Understanding Food Energy: How the Body Utilizes Calories

Understanding how food provides energy to the body is essential for maintaining good health. The energy content of food is usually measured in joules or calories, but the way this energy is utilized by the body is more complicated than measuring it with a bomb calorimeter. Factors that impact how food is digested, absorbed, and used in the body include biological factors, health status, and the loss of energy during excretion. In this article, we explore the different factors that impact how food energy is used in the body and how to measure metabolic rate, which is essential for maintaining a healthy weight.

Food contains potential energy which can be converted into work. The chemical bonds in our food give us a rich source of potential energy. Measuring energy in food is usually done in Joules or Calories. One Calorie equals 4.18 Joules. Small calories are smaller and use a lowercase c, while large Calories are 1000 times larger and use an uppercase C. We usually mean large C or kilocalories (kcals) when we talk about food Calories. Bomb calorimetry is a commonly used technique to determine the energy content of food. The process involves burning a small sample of the food in a sealed chamber surrounded by water. The heat generated by the combustion of the food is transferred to the water, causing a temperature rise. By measuring the temperature change and the amount of water present, scientists can calculate the energy content of the food in terms of calories or joules.

You may have heard that fat contains 9 kcal per gram, carbohydrates contain 4 kcal per gram, protein contains 4 kcal per gram, and alcohol contains 7 kcal per gram. These values are not just rounding errors but represent actual physiological values of how these substances behave in the body. However, the numbers mentioned earlier were obtained outside the body using a bomb calorimeter. So, this may seem like a contradiction but it is because the two sets of values are measuring energy content in different ways. Humans are not bomb calorimeters, and there are many factors that can impact how food is digested, absorbed, and used in the body. Biological systems are not simple machines, and even if we know the exact number of calories entering our bodies (which is rare), it is challenging to determine how many calories we will absorb and use. Therefore, precise calorie counting is not always useful for individuals or clients.

Factors that affect the digestion, absorption, and use of food include energy loss during digestion, the health and function of the gastrointestinal tract, and the loss of energy during excretion. Additionally, various factors can impact the nutrient and energy content of food, including resistant starches and fibers, outdated data, imprecise analytical methods, product variety, soil, and growing conditions, ripeness at the time of harvest, animals' diets, length of storage, and the preparation and cooking time.

These factors mean that the energy and nutrient information listed on food labels or in nutrient databases may not be entirely accurate, varying by as much as 25% more or less than what the package claims. Even some frozen foods can contain more calories than the package lists, and some restaurant meals may have more calories than claimed. As a result, it is challenging to determine precisely how many calories and nutrients are absorbed, used, or expended through metabolism and movement. The body’s need for energy:

As we’ve discussed previously, the total amount of energy required for each of our physiological actions is referred to as the metabolism. We can group these into five general categories:

1. Basal metabolic rate (BMR) Basal metabolic rate (BMR) is the minimum level of energy needed to maintain vital functions of the body, such as breathing and heart rate, during a state of rest and no digestion or movement. BMR is measured in a laboratory setting by determining the amount of oxygen consumed per minute using a metabolic cart and accounts for over 70% of daily energy expenditure.

2. Resting metabolic rate (RMR) Resting metabolic rate (RMR) is similar to BMR but is easier to measure since it doesn't require extreme conditions. RMR is also measured by determining the amount of oxygen consumed during rest but may be slightly higher than BMR due to small amounts of movement, different environments, and digestion.

3. Thermic effect of feeding (TEF) The thermic effect of feeding (TEF) refers to the energy used during the digestion, absorption, and assimilation of ingested food and nutrients. The thermic response varies depending on the macronutrient composition of the food, with proteins having the highest response and fats having the lowest. TEF typically accounts for around 10% of daily energy expenditure.

4. Exercise activity Exercise activity (EA) refers to the energy used during purposeful exercise and varies widely from person to person. Higher-intensity exercise can create a demand for energy transfer during and after the activity, resulting in increased daily energy expenditure. EA can make up anywhere from 10-15% of daily energy demand for sedentary individuals to over 30% for highly physically active individuals.

