Metabolism—what it is and how it works—is one of the least understood parts of weight management. No doubt your clients have presented you with all kinds of theories on how their metabolism has impacted their health and weight loss efforts. New research offers an enhanced understanding of metabolism and its role in combatting obesity.
An Obesity Crisis
The prevalence of obesity in the United States is increasing at alarming rates. According to the Centers for Disease Control and Prevention, approximately 42.4% of U.S. adults and 21.2% of U.S. teens (ages 12–19) are considered to have obesity (CDC 2021).
These statistics are concerning because of the serious comorbidities that can be induced by obesity, such as type 2 diabetes, hypertension, certain types of cancer, dyslipidemia (imbalance of lipids) and cardiovascular disease, all of which contribute to an increased risk of mortality (Abdelaal, le Roux & Docherty 2017).
There is an undeniable necessity to develop effective strategies for the management and prevention of this disease—a considerable task that requires a fundamental understanding of metabolism and the effects of exercise and diet interventions on the metabolic process.
What Is Metabolism?
Metabolism is a wide-ranging term that describes the constant work happening in each of the body’s cells. This includes pumping molecules from the blood into each body cell through the cell’s membranes, as well as sending other molecules out of the cell.
Very few of those molecules come into the cells ready to use. There is work to be done within the cell! Some cells build new molecules (a process called anabolism) while other cells break down molecules (catabolism) to be used for fuel. And all the work within each cell requires energy. Energy is released when the bonds within certain molecules in the cell (called adenosine triphosphate, or ATP) break apart. See “Understanding ATP,” below, for more about energy production in human cells.
We commonly use the terms calories and kilocalories to measure energy. A calorie is the total energy needed to bump up the temperature of 1 gram of water by 1.8° Fahrenheit. (Note: A 1/4 teaspoon of water = 1.42 g.) A kilocalorie is the total energy needed to increase the temperature of 1 kilogram of water (about a liter) by 1.8° Fahrenheit. While this may seem confusing, what’s most important for clients to know is that, for purposes of nutrition and exercise, kcal and calories are equal in number or amount.
Carbs, Fats and Proteins: The Raw Materials of Metabolism
Metabolism really starts with what you eat. The three macronutrients—carbohydrates, fats and proteins—are the raw materials of metabolism and fuel the process of energy production in your cells.
Carbohydrates come in three basic forms: sugars, starches and fibers. When digested, starches and sugars are used to build glycogen stores in the liver, brain and muscle, or they are used as fuel for energy. (See “Loving Carb-Loading,” below, for why the body prefers carbohydrates for high-intensity exercise.)
Dietary fiber is a nondigestible form of carbohydrate—our body doesn’t have the enzymes to break it apart. It comes in two main categories: soluble and insoluble. Soluble fiber, which dissolves in water, creates a gel-like substance in the gut that helps the body with blood glucose control, potentially reducing the risk of diabetes. Insoluble fiber, which does not dissolve in water, helps to promote regular bowel health by assisting in the transport of food through the stomach and intestines.
Fats are involved in chemical actions that control several aspects of metabolism including growth, immune function, reproduction and nutrient absorption. Fat is a major form of energy for the body. Each gram of fat provides the body with 9 kcals, which is twice that supplied by carbohydrates and proteins (at 4.5 kcal/g apiece).
Proteins, unlike carbohydrates and fats, are not a primary source of energy. Their principal role is to build and maintain muscle cells and other tissues of the body.
Key Components of Energy Expenditure
Total daily energy expenditure (TDEE) is a term that encompasses all of the components of human calorie expenditure. These components include resting metabolic rate (RMR), the thermic effect of food (TEF), exercise activity thermogenesis (EAT) and nonexercise activity thermogenesis (NEAT) (Aragon et al. 2017). All these elements combine to make up our total metabolism.
Resting metabolic rate is the energy required to maintain homeostatic processes (stable internal conditions in the cells) when a body is at rest. RMR is the largest element of TDEE, contributing about 60%–70% of TDEE.
The thermic effect of food (also called diet-induced thermogenesis) is the energy required for the digestion, absorption and storage of food. This accounts for approximately 8%–15% of TDEE.
