Globally, there's a growing concern about rising bodyweight and obesity in both developed and developing countries. Understanding the concept of energy balance is crucial for tackling this issue and designing effective interventions. To maintain a stable bodyweight, energy intake must precisely match energy expenditure over time, a state known as energy balance.
Understanding Energy Balance
The concept of energy balance is rooted in the fundamental thermodynamic principle that energy cannot be created or destroyed; it can only be gained, lost, or stored by an organism. Energy balance is achieved when energy intake equals energy expenditure. This concept can be used to illustrate how bodyweight changes over time in response to alterations in energy intake and expenditure. When the body is in energy balance, bodyweight remains stable.
Components of Energy Expenditure
Humans take in energy through food and drink and expend energy through three primary components:
- Resting Metabolic Rate (RMR): The energy expenditure required to maintain normal body functions and homeostasis. RMR is proportional to body mass, especially fat-free mass.
- Thermic Effect of Food (TEF): The energy required to absorb, digest, and metabolize consumed food, typically accounting for 8-10% of daily energy expenditure. Foods higher in protein can increase the thermic effect of food, while foods higher in fiber or volume may decrease energy intake.
- Energy Expended Due to Physical Activity (EEact): This includes voluntary exercise, shivering, postural control, and voluntary movement. It is calculated by multiplying the energy expenditure of an activity by the time spent performing it and is the most variable component of energy expenditure. The more sedentary the individual is, the lower the effect of physical activity, and vice versa.
The Interdependence of Energy Intake and Expenditure
It is commonly assumed that energy intake and energy expenditure can be independently modified through changes in food intake and physical activity to achieve energy balance. However, energy input and expenditure are interdependent and regulated at several levels. This involves a complex physiologic control system, which involves afferent neural and hormonal signals reaching the hypothalamus, with resultant efferent projections of the autonomic nervous system innervating the muscles, viscera, and adipose tissue. As a result of this physiologic control, components of energy intake and expenditure cannot be altered without compensatory changes in the other. The components of energy balance influence each other and serve to maintain a constant body mass. Despite this internal control system, the majority of adults gain weight over time.
Disturbances in energy balance cause changes in body mass, and the timeframe over which this occurs varies between individuals and may explain the large interindividual response to weight-loss interventions.
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Consuming excess energy does not result in continuous weight gain, since the weight gain is accompanied by an increase in energy expenditure that leads to a steady state of energy balance at this new, slightly higher bodyweight.
High-Energy Throughput: A Key to Easier Energy Balance
Theoretically, energy balance may be achieved at low or high levels of energy expenditure. However, it has been hypothesised that energy balance may be easier to achieve at high levels of energy expenditure, known as high-energy throughput. In the 1950s, cross-sectional studies found that energy intake was better matched to energy expenditure when people were physically active. Studies in rats found that the relationship between food intake and energy expenditure was only linear within certain limits. In rats, the matching of energy intake and expenditure was inaccurate at low or high levels of physical activity. The same observation was made in human studies: food intake did not drop when energy demand declined.
It has been proposed that a minimum threshold of either physical activity or energy throughput may exist. Above this threshold, termed the ‘regulated zone’, energy intake is raised to meet high energy expenditure. As a result, adaptive adjustments in energy intake and expenditure to achieve balance may be very sensitive in this zone. At low energy throughput, the ‘unregulated zone’, energy intake and expenditure are only weakly sensitive to each other. The unregulated zone is difficult for most people to sustain, and the result is weight gain, which returns the system to a high-energy throughput. Over the last century, the energy throughputs of Western human lifestyles have declined, pushing the majority of people into the unregulated zone.
The Role of Physical Activity
Strategies to combat obesity must target both energy input and expenditure, including food intake and physical activity. A recent report concluded that resting metabolism does not play a significant role in weight gain and that physical activity is a more important factor. Some authors have attributed the weight gain in the US population exclusively to the increased intake of food, arguing that leisure time physical activity has remained stable over the decades in which obesity rates increased.
However, this interpretation dismisses the decline in both lifestyle and occupational physical activity that has occurred over the past century. This continued decline in daily physical activity energy expenditure has created a ‘permissive’ situation in which any excess calorie intake is likely to promote weight gain (i.e. the unregulated zone). Weight gain in individuals maintaining a greater level of physical activity energy expenditure is less than among sedentary individuals, supporting the idea that higher levels of energy flux are protective against positive energy imbalance.
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Daily PA was inversely related to weight gain. A shift from a low PA to a moderate or high PA was necessary for weight loss over time. BMI, was inversely associated with PA spent in getting to work and practice of high-intensity recreational activities. Recreational PA was inversely related to bodyweight. Those who increased PA had less gain in bodyweight.
