Metabolism 101 A Look Inside Your Body’s Energy Engine

June 1, 2022

You probably don’t think about your body composition when you’re thinking about your metabolism. But you should.

You probably think about it in terms of speed: “My metabolism is fast” or “my metabolism is slowing down.”

People are naturally afraid of their metabolism slowing and the weight gain they know comes with it. To some extent, those worries are well-founded.

Metabolism is linked with weight gain and loss because of its a biological process involved with energy and calories.

The Mayo Clinic defines metabolism as: …the process by which your body converts what you eat and drink into energy. During this complex biochemical process, calories in food and beverages are combined with oxygen to release the energy your body needs to function.

Notice how it doesn’t mention anything about the speed you process your food. That would be digestion.

In medical terminology, metabolism is known as your Basal Metabolic Rate (BMR), which is the minimum number of calories your body needs to perform basic bodily functions. BMR is usually expressed in terms of calories. Your Basal Metabolic Rate also has another interesting quality: the more Lean Body Mass (which includes muscle, water, and minerals) you have, the greater your BMR will be.

When we talk about metabolism, we should always start the conversation with how many calories your body needs. But because your BMR and Lean Body Mass are linked, that means any conversation about metabolism becomes a conversation about your body composition.

Because your metabolism isn’t something that slows down or speeds up depending on things like age, this actually gives you some control over it.

Regardless of your age, weight, or body type, knowing the ins and outs of your metabolism is vital to obtaining a healthy lifestyle. Factors such as BMR, fluctuating temperatures and metabolic flexibility all contribute to the larger picture. It’s all about making your metabolism work for you.

Metabolism 101

Let’s take a deeper look at what you might call a “slow” metabolism. Far from being an issue of fastness or slowness, weight gain is almost always the result of a caloric imbalance that goes unchecked over a long period of time.

To clarify, your Basal Metabolic Rate is not the only factor that plays into your overall caloric needs, and it’s not the total amount of calories you need in a day. There are two other major influencers, which are:

  • Your energy level–how active you are
  • The thermic effect of food–the energy your body uses to digest your foot

These taken together with your Basal Metabolic Rate provide your Total Daily Energy Expenditure (TDEE). This is the number of calories your body burns in a day.

BMR is a necessary piece of information to estimate TDEE. Although they’re not exact, equations exist for estimating your TDEE based on your activity level and BMR. These are based on multiplying your BMR with an activity factor–a number between 1 and 2–that increases the more active you are (and decreases when you are less active, regardless of your appetite).

How Many Calories Do You Need?

You’re probably familiar with the 2000-calorie diet. This is a range set by the U.S. Food and Drug Administration (FDA) in 1993 for use on nutrition labels for packaged food.

So what are your actual caloric needs? A good way to start is by using a BMR calculator, which will determine the number of calories your body burns each day to perform its basic, life-sustaining functions. This includes all the involuntary processes in your body such as breathing, digesting food, pumping blood, brain activity, and much more. There is no shortage of online resources and apps that will provide you with a BMR calculator. Certain medical/fitness devices also feature BMR as an output during body composition analysis. However, there are a few things you should know about metabolism calculations before diving into the first option you find. Your caloric needs can be calculated in a couple of different ways and with a few different equations, including the revised Harris-Benedict equation and the Mifflin-St. Jeor equation.  These equations calculate BMR using your weight, with some adjustments for height, age, and gender. However, if you fall outside average assumptions for height, age, and gender (if you’re an athlete, for example), these formulas may not accurately produce your metabolic rate.

For people who do fall outside the assumed ranges for height, age and gender, there is a third option: use the amount of lean body mass you have to determine your metabolic rate. This is what the J.J. Cunningham equation will do. Using this method as a BMR calculator has a couple of benefits:

  • It won’t give you results that have been influenced by estimations derived from the typical representative member of your age and gender
  • As you increase lean body mass by developing your skeletal muscle mass, your caloric needs will increase, and the Cunningham equation will account for this.

Once you have your BMR in hand, you’re ready for the next step.

Total Calories and Dieting

Remember, your BMR is just the number of calories your body burns at rest and does not account for the calories you need to walk, talk, exercise, etc. When thinking about your caloric needs for a meal plan, you’ll need to convert your BMR to your Total Daily Energy Expenditure (TDEE). You can do this by multiplying your BMR by a factor that represents your estimated energy level. Those conversions are:

So, let’s take the example of a 171.1-pound male with 133.6 pounds of lean body mass and assume he is moderately active. Using the Cunningham equation, this person would have a BMR of around 1,679 Cal/day. Multiply that by the appropriate conversion, and you get 2,602.45.  This is how many calories this person needs to maintain his weight.