5. Non-exercise activity thermogenesis (NEAT)

Non-exercise activity thermogenesis (NEAT) refers to all daily-life movement that isn't deliberate exercise, such as housework, playing with pets, or carrying groceries. NEAT may contribute the least to daily energy expenditure for individuals without physically active jobs, but is an important component of weight loss or gain and can vary widely between individuals. Together, RMR, physical activity, and TEF make up total daily energy expenditure (TDEE), which can be represented by the equation: RMR + physical activity + TEF = TDEE.

Indirect calorimetry: This is the most common method used to measure metabolic rate in humans. It involves measuring the amount of oxygen consumed and carbon dioxide produced during respiration. From these measurements, the metabolic rate can be calculated using the equation:

Metabolic rate (kcal/day) = (3.941 x VO2) + (1.11 x VCO2)

Where VO2 is the rate of oxygen consumption (in liters/minute) and VCO2 is the rate of carbon dioxide production (in liters/minute).

Respiratory quotient (RQ): RQ is the ratio of carbon dioxide produced to oxygen consumed during respiration. It can be used to estimate the type of macronutrients being oxidized for energy (i.e. carbohydrates, fats, or proteins). RQ is calculated using the following equation:

RQ = VCO2/VO2

Where VCO2 is the rate of carbon dioxide production (in liters/minute) and VO2 is the rate of oxygen consumption (in liters/minute).

Estimating energy needs and energy intake: Indirect calorimetry: This is the most common method used to measure metabolic rate in humans. It involves measuring the amount of oxygen consumed and carbon dioxide produced during respiration. From these measurements, the metabolic rate can be calculated using the equation:

Metabolic rate (kcal/day) = (3.941 x VO2) + (1.11 x VCO2)

Where VO2 is the rate of oxygen consumption (in liters/minute) and VCO2 is the rate of carbon dioxide production (in liters/minute).

Respiratory quotient (RQ): RQ is the ratio of carbon dioxide produced to oxygen consumed during respiration. It can be used to estimate the type of macronutrients being oxidized for energy (i.e. carbohydrates, fats, or proteins). RQ is calculated using the following equation:

RQ = VCO2/VO2

Where VCO2 is the rate of carbon dioxide production (in liters/minute) and VO2 is the rate of oxygen consumption (in liters/minute). Direct calorimetry: This method involves measuring the heat generated by an organism in a sealed chamber. The amount of heat produced is directly proportional to the amount of energy expended by the organism. However, direct calorimetry is rarely used in humans because it is expensive, time-consuming, and requires specialized equipment.

Predictive equations: These are equations that estimate metabolic rate based on various anthropometric and demographic factors such as age, gender, weight, and height. Examples of predictive equations include the Harris-Benedict equation and the Mifflin-St. Jeor equation. These equations are less accurate than direct or indirect calorimetry, but they are useful for estimating metabolic rates in large populations or when direct or indirect calorimetry is not feasible. Energy balance and imbalance:

The body's weight is influenced by energy balance and imbalance, which can impact physiological function at the cellular level. When the amount of energy intake and output does not match, an energy imbalance occurs, leading to weight gain or loss. Although it may seem simple to adjust energy intake and output by changing diet and exercise, there are various other factors that come into play, such as genetics, environment, hormones, digestion, absorption, and stressors. These factors can affect eating habits, physical activity, metabolism, and energy usage. When we consume more energy than we expend, the excess energy must be stored somewhere in the body. Glycogen, primarily stored in the liver and muscles, is the main storage form of carbohydrates, while triglycerides, primarily stored in adipose and muscle tissue, are the main storage form of fat. Although protein is not stored in the same way as carbohydrates or fat, the body's amino acid pools and protein sources can still be thought of as a protein "reserve". When we gain mass due to excess energy intake, it can either be in the form of fat or lean mass. However, gaining only lean mass is preferred as it results in strong, valuable tissues.