Exercise activity thermogenesis (EAT) occurs during physical exercise that is performed with an objective of improving health. EAT accounts for 15%–30% of TDEE.
Nonexercise activity thermogenesis (NEAT) happens during spontaneous movement, such as standing, walking, stair climbing, fidgeting, cleaning, etc. NEAT comprises ~15% of TDEE in sedentary individuals and perhaps up to 50% or more in highly active individuals.
Various factors may prompt fluctuations in these components, as well as the variation between individuals, suggesting the development of different individual strategies to treat and prevent the onset of obesity in clients.
(Sources: Aragon et al. 2017; Chung et al. 2018.)
Interindividual Variations in Resting Metabolic Rate
Although RMR is a relatively static and involuntary component of TDEE, certain factors can cause large interindividual variations, which can impact a person’s tendency toward a healthy or obese body weight (Chung et al. 2018).
Chung et al. explain that body size, total body mass and body composition are responsible for these interindividual variations and that 80% of the variance in RMR is determined by body size. Lean body mass (muscle mass) is positively correlated with RMR.
Fat Mass and Fat-Free Mass
Two important components to consider when evaluating body composition are fat mass (FM) and fat-free mass (FFM).
Fat mass encompasses all the fat tissue in the body. Fat-free mass is skeletal muscle mass, body water, smooth muscle mass and bone.
Studies have shown that, of the two types of mass, FFM has a significant impact on RMR, whereas FM has a much smaller impact (Johnstone et al. 2005).
Perhaps surprisingly, Johnstone et al.’s study indicated that while the amount of fat in the body did account for some of the variation in RMR between individuals, it was minimal.
In fact, Johnstone and colleagues reported that each kilogram of fat-free mass had five times more of an impact on RMR than did each kilogram of FM! These conclusions are similar to those of other studies evaluating the impacts of FM and FFM on RMR.
Although the metabolic rate of adipose (fat) tissue is relatively small (4.5 kcal/kg/d), large changes in FM (gains/losses) do impact RMR to some degree, however, not to the extent of that of FFM (Johnstone et al. 2005; Aragon et al. 2017). The variation in RMR between individuals, then, can largely be explained by differences in quantities of FFM (Johnstone et al. 2005).
Specific Categories of FFM and RMR
With respect to RMR, the components of fat-free mass can be further categorized into one of the following: metabolically active FFM (heart, kidneys, brain, liver, skeletal muscle) and low-metabolic-rate tissues or organs (bone, extracellular mass).
Here are the metabolically active components of FFM and their energy expenditure (Aragon et al. 2017):
- heart: 400 kcal/kg/d
- kidneys: 400 kcal/kg/d
- brain: 240 kcal/kg/d
- liver: 200 kcal/kg/d
- skeletal muscle: 13 kcal/kg/d
Despite its relatively low metabolic activity, skeletal muscle is the largest organ in the body and accounts for approximately 40% of total body weight in humans, thus making it the largest overall FFM contributor to RMR (Frontera & Ochala 2015).
Weight loss implications suggest that resistance training, which can measurably increase skeletal muscle mass, may be a decisive strategy for fit pros to incorporate in a long-term weight management plan to help a client lose fat and then maintain fat loss.
Rethinking Energy Balance for Metabolism and Weight Management
Energy balance is the difference between calories consumed and calories expended over a sustained period of time. There is no doubt that obesity is the result of a prolonged positive energy balance, in which people consume more calories than they expend (Hill, Wyatt & Peters 2013). Decreasing the number of calories consumed and/or increasing the number of calories expended can stop, or even reverse, weight gain and, therefore, obesity.
Reducing calorie consumption is fairly uncomplicated and can be achieved with numerous strategies including decreasing the intake of high-calorie foods, eating smaller portions, eliminating sugar-sweetened beverages and eating less frequently throughout the day.
But, as fit pros know, establishing behaviors to maintain any of these strategies is complex. Energy expenditure of calories lost in exercise—and the effect on total daily energy expenditure—is a new area of intense research and surprising findings.
