People start working out for all sorts of different reasons, but weight loss is one of the most common. Despite being billed as necessary for weight loss, the effects of exercise on body weight are nuanced.
For example, an exercise program that includes both lifting weights and conditioning improves the body composition. In individuals who are losing weight, more muscle mass is preserved and more body fat is lost when those folks also exercise. However, the same relationship is true in those who are maintaining their weight. These individuals will tend to gain muscle and lose more body fat than those who are not exercising.1,2
Exercise also seems to increase how full people feel after eating a meal, e.g. their satiety response. While many claim that exercise will cause someone to “work up an appetite”, experimental data suggests that people will “eat back” only a small proportion of the amount of Calories used during exercise.3,4 This is one of the ways in which exercise helps prevent weight regain in individuals who have lost weight. 5 It’s also important to mention that exercise is beneficial to both health and performance regardless of whether someone loses weight or not.6
All else being equal, we’d expect that exercise would help individuals to lose more weight and body fat, while also building more muscle, strength, and endurance, as compared to not exercising. The bulk of the scientific evidence supports this idea, particularly when combined with the appropriate diet for the individual.7,8 This is because, in short – working out needs to be paired with the correct nutrition for the desired results.
In this article, we’ll cover three of the most common reasons that people have issues with weight management when they start working out: eating too many Calories, short-term changes in water retention, and measurement error.
Reason #1: Eating Too Many Calories
The number one reason someone is either gaining weight or failing to lose weight – regardless of how much exercise they’re doing – is because they’re eating too many Calories. Barring short-term changes to fluid status (e.g. from dehydration) or measurement error, any change in an individual’s body weight (and mass) is a direct result of energy balance.
Although this information is accurate, it’s important to note that changing dietary behavior can be extremely challenging. This is because eating practices are influenced by a variety of biological, psychological, social, and environmental factors – and much of this influence on appetite and related behaviors occurs outside of our conscious control.
We think that pretty much everyone could benefit from eating a health-promoting dietary pattern and regularly participating in exercise. However, it is important to avoid focusing exclusively on body weight or composition as a reflection of health or success. People have different values, preferences, and goals and we should respect those when treating individuals. There are often a number of lifestyle changes that can offer benefits independent of any effect on body weight or body fat.
Energy Balance
Energy balance represents one of the fundamental principles of human metabolism. This physiological mechanism measures the relationship between caloric intake—the energy we obtain from food and beverages—and caloric expenditure, which occurs through basic metabolic processes and physical activity.
When caloric intake exceeds expenditure, the body stores this surplus energy, typically as adipose tissue. When intake and expenditure are equal, body weight remains stable. During periods when caloric expenditure surpasses intake, the body draws upon its energy reserves, resulting in weight loss. Understanding this metabolic equation is essential for managing body weight.
Total Daily Energy Expenditure (TDEE) is the total amount of Calories used (“burned”) per day. The three components of TDEE are; 1) resting energy expenditure (REE), 2) diet-induced energy expenditure (DEE), and 3) activity-induced energy expenditure (AEE), modeled by the equation:
TDEE = REE + DEE + AEE
The relative contributions to total daily energy expenditure from each of these components are approximately 60% for REE, 10% for DEE, and 30% for AEE.9 If you’re wondering what your total daily energy expenditure is, check out our free TDEE calculator.
The effect of energy balance on body weight change takes place over a long period of time, e.g., many weeks and months, not hours and days. There’s also greater than 5-fold variability in response to the same energy deficit or surplus between individuals, which is mostly attributed to genetics. 10,11
To summarize, if someone is losing weight, they are in a negative energy balance or Calorie deficit. If someone is gaining weight, they are in a positive energy balance or Calorie surplus. Achieving meaningful, sustainable weight change takes time spent in either an energy deficit or surplus, and results often vary significantly between individuals.
Managing Your Energy Balance
At first glance, maintaining or manipulating your energy balance may seem simple. For example, if you want to lose weight, it would seem there are three basic, fool-proof ways to achieve your goal:
- Consume fewer Calories than you burn.