On the other hand, when we consume less energy than we expend, we lose weight. The body requires energy even when we are at rest, as about 70% of daily energy goes towards maintaining basic body functions. Exercise can increase energy output and signal the body to use nutrients for energy regeneration or to preserve lean mass, making it a crucial component of weight loss plans. Glycogen stored in the liver and muscles can be broken down into glucose for ATP regeneration, while triglycerides can be broken down into fatty acids and glycerol. However, if energy intake is too low or carbohydrate and fat stores are depleted, the body may resort to breaking down protein from muscles, bones, or internal organs, which is not desirable. Therefore, a combination of proper nutrition and resistance training is necessary for healthy and sustainable weight loss. A side note: Losing weight can be a daunting task, and it's easy to feel overwhelmed by the sheer amount of information out there. With so many fad diets, conflicting advice, and unrealistic promises, it can be hard to know where to start or what approach to take. That's why I've created a series of articles on weight loss - to provide a science-based overview of the topic, and to help anyone who's serious about losing weight do so in a safe and effective way. By reading these articles, you'll gain a deeper understanding of the biology of weight loss, the different approaches you can take, and how to navigate the maze of information out there so you can make informed choices about what's best for you. Whether you're just starting out on your weight loss journey, or you've tried everything and still haven't found success, this series is for you. So, let's get started! YOU Can begin here: Rapid Weight Loss: The Dark Side The impact of exercise on energy balance Approximately 70% of our daily energy demand is determined by our basal metabolic rate, which is beyond our control. However, we can manage how much and how intensely we exercise, which can affect how we use energy. The effects of different types of exercise on energy usage are as follows:

1. High-intensity, short-duration activities such as CrossFit, sprinting, speed skating, or hockey, burn a moderate amount of energy during the activity. However, after the activity, the total energy expenditure remains elevated for hours, which varies depending on the type and intensity of the activity. These activities use mainly fast-twitch muscle fibers that use glycolysis as the primary energy transfer pathway, and they also create an "afterburn effect." These exercises use mostly carbohydrates during the activity, but afterward, we use mostly fats.

2. Low-intensity, long-duration activities such as hiking, long-distance running or cycling, cross-country skiing, leisurely swimming, or walking, burn more energy during the activity because of their extended period. However, after the activity, total energy expenditure returns quickly to the resting baseline. These activities involve mostly slow-twitch muscle fibers that use slower pathways, and they use fat as the primary energy source.

Choosing an exercise that you genuinely enjoy and will consistently do is crucial to maintaining a healthy lifestyle. Don't feel pressured to find the "perfect" exercise form that fits everyone's body or lifestyle. The amount of fuel used during any activity is only a percentage of the total, and you may burn more calories during a long hike with a backpack than in a short, high-intensity exercise session.

Consider the amount of time you have available and prioritize higher-intensity activities, but don't overdo it. High-intensity workouts are tough on the body, and most people need time to recover from them. A mix of high- and low-intensity exercise and a variety of activities is likely the best choice for most people. This approach helps you recover well, enjoy what you're doing, and stick to your fitness plan.

It's also important to keep in mind that different bodies respond differently to exercise. If you're a beginner or switching sports types, your body may not be as efficient at exercising as a highly trained individual. However, being efficient at exercise isn't always the goal. Focus on finding an exercise routine that you enjoy and can stick to, and the benefits will follow. In conclusion, while measuring the energy content of food is relatively easy, determining how the body absorbs and utilizes this energy is more complicated. The body's metabolism is affected by various factors, such as age, sex, body composition, and physical activity levels, which make it challenging to accurately calculate how many calories and nutrients are absorbed and used by the body. Measuring metabolic rate is a useful tool for assessing daily energy needs, but this requires sophisticated equipment and laboratory conditions. Understanding how to balance energy intake and expenditure is essential for maintaining a healthy weight and preventing chronic diseases. By incorporating a balanced and varied diet with regular physical activity, individuals can achieve and maintain healthy body weight and promote overall well-being. In the next article, we will talk about Aerobic and Anaerobic Metabolism.