This also means that your diet must also match what your current goal is – losing fat mass and/or gaining lean body mass. This is incredibly important. People who don’t do this often end up sabotaging their goals by setting fitness and meal plans that are at odds with each other.

The most classic example is this: “I want to get in shape, so I am going to diet (eat less) and work out more (increase energy use).”

This isn’t a bad plan – if you’re looking to lose fat. If you’re looking to build muscle and get stronger, it’s very unlikely that you will achieve this by eating less than your TDEE while increasing your activity level beyond what you’re accustomed to.

As with any dietary plan, you will expect to see changes over time.  All this hard work has to produce results, right? So, how long will it take to see results? Unfortunately, that is going to vary for each individual. A good rule of thumb is to weigh yourself every 1 – 2 weeks.  If you are looking for a more precise analysis, you should get your body composition measured as well. Another important factor to consider: your BMR. Since your BMR is closely linked to your lean body mass, any changes will affect the number of calories you burn.

For example, if your plan is to gain lean body mass, and over a period of time you are successful in doing so, your energy needs are going to increase. This is why it is so important to be measuring body composition.

A diet is much more than creating a calorie deficit. It’s important to use a BMR calculator or body composition analyzer to understand how much energy your body needs. Without this information, you won’t know how much food you need to add or remove to your diet in order to achieve your goals. With this information, you’ll see quicker results and reach your goals faster.

Improving and Increasing Your Metabolism

With the correct exercise and dietary plan, you can make your metabolism work for you. It all goes back to improving and maintaining a healthy body composition.

Because your body needs more energy to support itself when it has more Lean Body Mass, working to increase your Lean Body Mass can actually increase your Basal Metabolic Rate, which can have a huge impact on your TDEE once you factor in your activity level.

Many people simply maintain their metabolism or avoid a “slowdown” (which as we’ve seen, is a myth right up there with muscle turning into fat) is an important goal. How can you be sure to stay away from creating a decrease in your metabolism?

In short: by maintaining the Lean Body Mass that you already have. That means maintaining your Skeletal Muscle Mass.

Your Skeletal Muscle Mass isn’t the same as your Lean Body Mass, but it is the overall biggest contributor to it. It’s the muscle that you can actually grow and develop through exercise, and increases/decreases in SMM have a strong influence on increases/decreases in Lean Body Mass.

Skeletal Muscle Mass is best developed through strength training and resistance exercise along with a proper diet. A regular exercise plan that includes strength training and resistance exercise will help you maintain your Skeletal Muscle Mass.

This can be especially important as you age. As people become older and busier, activity levels tend to drop and a proper diet can become harder to maintain as responsibilities increase. Poor diet and nutrition can lead to loss of Lean Body Mass over time, which leads to a decrease in overall metabolism–not a slowdown.

So what can be done to improve and increase your metabolic rate? Just like with most things in life, the key is finding your body’s perfect balance.

Balance Your Diet And Boost Your Metabolism – The Truth About Metabolic Flexibility

At times, a well-intentioned and thoughtful dietary plan doesn’t match the metabolism of the person practicing it. Even though someone may be led to believe that 1,800 calories is right for his or her diet regimen based on age and gender, our metabolisms might not require that caloric intake, ultimately resulting in weight gain despite our well-intended efforts.

In the end, most individuals will end up blaming their issues with weight loss on a “slowing metabolism,” illustrating the importance of understanding how the relationship between a balanced diet and body composition can actually lead to a boosted metabolism.

Contradicting health articles do not help the confusion. They preach that carbs should be eaten before you work out, as they will improve your performance. They also say to eat more fats like peanut butter or avocados. Others say that if you don’t eat at all, your body will burn more calories, and you should instead refuel with carbs or recover with protein to build muscle. Is your head spinning yet?

You’re not alone if you ever feel like every health article you see contradicts the one you saw the day before.

This is where the perpetual carbs-versus-fats debate comes into play. Everyone’s quick to tell you about their success with the Keto diet or how more balanced macros improved their performance in the gym.

In reality, there’s no perfect diet. Everyone’s preferred macronutrient ratio and calorie intake depend on their unique resting metabolic rate, activity level, food sensitivities, and more. But there is one thing everyone can count on: Your body will – or should – use what you give it.

What is Metabolic Flexibility?