- Increase your activity levels to burn more Calories than you consume.
- Increase your resting metabolic rate.
These strategies all generally work, but yet again, our physiology is less straightforward.
Reducing Calorie Intake
For example, reducing energy intake to achieve a Calorie deficit will indeed produce weight loss. However, many individuals’ bodies will fight back by reducing the amount of Calories used for basic metabolic processes and activity, while also increasing hunger cues, and decreasing feelings of fullness. 12, 13, 14
The combination of “metabolic adaptation”, where an individual using less energy, with increases in appetite leads to an inability to sustain an energy deficit, which then results in weight regain. Reducing hunger and increasing feelings of fullness to not only achieve, but also sustain an energy deficit is one of the ways GLP-1 Receptor Agonist (GLP-1 RAs) like semaglutide and tirzepaitide work.
Overall, reducing energy intake via changes to the diet are necessary to achieve a Calorie deficit. In many individuals however, their bodies will make adhering to the diet extremely challenging.
Increasing Activity To Burn More Calories
Increasing the amount of exercise performed in order to burn more Calories is another strategy used to achieve a Calorie deficit. With long-term exercise however, other components of energy expenditure tend to adapt and reduce energy utilization, thereby making the energy deficit created from more exercise less than predicted. This is still an area of active research and so, we’ll present both sides.
On the one hand, the constrained energy model suggests that humans and other animals share a set of evolved mechanisms to maintain total energy expenditure within a narrow range. When presented with higher activity levels that uses more energy, the body compensates by reducing energy expenditure elsewhere to keep daily expenditure in check.15
On the other hand, the additive energy model suggests that increased physical activity has its own energy or Calorie cost, and this energy is “added” to the total energy expenditure the individual had before undertaking an exercise program.
As of this writing, the answer is somewhere in the middle. With long-term exercise, there seems to be some reduction in energy expenditure from other elements of metabolism that overall “attenuates” the amount of “extra” Calories being burned during exercise, and results in only modest changes to total daily energy expenditure.16,17,18, 19, 20, 21, 22, 23 This is likely one of the main reasons why weight loss with exercise interventions produce less weight loss than predicted, especially when exercise is combined with dietary changes.
Overall, increasing energy expenditure by doing more exercise is likely to produce less weight loss than changes to the diet of a similar magnitude Calorie-wise. In other words, reducing intake by 500-Calories is likely to produce more weight loss than increasing exercise-related energy expenditure by 500-Calories. Still, doing more exercise is generally advisable for most regardless of the direct impact on weight loss, as it has significant health and performance benefits.
How-To Increase Metabolic Rate To Lose Weight
Increasing metabolic rate as a means to lose weight sounds good in theory, but is generally not possible without using potentially harmful drugs.
Resting Metabolic Rate (RMR) is the amount of energy used to sustain basic life processes such as breathing, heart function, and other essential tissue functions while an individual is awake. It’s typically measured after a 2-4 hour fast with the individual lying down in a neutral temperature environment (not hot or cold/shivering) for a period of 24 hours.
The majority (~80%) of an individual’s RMR is determined by the amount of lean body mass, which includes muscle, vital organs, extracellular fluids, and bone. 24 In general, higher amounts of lean body mass result in a higher RMR. About 20% of the variance in RMR cannot be explained by lean body mass, race, sex, age, measurement error, or environmental factors. This variance is primarily attributed to genetic differences.25, 26
Changes in body weight typically produce changes in both lean and fat mass. The proportional change in lean and fat mass varies among individuals and depends on many factors including level of energy intake, diet composition, baseline body fat level, activity level, genetics, health status, age, and more. All told, weight loss is estimated to result in a loss of approximately 75% fat mass and 25% lean tissue, though this varies significantly amongst individuals.27
With weight loss, the reduction in active body tissue also reduces energy expenditure by about 19 and 4.5 Calories/kilogram/day for lean and fat mass, respectively. 28 If an individual loses 10 kg consisting of 75% fat and 25% lean mass, we’d expect a reduction in RMR of approximately 81 Calories/day. RMR reductions beyond this level could represent adaptive thermogenesis due to changes in hormonal and other cellular signals that act to preserve energy balance. 29
Building muscle while reducing body fat commonly occurs in untrained individuals and those with obesity. 30,31 However, RMR does not seem to change that much when swapping a pound of fat for a pound of muscle. No, one pound of muscle does not weigh “more” than one pound of fat. Rather, the density of fat is ~ 0.92 kg/L and the density of muscle is 1.06 kg/L, 32,33 So, we could say that 1 kilogram of fat will take up a bit more space than 1 kilogram of muscle.
