“Yesterday I was clever, so I wanted to change the world. Today I am wise, so I am changing myself.” - Rumi
For almost twenty years, I preached the gospel of cardiovascular fitness, repeating the dogma all fitness professionals held as sacred to the principles of weight loss: exercise like you are a Porsche, not a Prius. The message underscored the importance of firing up our metabolism with different exercise intensities, durations, and frequencies; higher and more variable heart rates were associated with increased caloric expenditure both during the workout and one to two hours after. Metabolism was thought to be either a friend or foe in the pursuit of weight loss. The goal was to work harder and use more gas to make us more like a gas hog Porsche. Working at the same intensity, duration, and frequency was thought to slow metabolism, making us more like an energy efficient Prius, a foe to a goal of burning more calories. This car analogy was simple, easy to remember, and often repeated in the gym. Unfortunately, it had no basis in truth. For years, many in the health arena had it all wrong about metabolism.
Anyone who has ever struggled to lose weight may have wondered if they have a slow metabolism or envied the fast metabolisms of thin people who ate whatever they wanted. But what exactly is metabolism? At a very basic level, metabolism is all the work our cells do to keep us alive. The body is a busy place, a nonstop manufacturing center in which molecules floating through the bloodstream are drawn into cells and transformed into either fuel or building blocks that will be used to make another substance. The remnants of the transformed molecule are pushed back out into the bloodstream to be used elsewhere in the body. It is a relentless process, one that is churning away continuously in all our organs, requiring a tremendous amount of energy.
There are many kinds of metabolism, and each creates different substances that perform specific bodily functions. On the surface, metabolism seems to be a fairly straightforward process, but its simplicity masks its elegance and sophistication; it is an intricate process that has been honed over billions of years of evolutionary trial and error. All cellular activity is driven by the food we eat—the water, minerals, vitamins, and macronutrients taken in through our diet. Macronutrients are the carbohydrates, fats, and proteins found in our food; they make up the raw materials used to fuel all our cellular processes. Think of metabolism as the work horse of the body: whatever you ingest will be broken down and separated into its macronutrients, minerals, and vitamins, while any water will be absorbed. Food is transformed methodically, without judgment; “good” foods are broken down using the same process as junk foods. Carbohydrates are transformed into glucose, proteins into amino acids, and fats into fatty acids and glycerides. These substances are then sent into the bloodstream where their fate is determined by whether they will be used in building or fueling the body.
Can the type of food we eat change our metabolism? We have been told that eating more protein will speed up our metabolism and that carb heavy diets will slow us down. Dr. Kevin Hall, a researcher at NIH, has conducted several studies that examined how diet composition impacts metabolism. Participants in the study lived on the NIH campus and were fed specific diets with carefully calculated nutrients that provided a consistent number of calories, about 2000 calories per day. Every two weeks they were fed foods that were high in one of three groups--protein, carbohydrate, or fat. At the end of each two-week period, their metabolisms were measured before they switched to a new menu with a different macronutrient focus. The meals were prepared by a trained chef so that they were tasty and satisfying, regardless of which menu they followed. At the end of the six-week period, participant’s metabolisms were compared to each two-week period. Can you guess which one of the diets changed the participants’ metabolisms the most? None of them. Except for a few minor variations, metabolism rates generally remained steady, regardless of whether meals were high in carbohydrate, protein, or fat. Food intake remained consistent as well, as participants ate about the same number of calories per meal, regardless of whether it was high in fat, carbohydrates, or protein.
For decades, weight loss strategies focusing exclusively on diet and caloric intake have failed or, at best, yielded dismal results. Although some people have lost weight, the failure rate is staggering—about 80 – 90% of those who lose weight gain it back within two years. Over the years, much of the research showed that restricting caloric intake to levels below 1200/kcal per day causes our metabolic rates to plummet, making it hard for people to maintain their weight loss over time. That is why obesity researchers shifted their focus to exercise which, combined with more modest caloric restrictions, was seen as a way to counteract the downshift in metabolism that occurs with caloric restriction.
