Caloric Balance
The Physiology of "Calories In vs Calories Out" and Its Impact on Body Weight
What Determines Your Weight?
When you are growing during childhood and adolescence, your body uses the calories and nutrients you consume to grow your body. Your body can also grow larger if you eat and exercise in a way that facilitates muscular hypertrophy, the growth of muscle fibers.
In contrast, when you lose weight, your body breaks down molecules to access their energy stored in the chemical bonds that form them (Widmaier, 2019).
How Does Your Body Access the Energy in Your Food?
Within the food we eat, there are three types of macromolecules that we use for energy and growth:
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Carbohydrates
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Proteins
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Fats (Triglycerides)
In order to use these macromolecules, our digestive systems have to break them down into smaller components that can enter the blood stream to be shuttled to cells in the body that need them.
When the body has excess calories available, carbohydrates and fats, the main sources of energy in the body, are broken down into smaller units and stored. Protein is broken down into amino acids, which are used to build protein chains to form muscle and other structural components in the body.
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Meal
Meal content:
CARBS
PROTEINS
FATS
Digestive System
Broken down into:
GLUCOSE
AMINO ACIDS
FATTY ACIDS
Blood Stream
Sent to:
MUSCLE & LIVER
MUSCLE
ADIPOSE TISSUE
Body Cells
Stored as:
GLYCOGEN
PROTEIN
ADIPOSE
Hormonal Control of Energy Storage and Energy Usage
There are two main hormones responsible for regulating energy storage and energy usage in the body: insulin and glucagon (LibreTexts, 2019; Widmaier, 2019).
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Insulin is stimulates cells to absorb the building blocks in the digestive system and transported in the blood stream (Widmaier, 2019). The actions of insulin allow:
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Muscle cells to take in glucose and amino acids
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Liver cells to take in glucose
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Adipose cells to take in fatty acids
After a meal, digestion causes your body's blood sugar levels to rise, which triggers the release of insulin from the pancreas. Insulin's job is to take the excess glucose (along with free amino acids and fatty acids) and make sure they are stored for later use. When needed, glycogen (made from glucose) and triglycerides (made from fatty acids) are then available to fuel high-intensity and low-intensity activity, respectively (Widmaier, 2019).
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At the same time, insulin also helps your body build muscle, as it shuttles amino acids to muscle cells for storage and protein production. If you've been performing exercises that create enough micro-tears in your muscle fibers, those muscles repair themselves with the amino acids insulin allows into cells and grow larger (LibreTexts, 2019).
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Glucagon, which is also released by the pancreas, essentially reverses these processes. When you blood glucose levels drop, glucagon is released so that stored macronutrients can be broken down to provide energy for your cells and keep your blood sugar at normal levels (Widmaier, 2019). This can happen as a result of:
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Starvation
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Intense exercise/exertion
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Low carb meals
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The Calories In vs. Calories Out Equation
As previously mentioned, the "Calories In vs Calories Out" concept is an accurate but simplistic description of how calories dictate changes in body weight. In reality, the "calories out" side of the equation is a bit more complex. Your body expends energy in your food in 3 ways (Wedmaier, 2019):
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To perform work
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By releasing energy as heat
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By storing energy as glycogen and adipose for future use
Overall, the calories you consume equal the energy used in these three ways, resulting in the following equation: Energy Intake = Work Done + Heat + Energy Stored
The number of calories you eat can be influenced by:
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Energy levels (LibreTexts, 2019; Widmaier, 2019)
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Perception of satiety (Cassidy & Tong, 2017; Lennerz & Lennerz, 2018)
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Cognitive-behavioral conditioning (Apolzan et al., 2017)
The amount of "work" your body performs is made up of two related components: life-sustaining metabolism and movement. The basic processes (i.e. cell repair, digestion, breathing, circulation) that allow you to live take a certain amount of energy to sustain; this is known as your resting metabolic rate (RMR), which increases with higher body weight and lower body fat percentage (LibreTexts, 2019).
Additionally, while your body performs work, some energy is lost as heat (Widmaier, 2019):
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Exercise activity thermogenesis (EAT)
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Non-exercise activity thermogenesis (NET)
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Diet-induced thermogeneis (DIT)
When you eat more calories or move more in your daily life, the amount of heat lost and the amount of energy consumed to perform work increases, raising your Total Daily Energy Expenditure (TDEE):
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Weight maintenance: Calorie intake = TDEE
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Weight loss: Calorie intake < TDEE
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Weight gain: Calorie intake > TDEE
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Next, let's take a look at how your diet, eating habits, and physiology can affect your hunger and appetite.