If you eat a lot of fats, fat will be your body’s primary source of fuel. If you eat a lot of carbohydrates, glucose will be your body’s primary source of fuel. If you eat more protein than anything else – you guessed it – your body will burn more protein for energy.

Metabolic flexibility is your body’s ability to adapt to metabolic demands. When things change – such as changing what time you eat breakfast or what you eat for breakfast – your body is forced to change the way it metabolizes your food.

When you eat, your food is either burned for energy or stored if what you ate was in excess of your energy needs. Any excess carbohydrates are stored as glycogen(which serves as quick energy reserves) or fat tissue if glycogen stores are full. Any extra fat is also put away for later use, as fat tissue.

How flexible your metabolism refers to how efficiently your body can switch between using carbs or fats for fuel – or, more importantly – how efficiently it can use what is already available.

Someone with great metabolic flexibility can burn carbs when they eat them. They can also burn fat when they eat it, or when they don’t eat at all. People with flexible metabolisms can “flex” between carbohydrate metabolism and fat oxidation relatively easily.

For example, a person with great metabolic flexibility does cardio in the morning on an empty stomach. Because their metabolism is highly flexible, their body powers through the workout on fat tissue that already exists. The same person, however, could eat oatmeal and a banana an hour or two before their workout and instead use those carbs as fuel (instead of storing them for later use).

A flexible metabolism gives your body more leeway when determining what fuel source to use. Take the example of the person doing fasted cardio in the previous section. If that person was metabolically inflexible and exercised on an empty stomach, he or she would first burn through the glycogen (stored carbohydrates) in their body – leaving the fat untouched.

This is why it’s so hard for many people to burn fat: They are metabolically inflexible.

If you’re very metabolically flexible, eating calorie-dense, sugar-laden, or very fatty foods from time to time won’t always be an issue for your body. Your body can convert those calories into energy without much negative aftermath.

Those who are metabolically flexible carry more mitochondria in their muscles, which allows them to produce energy more efficiently. Having too few mitochondria, or having dysfunctional mitochondria, limits the amount of energy a person can produce. It makes switching between fuels difficult, which makes utilizing any stored body fat between meals almost impossible. This is why metabolically inflexible people tend to snack often.

Training Your Metabolism to be More Flexible

The key to understanding metabolic flexibility is understanding how insulin regulates our energy. A healthy person with normal insulin action can effectively switch between fats and carbs as fuel. An insulin-resistant person cannot do this as effectively.

Insulin is the hormone that regulates your blood sugar by taking glucose into your bloodstream. There are two primary states of being as it relates to a person’s metabolism: fed and fasting.

During fed conditions (e.g., having just eaten,) your insulin levels will be high due to incoming food. During fasting conditions, someone who is very metabolically flexible will easily be able to tap into stored body fat. The workings of insulin are vast, and you can learn more here, but these points are essential to understanding metabolic flexibility:

  • All of the food you eat, regardless of its macronutrient composition, sparks the release of insulin.
  • Insulin is a factor in deciding which fuel source your body uses.
  • When insulin levels are low, your body primarily burns fat.
  • When insulin levels are high, your body primarily burns carbs and stores fat.

One of the best–and easiest–ways to improve your metabolic flexibility is to exercise. If you’re generally sedentary, adding exercise to your days is a surefire way to kick your body into a fat-burning mode it’s never experienced before. If you already exercise regularly, add more variety to your workouts.

Varying the type of training you do (strength training, intervals, and some sort of aerobic or endurance activity) might just give your body the nudge it needs to tap into your fat reserves. Different types of exercise use different mixes of fuel and eventually may train your body to use different fuel sources during day-to-day activities.

Fasted cardio is one type of exercise intended to tap heavily into your body’s fat reserves. Doing high-intensity cardiovascular exercise with little to no glycogen stored can train your metabolism to be more flexible.

Another way to improve your metabolic flexibility is intermittent fasting (IF). Remember the person who used body fat as fuel during their morning fasted cardio? That worked because his or her body was trained to use the available energy in the absence of food.

Metabolically inflexible people would just feel atrocious in the same scenario because their bodies don’t know how to function without incoming energy (food). If you can’t make it more than two-to-three hours without food, you’re impairing your body’s ability to utilize your body fat. Work slowly to increase your spacing between meals.

Timing your nutrient intake can help your body use fat more efficiently, and the longer the fasting period, the more your body has to tap into its fat reserves, which may be one way to combat obesity.