Urbanisation, industrialisation, and use of mechanised transportation have led to a general decrease in physical activity. While leisure time physical activity has remained fairly constant since 1988, physical activity in terms of lifestyle has declined significantly. A study of an Old Order Amish community found that the average number of steps per day taken by men in the community was 18,425 versus 14,196 for women. By contrast, in Colorado, the average male takes 6,733 steps per day, and the average female takes 6,384 steps per day, a difference in daily energy expenditure of 400-600 kcal per day.
As physical activity has decreased, bodyweights have increased, which is accompanied by a corresponding increase in energy expenditure. In terms of our understanding of energy balance, individuals with low levels of physical activity are at greater risk of positive energy balance and obesity compared with those with higher levels of physical activity. Those with a low level of physical activity have trouble achieving energy balance because they have to restrict their food to match their energy intake to a low level of energy expenditure.
In summary, there are clear benefits associated with increasing physical activity among the sedentary. A healthy weight is best maintained with a relatively high level of physical activity and a high energy intake. This does not mean that the need to control food intake should be forgotten.
Prevention vs. Treatment: A Matter of Scale
An understanding of energy balance leads to the conclusion that prevention of weight gain should be easier than treatment of obesity. Components of energy balance are interdependent, and weight loss requires major behaviour changes, which trigger compensatory decreases in energy expenditure that facilitate weight regain. Prevention of weight gain can be accomplished by smaller behaviour changes. In addition to being easier to sustain than larger behaviour changes, smaller ones produce less compensation by the energy balance regulatory system. It has been estimated that relatively small changes in energy intake and expenditure totaling 100 kcal per day could arrest weight gain in most people.
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Practical Strategies for Achieving Energy Balance
The evidence discussed above suggests that strategies to reduce the obesity epidemic should aim to push the population into the regulated zone of energy balance, and this can be achieved through an increase in physical activity. Interventions and recommendations that do not take into account both energy intake and expenditure tend to be unsuccessful in combating obesity in the long term. However, the majority of current weight loss interventions place the emphasis on food restriction rather than energy balance.
Small Changes, Big Impact
Strategies to prevent weight gain are more likely to be successful in the long term compared with strategies to promote weight loss because the physiologic systems involved in energy balance system respond more strongly to negative energy balance than to the prevention of positive energy balance. Weight loss interventions trigger compensatory mechanisms to maintain energy balance.
We are much more successful in producing weight loss than in maintaining it over the long term. Weight loss can be achieved through temporary changes in either diet or physical activity, but long-term weight loss maintenance requires permanent changes in both diet and physical activity.
The change in energy balance required to prevent primary weight gain is relatively small: a mathematical modeling approach was applied to US population models and concluded that the obesity epidemic could be explained by an average daily energy imbalance between intake and expenditure of about 10 kcal. Hill estimated that the median weight gain of the population of 0.5 to 1 kg a year over the last two decades can be accounted for by a positive energy balance of 15 kcal per day. At the 90th percentile of weight gain, this was 50 kcal per day.
Based on the assumption that excess energy is stored with 50 % efficiency, it was suggested that by reducing positive energy balance by 100 kcal/day by a combination of reductions in energy intake and increases in physical activity, weight gain in 90 % of the adult population could be prevented. Public health interventions may benefit from changing energy balance by using a specific quantifiable behavioural goal.
However, changing any behaviour is difficult and using a small-changes approach is useful for promoting incremental improvements that build self-efficacy along the way. Based on the concept of small-changes approaches, the America on the Move programme advocates walking 2,000 more steps each day and eating 100 kcal less each day (www.americaonthemove.org).
Dietary Approaches for Weight Loss
When active individuals want to lose weight, it is imperative that the risk of introducing disordered eating behaviors is minimized. In addition, inappropriate weight loss can introduce nutrient deficiencies important for sport performance, such as dehydration, inadequate protein and carbohydrate intake, and low micronutrient intakes.
- Low-Energy Dense Diet: A low-energy dense diet is high in whole fruits and vegetables, whole grains, and incorporates low-fat dairy, legumes/beans, and lean meats and fish. Overall the diet is lower in fat and reduces or eliminates beverages containing kcal, especially sweetened beverages and alcohol. It is high in fiber and water, and lower in fat, which means one can consume a greater volume of food for an overall lower energy intake and yet feel satisfied. A 10% decrease in dietary energy density will result in a ~10% decrease in energy content. There is less reliance on reducing portion size and counting calories. These diets make one feel full and satisfied after a meal, thus reducing the risk of recidivism. For athletes trying to lose weight, it may be easier to focus on changing the energy density rather than portion sizes. This approach reduces hunger and increases adherence to the weight loss diet plan.