Unfortunately, weight loss strategies that focus on increased exercise have not been shown to be any more effective than other approaches. But new research suggests that it may be our misunderstanding of how metabolism works that explains why these strategies fail for most people. For a long time, we have thought of metabolism as the body’s calorie burning engine that could be manipulated—switched from being a Prius to a Porsche--to help us tone up and trim down. Newer research that uses more sophisticated methodologies is suggesting that our “calories in, calories out” description of metabolism is just simply wrong. Rather, our metabolisms are incredibly adaptable, dynamic, and flexible, not just an energy piggy bank from which we make deposits by eating food and withdrawals with exercise. Rather, metabolism is like a shrewd money manager who juggles all our investments. Like a money manager, it is constantly monitoring and assessing the body’s energy needs and shuffling resources to those organs where more resources may be needed. The goal of metabolism is broader than energy withdrawals and deposits; it seeks to protect our resources while maintaining balance in our energy reserves.
Our metabolisms are always making tradeoffs. While you are sitting and reading this, your body is expending energy to keep the brain, liver, kidneys, intestines, skin, muscles, and bones functioning smoothly. This energy requirement is known as the Resting Metabolic Rate—RMR, which reflects the energy your body requires at rest to keep all your organs functioning properly. If you decide to stand up and go for a hike, your metabolism will adjust, but does so not by revving up but by diverting calories from these basic functions to fuel the muscles you’ll need for your hike. In other words, increased activity results in a tradeoff made by your metabolism which shifts which organ gets more energy, and which can do with less.
It seems that human metabolism may be a type of evolutionary adaptation our species made to meet the large energy demands of our body. Humans need more calories than other primates of similar size, most likely due to our bigger brains, longer life span, and the extended amount of time required to raise our offspring. And although the work of metabolism is quite sophisticated, it has but one mission—to keep us alive. Turns out, it is quite good at it. A calorie is utilized by your metabolism only once, and it is done so in one of five ways: building or repairing tissues, maintaining an organ, storing energy as fat, physical activity, and reproduction. What makes metabolism so dynamic and flexible is its ability to prioritize which functions get the most energy, based on the body’s ever-changing circumstances and the demands of the environment.
Although it is still true that increased levels of physical activity and exercise burn more energy, these spikes, over time, are not permanent. Metabolism understands its mission well, so it consistently searches to stay within a steady and consistent range. Several anthropological studies have shown that humans, regardless of culture, location, or socioeconomic status, expend about the same number of calories per day to stay alive. Hunter-gatherer societies who forage for hours a day use the same amount of energy as urbanites sitting at a desk all day. A farmer in the Amazon expends just as many overall calories as a seasoned ultramarathoner. We can all be a Porsche, but we’ll eventually go back to being a Prius.
All this data seems to fly in the face of what we have thought to be true about the role exercise plays in weight loss. But it helps to explain why we start out losing weight when we exercise but then it levels off after a short period of time. It also explains why sudden or dramatic weight loss is so impossible to maintain. The fast and dramatic changes created by long periods of exercise and extreme caloric constriction put our metabolisms on high alert. Thinking we are in extreme danger, it shifts into survival mode, ramping down our energy requirements so it can shuttle limited energy to areas in need. Any function that isn’t necessary for survival has energy diverted from its organs: reproductive hormones drop, skin cells turn over more slowly, bones become brittle, muscles start to shrink.
This nuanced understanding of metabolism does not negate the benefits of exercise; in fact, it makes exercise and physical activity even more important to our health. The tradeoffs our metabolisms make to provide the energy for physical activity and exercise divert resources from processes that cause disease, such as tumor growth, inflammation, and stress. Exercise and physical activity positively affect every system in the body in ways we are just beginning to understand, from making blood vessels more pliable, to stabilizing our mood, preventing cancer tumor growth, and delaying age-related cognitive decline. And whereas exercise may not be the Holy Grail of weight loss, it has been shown to be vital in regulating appetite and mood, an important component of weight maintenance.
A teacher once told me that wisdom comes not from knowledge, but from recognizing when our beliefs no longer reflect reality, and having the humility necessary to accept the truth. Using the calories in, calories out formula made fitness a straightforward process for me, easy to communicate, measure, and plan for, but regardless, it was simply not true. Reluctantly, I needed to let it go. Embracing metabolism as an evolutionary process that is all about balance and tradeoffs is much more complex and harder to wrap my head around; it defies catchy analogies that can be dropped during a rest break in a cycling class. Perhaps most importantly, metabolism is an ancient and deeply human process that transcends geography, socioeconomic class, and culture, one that is not better in one person or worse in another, but equal to us all. At a very basic level, it is what made--and keeps—us connected to one another.
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