Measuring Energy Expenditure: Doubly Labeled Water
To begin the discussion on energy expenditure in humans, it’s essential to discuss the primary measurement tool now being used in recent research. The most accurate and safe way to measure TDEE is a costly approach known as doubly labeled water. Volunteers drink a small amount of water that has been enriched in two isotopes, deuterium and oxygen-18. Researchers analyze the concentration of each of these isotopes in urine and are able to calculate the daily rate of carbon dioxide production. Carbon dioxide production is directly related to daily energy expenditure. The doubly labeled water technique is the gold standard for measuring TDEE.
The Human Metabolism Paradox: Surprising Research Findings
Research indicates that people tend to burn a similar number of TDEE kcals, regardless of how physically active they are (Pontzer 2018). Pontzer continues that people adapt dynamically to changes in their daily physical movement, maintaining total energy expenditure within a narrow range.
These surprising research findings initially appeared in a 2009 study (Luke et al.) measuring the energy expenditure of 149 women from rural Nigeria (7% of them obese; involved in physically active professions and lifestyles) and 172 African American women (50% of whom were obese; involved in less active professions and lifestyles). The Luke et al. study found no significant difference in daily energy expenditure between the two groups.
In another large doubly labeled–study, Dugas et al. (2011) did a meta-analysis of data from 98 studies around the world, which showed that populations in developed nations (spoiled by modern conveniences) have similar energy expenditures to those in less-developed countries.
These surprising new observations in energy expenditure across populations of different activity levels have led Pontzer to introduce the Constrained Total Energy Expenditure model for energy compensation.
The constrained total energy expenditure (constrained TEE) model submits that humans share a set of evolved mechanisms to maintain total daily energy expenditure within a narrow range, which dynamically compensates to changes in physical activity to maintain daily energy expenditure (see “Constrained TEE Model,” below).
New Applications for Fit Pros Regarding Metabolism and Weight Management
First and foremost, as Pontzer emphasizes, regular exercise is essential for a healthy life, summarizing research from several studies that consistent exercise has been shown to decrease the risk of cardiovascular disease, type 2 diabetes, many cancers, cognitive decline and mental illness. Pontzer continues, stating that a sedentary life is associated with an increase in cardiometabolic disease and shortened lifespans.
Linking this with the new constrained TEE model information leads to the idea that fit pros should assess dietary interventions and exercise as uniquely different tools in weight management strategies. Pontzer clearly states that initially (and perhaps for several months thereafter), cardiovascular exercise in a weight management program provides a meaningful “additive” benefit to total daily expenditure.
“Certainly, in the immediate term, exercise can have profound effects on energy expenditure, with metabolic rate rising by an order of magnitude during strenuous exercise,” he wrote. However, at some time, which has not been identified in any research yet, there is a leveling off of the total daily energy expenditure, regardless of the physical activity of a person. This research suggests that for further gains in weight management, fit pros should incorporate portion control interventions that create a caloric deficit for clients.
What Diet Interventions Are Best for Various Metabolism and Weight Management Goals?
The International Society of Sports Nutrition recently published a critical analysis position stand on the literature regarding the effects of diet types and their influence on body composition (Aragon et al. 2017). (The authors define a diet as the sum of energy and nutrients obtained from foods and beverages consumed regularly by individuals.)
This research assessed very-low-energy diets, low-energy diets, low-fat diets, low-carbohydrate diets, high-protein diets, intermittent fasting, and ketogenic diets. (The researchers note that diets with qualitative themes or commercial brands were grouped under the appropriate classification of diet category above.) Highlights of the findings of this research include the following:
1) There are a great variety of eating styles and diet types. Fit pros are encouraged to always read the published research on a diet to separate personal claims or testimonials from evidence-based research.
2) Diets centrally focused on fat loss are driven by a sustained caloric deficit. More aggressive caloric deficit strategies may be indicated for people with higher baseline fat levels.
3) As clients get leaner, slower rates of weight loss will better preserve lean mass.
4) Diets designed to gain muscle mass are driven by a sustained caloric surplus to facilitate anabolic processes and support increasing resistance-training demands. The training status of the participants, as well as diet composition and magnitude, will influence gains made by the client.