- Strategic Nutrient Timing: Timing of food intake around exercise training and spreading food intake throughout the day will ensure that the body has the energy and nutrients needed for exercise and the building and repair of lean tissue. Refueling after exercise is especially important for the athlete who wants to lose weight. The post-exercise dietary routine needs to include fluids for rehydration, carbohydrate in the form of low-energy dense foods (e.g., whole fruits and vegetables, whole grains) to replenish glycogen, and high-quality low-fat protein for building and repair of lean tissue. An early morning workout can be immediately followed by a hearty breakfast, which then refuels and rehydrates.
- Adequate Protein Intake: Higher protein diets have been associated with increased satiety and reductions in energy intake.
Energy-containing sport drinks are appropriate to use around exercise, especially intense exercise of long duration and in extreme environments. However, other high energy sweetened beverages and alcohol can derail any individual trying to lose weight, including the athlete. They add extra energy to the diet without increasing satiety or reducing the amount of food consumed with these beverages.
Avoiding Extreme Dietary Practices
Although it is tempting to use extreme dietary practices, especially very low energy diets (< 1,200 kcal/d) that result in rapid weight loss, the athlete should avoid these diets. Combining severe energy restriction with an intense endurance and/or strength training program can actually result in metabolic adaptations that reduce the effectiveness of these two factors on weight loss.
Research has shown that slower, more reasonable weight loss in athletes (~0.7% loss of body weight/week) helped preserve lean tissue while improving strength gains over more severe weight loss (1.4% weight loss/week).
The Dynamic Nature of Energy Balance
The static energy balance equation does not work well for predicting weight loss. To better predict weight change in response to changes in energy intake or expenditure, one must account for the dynamic energy imbalances that occur.
The Challenges of Weight Loss
Weight loss - particularly extreme weight loss - is more complicated than consuming fewer calories than you burn. As many as 90 percent of people who have lost a considerable amount of weight will gain it back.
- The more you work out or manage your calorie intake to lose weight, the more your metabolism wants to compensate by slowing down to maintain your current weight, this is called metabolic compensation. It kicks in to preserve and store fat for future energy. Research shows that this happens because the human body has evolved to value storing fat and energy and to interpret a shortage of calories as sign of distress.
- Metabolic compensation isn’t the only way that your body’s can prevent weight loss or encourage weight gain. Fat cells produce leptin, which tells your brain when you’re full. Fat cells shrink when you lose weight, producing less leptin, which means that you don’t feel as full. Your stomach produces ghrelin, which tells your brain when it’s time to refuel. When you lose weight, your ghrelin levels rise, making you want to eat more often.
- When you lose weight, the part of your brain that regulates food restraint becomes less active - meaning that while you’re eating more to feel full (courtesy of leptin), you’re also less aware of how much you’re eating.
- More than 400 genes have been linked to obesity and weight gain, and they can affect appetite, metabolism, cravings and body-fat distribution. If you have a genetic predisposition for obesity, it is easier to take a proactive stance to weight management. A preventive approach is more effective because you are preventing obesity from occurring in the first place.
- Some scientists think that your body has a set point weight and your metabolism, hormones and brain will adjust to maintain that weight. People may have naturally higher or lower set weights than others; their set points can be impacted by genetics, aging, history of weight loss and hormonal shifts. The theory suggests that your set point weight can rise but rarely lower. It is easier to maintain your set point weight because your body wants to remain at that point - not lose weight.
- After successful weight loss, your body may look different than you were expecting.
- People often tie happiness and emotional health to weight loss. When they have successfully lost weight but remain dissatisfied with other parts of their life, they can fall into a cycle of dissatisfaction. Guilt at not feeling happy after weight loss can be a factor, as well as the temptation to eat to cope with these feelings.
Tips for Striking the Right Energy Balance
- Reshape your plate: Increase the proportion of vegetables, salads, fruits, beans, and whole grains to cover two-thirds of your plate. Fill the other third of the plate with lean meats and poultry, fish (especially fatty fish like tuna or salmon), or low-fat dairy.
- Control portions: Once in a while, get out the food scale and measuring cups to remind yourself of what a normal portion should look like.
- Schedule exercise: Schedule your exercise first thing in the morning so it won't get crowded out of your day.
- Vary your workouts: Alternate your exercise activities (maybe try a new class) so you won't get bored and your muscles will get a better workout.
- Increase intensity: Power past any weight loss plateaus by increasing the intensity of your workouts.