You should also test out different macronutrient ratios. The fewer carbohydrates you consume, the more your body will have to rely on fat sources for fuel. This is the premise behind the ketogenic diet. Try cutting out added sugars and overly processed grains first. Then you can try reducing your carb intake even further by cutting out starches like potatoes. If keto’s not for you, try out the paleo diet, which is also low in carbs and higher in fat and protein.

While it’s encouraged to consume fats, avoid trans fats and too many saturated fats. Try to get your fats from nutrient-dense sources like olive and coconut oils, avocados, nuts, seeds, and fatty fish. Another good source is foods containing high antioxidants. Antioxidant consumption has been linked to a reduced risk of metabolic syndrome, as well as an increase in insulin sensitivity.

Just like keeping your diet in check, there are other factors that can use to impact your metabolism.

How Hot and Cold Exposure Change Your Metabolism

In today’s society, much of our technology – air conditioning, heated pools, goose-down jackets, portable misting fans – is designed to keep us within a comfortable band of temperatures. But humans did not evolve with these modern comforts. Instead, we evolved on hot grasslands, cold tundras, and every climate in between. As a result, our bodies developed adaptive mechanisms to deal with the extremes of hot and cold.

These mechanisms are actually beneficial to our health. Outside our comfort zone, hormones are released, metabolic changes occur, inflammation is dampened, and that’s just a few of the benefits. With enough cold exposure, in fact, even our fat tissue changes color and characteristic and starts using more energy. Not bad.

Let’s start off with proven metabolic changes that cold exposure can deliver.

In one study, exposing mice to 4℃ air (39°F) for 1-8 hours 3 times per week increased their metabolism and improved their blood sugar response. The cold hungry mice ate so much to compensate for their higher energy burn that they didn’t lose any weight.

In another study, this time on humans, young men were immersed up to their necks in cold water of various temperatures – 32°C (89° F), 20℃ (68°F), and 14°C (57°F) – for one hour. Afterward, various biomarkers were measured.

The results? Under the cold conditions, the young men’s resting metabolism increased by 93% in the 68°F water and 350% in the 57°F water. Under the warm condition, though, no metabolic change was observed.

If you wallow in cool water long enough or brave the winter frost wearing only a Speedo, eventually your muscles will start to quiver involuntarily. It’s your body saying hey, I’m going to keep you warm whether you like it or not.

This is called shivering thermogenesis, and it’s one way your body maintains its core temperature in frigid environments. Shivering generates heat (that’s the thermogenesis part), and this increase in heat means a boost to your metabolism.

Although shivering thermogenesis accounts for some of the metabolic benefits of cold exposure, the more lasting benefits come from another adaptive mechanism: non-shivering thermogenesis.

Staying Warm Without Shivering

No, shivering isn’t the only way your half-naked body stays warm on a blustery winter day. You’re also kept warm by a special, metabolically active form of fat tissue called brown fat.

Brown fat, also called brown adipose tissue, converts food energy into heat energy. It keeps you warm without you having to shiver. Brown fat, then, is your secret weapon for nonshivering thermogenesis, which is way more fun than shivering to stay warm and burn more calories.

From a body composition perspective, having more brown fat is generally a good sign. Overweight and obese men, one study revealed, had less brown fat than healthy men. In that same study, brown fat activity was also positively correlated with metabolic rate.

It makes sense that higher brown fat activity is linked to higher metabolism. That’s because brown fat is packed full of mitochondria, our cellular energy centers. More mitochondria means more energy production.

But the mitochondria that give brown fat its distinctive brown color aren’t just any old mitochondria. These special mitochondria contain a protein called uncoupling protein 1 (UCP1), which creates energy in the form of heat. That’s how you stay warm without shivering.

A bit more detail on how this works. Mitochondria need food (like fatty acids and glucose) in order to produce energy. UCP1 super mitochondria, found in brown fat, are great at turning food into energy – specifically, heat energy.

In other words, when active, brown fat burns both fat and sugar to keep you warm.

From a metabolic perspective then, brown fat is pure gold. How do you get more of it? If you’re thinking cold pool, snow angels, cold showers, ice baths, or cryotherapy, you’re on the right track.

Browning Your Fat

If you want to turn your fat brown, cold exposure is the quickest way to accomplish that goal. You don’t need to shiver, but you might have to get a bit uncomfortable.

The good news is: the more brown fat you have, the less uncomfortable cold will make you. That’s because, as we covered earlier, the super-mitochondria in brown fat keep you warm through their special, UCP1-powered nonshivering mechanism. This, in turn, boosts your metabolism.