Semantics aside, it takes quite a long period of time to build a substantial amount of muscle, especially when someone isn’t trying to gain weight. In one study, a group of 18 elite rugby athletes trained for 14-weeks before their season started, which produced an average gain of 2.0 kg of lean body mass. There was no significant change in RMR. 34 Gaining muscle mass is certainly a good goal to chase from exercise, but it’s probably not going to increase RMR to a point where it helps in active weight loss efforts.
As mentioned at the beginning of this section, there are some drugs that can increase RMR. Perhaps the most infamous is 2,4 dinitrophenol, or DNP.
DNP is a chemical originally used to make explosives in World War I, but it has also been used as a dye, wood preservative, herbicide, and even to develop photographs. DNP works by uncoupling oxidative phosphorylation, which is the process our mitochondria use to create energy as our body is breaking down carbohydrates and fats. Instead of creating adenosine triphosphate (ATP), which is what our body uses for energy, DNP creates heat instead. This produces a large elevation in RMR (and heat) that can lead to weight loss. In the 1930’s, DNP was used for weight loss before studies showed that the drug was very toxic, leading to many medical complications including death. A number of DNP toxicity cases pop up each year, predominantly in bodybuilders who have purchased the powder over the internet despite the known harms. Of note, data on DNP shows that weight loss is generally smaller when compared to the newer GLP-1 RA medications.
Again, increasing resting metabolic rate in order to create a Calorie deficit is not generally a viable strategy for weight loss.
Reason #2: Increased Water Retention
Short-term changes in body weight are primarily related to changes in total body water, not actual changes in fat or muscle mass.
Humans are about 60% water by mass, though this ranges between 45 to 75% amongst individuals primarily due to different body compositions. Fat-free mass is typically between 70 to 80% water, while fat mass is only 10%.
Changes in hydration are broken up into three main categories:
- Euhydration: This is when your body’s water levels are just right, requiring minimal adjustment by physiological mechanisms. The body tends to function most efficiently here and is considered to be the normal baseline.
- Hypohydration: This is when total body water is decreased below baseline. It can vary in severity based on how low total body water is. People here have lost body mass via water.
- Hyperhydration: This is when total body water is above baseline. People here have gained body mass via water.
Dehydration and hypohydration are often used interchangeably, but they are not really the same things. Dehydration is the process of losing body water, e.g. sweating during exercise, whereas hypohydration describes total body water status.
While total body water is maintained within a relatively narrow range, small changes in hydration status can produce significant changes in body weight. In a 70kg human, a 2% change in total body water range represents a predicted weight change of nearly two pounds. Experimental data shows that some individuals, particularly women, experience even greater fluctuations in total body water and body weight. 35
The Effects of Exercise on Hydration
As you exercise, your muscles generate heat, which moves the blood to your core. This heat needs to be passed on through your skin to the environment. The small blood vessels in your skin help transmit this heat, which is when sweating occurs.
When all that heat is generated and transferred through your skin, you’ll need to cool down. Thankfully, your body knows how to do that. We sweat because the capillaries in our skin send the heat from our skin out to the environment. The vaporization of sweat accounts for ~80% of heat loss in hot, dry conditions. Therefore, our bodies produce sweat to cool us off when we are feeling the heat.
