naturally thin individuals with Body Mass Index (BMI) counts between 19 and 23 who considered themselves to have a fast metabolism were compared in a carefully monitored environment to 23 individuals with BMI’s of 23-27 who identified themselves and family members as ‘obesity prone’ to see if they could overeat and not gain weight since they appeared to have a naturally higher metabolic rate than those who had problems with weight gain.
In both groups metabolic rates were tested after the subjects ate a normal diet and again after three days of eating 40% more food than their individual energy requirements. All subjects underwent two one-week dietary study periods, with four days of a control diet followed by three days of either eating exactly the amount of calories for their calculated energy expenditure or overeating. Metabolic rates were tested using the indirect calorimetry method and all foods were carefully prepared and monitored following a standard of 20% protein, 30% fat and 50% carbohydrates. At the end of the study researchers found that both groups had higher metabolic rates at rest after overeating for three days compared to when they ate a normal diet but the increase was the same for both groups. Researchers had theorized that energy expenditure and fat oxidation would increase following overfeeding in the ‘fast metabolism’ group, which would explain the process protecting them from gaining weight, but this turned out not to be the case at all.
Difficulties in Estimating Food Intake As A Weight Gain Factor
These findings were consistent with what many other studies have found time and time again, which is that some people have a hard time balancing their daily caloric intake with how it relates to their actual energy needs. Thin people don’t have faster metabolisms that somehow save and protect them from weight gain, and those with problems losing weight don’t have slow metabolisms. Over the past twenty years of my career as a personal trainer I have had very similar experiences. I started out being convinced that there were people who suffered from slower metabolisms and that they would gain weight easily while others (myself included) found it ridiculously hard to gain weight. As time went on my observations and personal experiences proved both concepts to be flawed as everyone who trained consistently and ate in accordance to their individually designed dietary plans were able to get into fantastic shape as long as they stuck to the program. It didn’t matter if they were overweight all their lives or if they were previously unable to lose weight, the only differences were that those who had more weight to lose took longer to get to a point where they had a high degree of definition as compared to those who started at a lower body fat percentage to begin with. On the other hand- everyone who had trouble gaining weight because they thought they had a ‘fast metabolism’ was able to gain significant amounts of muscle mass as long as they adhered to the diet prescribed for them and a training program designed to stimulate muscle growth.
I always believed that I had a fast metabolism as I was painfully thin for the earlier part of my life. I started weight training at 125 lbs at six feet even, but my metabolism had nothing to do with my ability to gain weight. As long as I ate in accordance with the dietary plan designed to help me get bigger I gained weight. It took a lot longer for me to do so to the point where it would show but I was able to put on over a hundred pounds of lean muscle mass over the course of 17 years or so without the use of any drugs whatsoever. As for the protective aspect of a ‘fast metabolism’, in the days when I ate junk- I got fat like everyone else. Today I stay steady at 225lbs at about 6% body fat not because of a fast metabolism but because I am always on point with what I eat and my program of high intensity training. When I didn’t eat as well as I do now I tipped the scale at an ungodly 252 lbs looking nothing like how I do today. If I ‘relax’ my eating there isn’t a magic parachute waiting to protect me from getting fatter if I overeat and indulge in the wrong foods and the same applies to everyone else.
Beyond The Energy Balance Theories- How What You Eat Plays A Part In What You Weigh
As straightforward as this may sound it isn’t the end of the story, as weight gain is even more complex than simply saying that eating too many calories will make you gain weight. The laws of thermodynamics and the theory of energy expenditure versus energy requirements don’t completely explain why people keep gaining weight as the years go by. Given the energy balance principles there should be a finite point where weight gain reaches a plateau as energy demand rises with increases in body mass but this doesn’t happen at all as many continue to gain weight throughout the course of their lives. Alternate and perhaps more accurate models take a look at the role of modern food ingredients as a reason some people keep accumulating those extra pounds. Fructose derived sweeteners (sucrose and high fructose corn syrup) can trigger increased fat tissue accumulation in addition to increasing appetite- a vicious cycle which usually leads to a decrease in activity. This model for why we gain weight places emphasis more on what you eat in addition to how much you eat as the two seem to be very closely related.
In the end it is hard not to make simple cause and effect judgments. The slow/fast metabolism neatly explains everything and in many cases places an uncontrollable factor into the equation. One that can be easily blamed for the extra pounds around your waist, but this simply isn’t the case for the vast majority of individuals. Hormonal disorders and some medications seem to have a part in weight gain as well but in most instances it really comes down to an awareness of what you eat, how much of it you eat and how active you are to deal with the calories you are taking in.
1. Kaiyala KJ, Ramsay DS: Direct animal calorimetry, the underused gold standard for quantifying the fire of life. Comp Biochem Physiol A Mol Integr Physiol. 25 April 2010
2. Canadian clinical practice guidelines on the management and prevention of obesity in adults and children
3. Kushner, Robert (2007). Treatment of the Obese Patient (Contemporary Endocrinology). Totowa, NJ: Humana Press. pp. 158. ISBN 1-59745-400-1. Retrieved April 5, 2009.
4. Peter G. Kopelman, Ian D. Caterson, Michael J. Stock, William H. Dietz (2005). Clinical obesity in adults and children: In Adults and Children. Blackwell Publishing
5. Björntorp P. Obesity. Lancet 1997; 350: 423–6
6. Carlsson B, Lindell K, Gabrielsson B, et al. Obese (ob) gene defects are rare in human obesity. Obes Res 1997; 5: 30–5
7. Wells JC, Siervo M. Obesity and energy balance: is the tail wagging the dog? Eur J Clin Nutr. 2011
Kevin Richardson is an award winning health and fitness writer, one of the most sought after personal trainers in New York City and creator of Naturally Intense High Intensity Training™. Get a copy of his free weight loss ebook here. If you live in the New York metropolitan area and need help losing weight or taking your body to the next level give Kevin and his team a call at 1-800-798-8420 or click here to get started with 50% off your trial personal training session.