5) A wide range of dietary approaches (low-carbohydrate/ketogenic to low-fat and all points between) appear to be similarly effective for improving body composition. This allows great flexibility for fit pros to help clients find a suitable strategy that works individually for them.
6) Increasing dietary protein to levels significantly beyond current recommendations for athletic populations may result in improved body composition. Higher protein intakes (2.3–3.1 g/kg FFM) may be required to maximize muscle retention in lean, resistance-trained clients under hypocaloric conditions. Emerging research on very high protein intakes (>3 g/kg) has demonstrated that the known thermic, satiating and lean mass–preserving effects of dietary protein may be better in resistance-training clients.
7) The collective body of intermittent caloric restriction research demonstrates no significant advantage over daily caloric restriction for improving body composition.
See also: Big Breakfast and Metabolism
8) Sufficient protein intake combined with consistent resistance training and an appropriate rate of weight loss should be the primary focus for achieving the objective of lean mass retention (or gain) during fat mass loss.
9) The long-term success of a diet depends upon compliance and circumvention of mitigating factors, such as adaptive thermogenesis (also called “metabolic adaptation”), which refers to an undesirable decrease in resting metabolic rate.
10) More research is needed on women and older populations on dietary interventions with different training protocols.
11) Behavioral and lifestyle modification strategies are still poorly researched areas of weight management.
What Exercise Strategies Are Best for Metabolism and Weight Management?
A recent study (Ostendorf et al. 2018) comparing exercise patterns of participants (ages 18–65) provides excellent real-life and data-based direction for fit pros to incorporate with weight-management-driven clients.
Ostendorf et al. objectively compared three different groups: successful weight loss maintainers (maintaining >30 lb weight loss for ≥1 year), normal weight controls, and overweight/obese controls. Physical activity patterns were tracked for all volunteers for 1 week. Results clearly showed the weight loss maintainers spent significantly more time in sustained moderate-to-vigorous physical activity than did the other two groups (controls).
Another big finding is that weight loss maintainers and normal weight controls spent significantly more time doing light physical activity and significantly less time in sedentary behaviors. Ostendorf et al. summarize that high levels of moderate-to-vigorous physical activity are strongly correlated with successful long-term weight loss maintenance. Registered dietitian Nadine DeMone, RD, LD, (co-author of this manuscript) confirms this research from her real-life profession working with obese patients. She tells her patients, “Weight loss happens in the kitchen. Weight maintenance happens in the gym.”
The current Physical Activity Guidelines for Americans state the following for weight management: “People who want to lose a substantial amount of weight (more than 5% of body weight) and people who are trying to keep a significant amount of weight off once it has been lost may need to do more than 300 minutes of moderate-intensity activity a week to meet weight-control goals. Muscle-strengthening activities can also help maintain lean body mass during weight loss” (DHHS 2018). As stated earlier, muscle-strengthening exercises are a most effective way to maintain and/or increase a person’s resting metabolic rate.
See also: How Strength Training Impacts Metabolism
Use NEAT Strategies to Reduce Sedentary Behaviors
Ostendorf et al. emphasize that a major finding from their research is that weight loss maintainers spend significantly less time in sedentary behaviors. A reminder: Nonexercise activity thermogenesis (NEAT) encompasses energy expenditure related to unplanned and unstructured movements (such as maintaining and changing posture) and activities of daily living (such as walking, working and stair climbing). NEAT tends to be a predominant contributor to physical activity for people who also exercise regularly (Chung et al. 2018).
Given that the physical demand of NEAT can be relatively low, NEAT can be sustained for long periods of time, even for individuals who are generally inactive. These activities performed for sustained periods of time and/or intermittently throughout the day can contribute considerably to TDEE and, as such, can and should be incorporated into prevention and treatment strategies for obesity. See “30 Ways to Add More Movement to a Client’s Daily Life,” right, for suggestions.
Metabolism, Obesity and Weight Management Research Highlights
From this review on metabolism, here are some highlighted findings:
- Body size, total body mass and body composition are responsible for the major interindividual variations in resting metabolic rate (Chung et al. 2018).