So what kind of cold exposure is required to brown your fat? Let’s look at some peer-reviewed research.

In one 2013 study, 8 men and 9 women were exposed to cool temperatures once a day for 10 days. While the exposure wasn’t very cold, it was enough to induce a response. And the response, in both men and women, was increased energy expenditure during the procedure.

The cold group also showed significant increases in nonshivering thermogenesis after the cold acclimation period. This means that, after being in a cold environment, these men and women became better able to convert food energy to heat – an adaptive response to the cold.

Along these lines, the final (and probably most interesting) finding in the study was the robust increase in brown fat tissue observed in the 17 subjects (a 37% increase, to be precise). “The current study shows, for what we believe is the first time,” write the authors, “highly significant [brown fat] recruitment in human adults after a 10-day period of cold acclimation.”

This research proves that it doesn’t take Game of Thrones, “winter is coming”, levels of cold to induce brown fat formation. Good news for all you non-polar bears out there.

There are also benefits on the opposite end of the spectrum. For those who can handle the heat, extra health benefits might be in store. Heat affects your metabolism during exercise. In fact, prolonged exposure to heat, in activities such as hot yoga, can actually speed up your metabolic rate, one pose at a time.

Hot Exposure–Hot Yoga and Your Metabolic Rate

Although hot yoga might seem like a crazy fad, it could actually be onto something.

Both your internal and external temperatures influence your metabolic rate. The chemical reactions that occur in your body and make up your metabolism happen more quickly if the temperature is higher, as the body works harder to restore your normal temperature balance. For example, if you have a fever, your BMR will predictably jump up to a rate that’s much higher than normal to increase the rate of cellular metabolic reactions aimed at tackling that fever and getting your body back to a healthy state.

When it comes to external temperature, though, it’s only prolonged exposure to heat that raises your BMR significantly. A brief exposure to heat isn’t enough to do much to your metabolism. To really raise BMR, a longer exposure to heat is necessary. This is something more easily changeable than certain genetic or biological factors, such as age, height or gender. And that’s why so many have turned to hot yoga in the hopes of helping their BMR.

Hot yoga is one of the latest trends in the world of exercise.

Traditional yoga, the kind that’s not done in a 105-degree room, has been around for a while. Experts say it originated in India more than 5,000 years ago. It’s still popular today, and for good reasons. Practicing yoga has real health benefits like increasing flexibility, managing high blood pressure, and improving cardiovascular (heart) health. But hot yoga might be overtaking its predecessor on the popularity scale because of something even better–its link with burning more calories by increasing metabolism.

The idea behind hot yoga is simple but might seem crazy to some. Hot yoga involves performing a set sequence in a studio that’s 105 degrees Fahrenheit with a 40% humidity rate. It’s an intense workout that involves a lot of sweating, and it scares many wannabe yogis away. The founder of the practice maintains that the high heat helps increase blood flow, warm the muscles for deeper stretching, and helps the lymphatic system to release toxins through sweat.

With hot yoga, all the same benefits of traditional yoga are still present–and a few extra advantages are added, too. Hot yoga tends to get a bad rap; either people think it’s crazy, dangerous, or both.

But in reality, there’s nothing wrong with hot yoga. It can be dangerous because you will be sweating a lot in a heated room, but as long as yogis consult with their doctor beforehand, hydrate properly and watch for any signs of heat intolerance, hot yoga is safe… and remember, it’s a great way to get a workout.

So how exactly does hot yoga help? Hot yoga is often performed in an environment where the temperature is much higher than your body temperature–for example, although most yoga studios keep the temperature below 100 degrees Fahrenheit, some are conducted in 105-degree heat. Because prolonged exposure to heat can raise your BMR, hot yoga can raise your BMR, too.

Yoga is extremely beneficial for many reasons. And although hot yoga may seem like an undue, sweaty punishment, the rewards could be worth it- because of the exposure to heat, hot yoga can help speed up your metabolic rate. Not only does hot yoga reap the benefits of increased flexibility and mental strength, but it can also help support your body composition goals. Next time you are looking for a new challenge, why don’t you roll out your mat and get your sweat on?

Wrapping Up

The link between our metabolism and body composition is strong, and it prompts many questions. How much Lean Body Mass do you have? What might your Basal Metabolic Rate be? What are the benefits of hot and cold exposure when it comes to your metabolism? These questions should be answered first before starting any weight loss or diet program. Getting the information you need and the answers to these questions by getting your body composition accurately tested is a great first step in your journey.

About the author : Colin Anthony

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