A certain amount of water leaves our bodies to produce sweat. Therefore, if we lose more water through sweating than we consume, dehydration occurs, which may have a detrimental effect on both the heat transfer from the muscles and the whole process of sweating. For example, hypohydration of ~ 2% or more decreases the rate in which you sweat, the onset of sweating, and in many cases, subsequent performance. 36,37
In response to regular exercise, the body tends to retain more water to be better prepared for future bouts of exercise. This is especially true in hot, humid environments that promote a higher sweat rate in most individuals.
Overall, changes in body water are common and can result in significant, short-term changes to body weight that are not associated with changes to the amount of fat or muscle someone has.
Recommendations for Hydration Before, During, and After Exercise
Maintaining an appropriate level of hydration is important for both performance and safety. 38 To establish a reference for the euhydrated state in an individual, measure body weight first thing in the morning, after going to the bathroom, but before consuming any food or drinks for three days. The average weight calculated after collecting 3-day data is an individual’s reference rate for their euhydrated body mass.
Once an individual’s euhydrated body mass is identified, our recommendations are as follows:
- Aim to start a workout within 1% of euhydrated weight.
- During a workout, aim to avoid losing more than 2% from euhydrated weight.
- After a workout, no specific hydration protocols are necessary for most people. Eating and drinking normally will be sufficient.
- For those working out multiple times a day with limited recovery time, aim to consume ~1.5L of water for each kilogram of body weight loss during exercise at a rate of ~0.4 to 1.5 liters per hour. The correct rate can vary significantly amongst individuals.
To practically apply the recommendations above, an individual would need to establish their euhydrated reference weight, as well as weigh themselves before and after a workout in order to assess hydration status and make the appropriate changes.
Reason #3: Measurement Error
Weight is accurately and precisely assessed with a scale, meaning there’s little reason to believe the scale is wrong. However, this doesn’t mean we can feel confident about a single weight.
Normal Fluctuations in Weight
In most humans, there is substantial variation in day-to-day weight due to changes in dietary practices, physical activity, and the body’s subsequent response. For example, a weekly rhythm in body weight has been observed, with individuals typically weighing more on Sundays and Mondays, and less on Fridays 39 The range of weight variation is typically on the order of a few pounds or so, or about 1-2% of someone’s weight, but this can vary by season, holidays, and more. 40, 41, 42
Additionally, significant changes in body weight due to loss or gain of muscle and/or fat generally takes place over a long period of time, e.g. weeks and months,not hours and days. As described in the previous, shorter term weight changes are typically due to differences in body water.
Frequent weighing that is done under similar conditions can produce a rolling average that can be used to analyze longer trends in weight change. These inputs would be a bit more indicative of what is happening at the tissue level. If you’re monitoring your weight and using it to guide the changes you make to your diet, we recommend combining two to four weeks of data to determine your energy balance and the effect it has on your body weight.
Take-Home
Gaining weight while exercising can be quite confusing and frustrating, especially if your main goal is losing or maintaining your current weight. Even so, knowing why the weight gain occurs, can help you navigate these changes more effectively. The main reasons for gaining weight while following a training program can be boiled down to eating too many Calories, fluctuations in hydration levels, and measurement error.
If you’re exercising to lose weight, but are still gaining, we recommend that you do the following:
- Consider reducing your daily intake by 200 to 300 Calories daily. Use our free calorie calculator.
- Consume ~ 1.4 to 1.6 grams of protein per kilogram of total bodyweight per day coming from lean, predominantly unprocessed or minimally processed sources.We recommend using our free macronutrient calculator.
- Change the food environment by reducing access to foods with added sugar, added sodium, and increase availability of minimally processed foods in the home and work place to the extent that’s possible.
- Make sure you’re meeting or exceeding the exercise guidelines for both lifting and conditioning.
- Use a rolling average of bodyweight values to accurately assess trends in body weight.
And perhaps our most important recommendation…KEEP EXERCISING! Regardless of weight change, exercise is beneficial to health. If you have questions or need a helping hand, shoot us an email at support@barbellmedicine.com.