- Each kilogram of muscle mass has five times more of an impact on resting metabolic rate than does each kilogram of fat mass (Johnstone et al. 2005).
- There is no doubt that obesity is the result of a prolonged positive energy balance, in which an individual is consuming more calories than they expend (Hill, Wyatt & Peters 2013).
- A wide range of dietary approaches (low-carbohydrate/ketogenic to low-fat and all points between) appear to be similarly effective for improving body composition (Aragon et al. 2017).
- A decisive strategy for fit pros to incorporate in a long-term weight management plan is to help a client increase muscle mass (and prevent its loss) to maintain and or elevate resting metabolic rate (Frontera & Ochala 2015).
- Certainly, in the immediate term, exercise can have profound effects on energy expenditure for weight loss success (Pontzer 2018).
- At some time, which has not been identified in any research yet, there is a leveling off of the total daily energy expenditure, regardless of the physical activity of a person. At this point, for attainment of further weight management goals, fit pros should incorporate dietary interventions that create a caloric deficit for clients (Pontzer 2018).
- High levels of moderate-to-vigorous physical activity are strongly correlated with successful long-term weight loss maintenance (Ostendorf et al. 2018).
- Current exercise guidelines recommend more than 300 minutes of moderate-intensity activity a week to meet and maintain weight-control goals (DHHS 2018).
- Wherever possible, add NEAT movement strategies to a client’s lifestyle (Chung et al. 2018).
Most importantly, get your clients moving for quality of life! You can do this!
The work that happens within each cell requires energy. Chemical energy is released when the bonds between molecules break apart. This energy can take on many forms in biological systems,
but the energy currency that’s most useful is adenosine triphosphate (ATP).
Cells cannot create ATP from scratch. The process begins with the raw materials of metabolism (digested food)—specifically in the chemical bonds that hold together the organic (carbon-based) compounds in that food. These compounds include 1) glucose (a simple sugar, or monosaccharide), 2) glycogen (a complex sugar, or polysaccharide, composed of hundreds or thousands of glucose molecules stored in the muscles, liver and brain) and 3) fatty acids (saturated or unsaturated acids produced during the breakdown of triglycerides).
When these compounds enter energy pathways, some of the molecules break or become
rearranged. The energy is released and captured in the formation of ATP. That energy is then used
to power every activity in the cell, such as supplying the energy for muscle contraction, building other complex molecules (in conjunction with enzymes), generating electrochemical messages in nerves, and transporting substances across cell membranes.
Different types of food produce different levels of ATP: This is helpful to know as you’re guiding your clients (within your scope of practice) on the best food choices for their needs (see “Loving
Carb-Loading,” page 23).
Think of ATP as a microscopic rechargeable battery. It’s an energy currency constantly replenished to give our cells what they need to function.
- RMR: resting metabolic rate
- TDEE: total daily energy expenditure
- Constrained TEE: constrained total energy expenditure
- EAT: exercise activity thermogenesis
- NEAT: nonexercise activity thermogenesis
- FM: fat mass
- FFM: fat-free mass
Constrained TEE Model
Most of your clients have heard of carb-loading, but they should understand that it’s not as simple as eating donuts the day before a long run! Yes, the body does crave carbohydrates for intense exercise, but here’s the real story:
As exercise intensity increases from rest to near-maximal levels, there is a gradual transition to use more glucose and glycogen as the predominant sources of adenosine triphosphate (ATP), the body’s preferred energy currency (see “Understanding ATP,” page 20).
From a metabolic standpoint, more ATP can be produced aerobically from the breakdown of carbohydrate than from the breakdown of fat. However, and most importantly, many more fast-twitch muscle fibers are recruited as exercise intensity increases. Because of the enzymes in fast-twitch muscle fibers, they are much more suited to utilize carbohydrates for the needed ATP production.
In addition, higher-intensity exercise stimulates epinephrine production, which also enhances carbohydrate metabolism.
30 Ways to Add More Movement to a Client’s Daily Life
Here are 30 suggestions to help clients develop physically active lifestyles and increase their nonexercise activity thermogenesis (NEAT).
- Get up and walk around your house upon awakening each day.
- Walk during your lunch hour (before or after you eat—or both!).
- Walk instead of drive whenever you can.
- Take a family walk after dinner.
- Take a brief walk every time you refill your water glass.
- Vacuum more frequently.
- Walk around the grocery store before you start shopping.
- Walk your dog regularly.
- Replace the Sunday drive with a Sunday walk.
- Walk and talk while making phone calls (in a safe area).
- Bring your groceries into your house one bag at a time.
- Plan some play time with your dog at a park.
- Wash the car by hand.
- Walk fast when doing errands.
- Pace the sidelines at your kids’ athletic games.
- Keep walking around in the airport departure area instead of sitting to wait.
- Walk to a co-worker’s desk instead of emailing or calling.
- Take a brief walk before and/or after a coffee/tea break.
- Try doing some walking meetings with co-workers.
- Take a brief walk after work (or before if you have more time).
- Take the stairs instead of the elevator or escalator when possible.
- Walk in place or do some alternating knee lifts during TV commercials.
- Walk in place or do some alternating knee lifts between TV shows.
- Do toe lifts or step-touches while brushing your teeth.
- Play with your kids for 30 minutes.
- Dance to music while you brush your hair.
- March in place when doing your laundry or dishes.
- Every 30 minutes get up and move briskly for 1–3 minutes.
- Park at the far end of the lot (in a safe area).
- Turn on music and dance while you prepare a meal.
Abdelaal, M., le Roux, C.W., & Docherty, N.G. 2017. Morbidity and mortality associated with obesity. Annals of Translational Medicine, 5 (7), 161–72.
Aragon, A.A., et al. 2017. International Society of Sports Nutrition position stand: Diets and body composition. Journal of the International Society of Sports Nutrition, 14 (16).
CDC (Centers for Disease Control and Prevention). 2021. National Health and Nutrition Examination Survey. Accessed Jan. 12, 2023: cdc.gov/nchs/nhanes/visualization/.
Chung, N., et al. 2018. Non-exercise activity thermogenesis (NEAT): A component of total daily energy expenditure. Journal of Exercise Nutrition & Biochemistry, 22 (2), 23–30.
DHHS (U.S. Department of Health and Human Services). 2018. Physical Activity Guidelines for Americans, 2nd Edition. Accessed Feb. 8, 2023: health.gov/sites/default/files/2019-09/Physical_Activity_Guidelines_2nd_edition.pdf.
Dugas, L.R., et al. 2011. Energy expenditure in adults living in developing compared with industrialized countries: A meta-analysis of doubly labeled water studies. The American Journal of Clinical Nutrition, 93 (2), 427–41.
Frontera, W.R., & Ochala, J. 2015. Skeletal muscle: A brief review of structure and function. Calcified Tissue International, 96 (3), 183–95.
Hill, J.O., Wyatt, H.R., & Peters, J.C. 2013. The importance of energy balance. European Endocrinology, 9 (2), 111–15.
Johnstone, A.M., et al. 2005. Factors influencing variation in basal metabolic rate include fat-free mass, fat mass, age, and circulating thyroxine but not sex, circulating leptin, or triiodothyronine. The American Journal of Clinical Nutrition, 82 (5), 941–48.
Luke, A., et al. 2009. Energy expenditure does not predict weight change in either Nigerian or African American women. The American Journal of Clinical Nutrition, 89 (1), 169–76.
Ostendorf, D.M., et al. 2018. Objectively measured physical activity and sedentary behavior in successful weight loss maintainers. Obesity, 26 (1), 53–60.
Pontzer, H. 2018. Energy constraint as a novel mechanism linking exercise and health. Physiology, 33, 384–93.
Len Kravitz, PhD is a professor and program coordinator of exercise science at the University of New Mexico where he recently received the Presidential Award of Distinction and the Outstanding Teacher of the Year award. In addition to being a 2016 inductee into the National Fitness Hall of Fame, Dr. Kravitz was awarded the Fitness Educator of the Year by the American Council on Exercise. Just recently, ACSM honored him with writing the 'Paper of the Year' for the ACSM Health and Fitness Journal.