What is a Calorie?

A calorie is a unit of measurement for energy. More simply, it is a way to measure the food you consume. When you eat and drink, you take in calories. Your body uses those calories for energy to fuel your daily activities. The number of calories you take in from food and drinks represent how much energy is available for your body to use. For example: a serving of cereal may have 100 calories, which means that serving provides 100 units of energy for your body to use.

How do Calories Affect the Body?

Your body needs a certain number of calories to function properly, and that amount varies depending on factors such as your age, sex, activity level, and goals. When you eat more calories than your body needs to maintain its current activity level, those extra calories are stored as fat. Alternatively, when you take in less calories than your body needs to maintain its current activity level or you increase your activity level beyond what it is used to, you will lose weight/fat. This is called creating a calorie deficit, and basically means that you burn more calories than you take in whether it's through reducing the number of calories consumed, increasing the number of calories burned through exercise or a combination of both.

How are Calories used for Energy?

As we have learned, calories are a way to measure the food (energy) we consume which can be classified into different groups called macronutrients. A macronutrient is a type of nutrient that is required in large amounts by the body to function properly. The three main macronutrients are Proteins, Fats, and Carbohydrates. Additionally, the body can derive energy from Alcohol and Fiber as well.

Proteins contain 4 calories per gram, but their primary function is not energy production. Instead, they are used for important bodily functions such as building and repairing tissues, producing enzymes and hormones, and supporting a healthy immune system. Carbohydrates and fats are the body's preferred sources of fuel as they are more efficient.

Proteins are typically only used for energy in situations where there is a shortage of other fuel sources, such as during prolonged fasting or intense exercise when glycogen stores are depleted. In these situations, proteins are broken down into amino acids that can be used for energy production.

However, using proteins for energy can have negative consequences such as muscle wasting and decreased availability of proteins for essential functions. Therefore, it is important to ensure that the body is receiving enough carbohydrates and fats to meet its energy needs and spare proteins for their primary roles.

• One gram of protein contains ~4 calories. Think meat, poultry, whey, fish, seafood, dairy, and eggs.
  

Fat is the most energy-dense macronutrient, providing 9 calories per gram. Fat is the primary source of energy during most of our daily activities, such as working, walking, light exercise, and rest.

• One gram of fat contains ~9 calories. Think avocado, nuts, oils, seeds, dairy, and eggs. 
  

Carbohydrates are a rapid source of energy, providing 4 calories per gram. The body prefers to use carbohydrates as its primary source of energy during high-intensity activities, such as sprinting or intense exercise, because carbohydrates can be broken down more quickly than fats and provide a fast source of energy.

• One gram of carbohydrates contains ~4 calories. Think fruits, vegetables, grains, and sweets.
  

Fiber is a plant-based compound, providing 2 calories per gram. Fiber includes a wide range of non-starch polysaccharides that are not fully digested in the human gut.

• One gram of fiber contains approximately 2 calories and, although it belongs to the carbohydrate family, it is not usually counted in the total carbohydrate content. For example, if a food has 10 grams of total carbohydrates but 7 grams of those are from fiber, then only 3 grams are considered as net carbs. Net carbs refer to the total amount of carbohydrates in a product or meal that are fully digestible.
  

Alcohol is not a necessary nutrient, but it provides the body with 7 calories per gram, which is more than carbohydrates and protein but less than fat. When consumed, alcohol is quickly absorbed into the bloodstream and transported to the liver, where it is metabolized. The metabolism of alcohol takes priority over the breakdown of other macronutrients because there is no mechanism to store alcohol in the body. As a result, the metabolism of carbohydrates and fats for energy production is slowed down when alcohol is consumed. This means that if you consume alcohol during a meal that includes carbohydrates or fats, the body will prioritize the metabolism of the alcohol over the other macronutrients. Any excess calories consumed will be preferentially stored until all the alcohol is metabolized.

• One gram of alcohol contains ~7 calories. Think about your Friday nights.
  

All macronutrients (except alcohol) are essential for our body to function optimally, but the specific amount and ratio of macronutrients that is best for each individual will vary based on their individual needs and goals.

What is the Thermogenic Effect of Different Foods?

The thermogenic effect of food refers to the energy your body expends to digest, absorb, and process the nutrients in the food you eat. Some foods require more energy to digest than others, which can cause your body to burn more calories. This is known as the thermogenic effect of food.

For example, protein has a higher thermogenic effect than carbohydrates or fats. This means that your body burns more calories digesting protein than it does digesting carbs or fats. Also, fiber-rich foods like fruits, vegetables, and whole grains have a higher thermogenic effect than processed foods. This is because these foods are more difficult to digest, so your body has to work harder to break them down, burning more calories in the process.

So, while all calories provide energy, the source of those calories can have a big impact on the number of calories your body burns through the thermogenic effect of food. Eating a diet high in protein and fiber-rich foods can help you burn more calories and maintain a healthy weight.

Protein digestion has the potential to burn 20–30% of its calories.

• Ex: 300 calories of pure protein consumed, around 60–90 calories are burned just through digesting it. NOTE: Consuming protein from whole sources like meat, eggs, or dairy will impact thermogenesis to a significantly higher degree than consuming protein from powders, which likely has little to no thermogenic effect.
  

Fat digestion can potentially burn ~5% of its calories.

• Ex: 300 calories of pure fat consumed, around 15 calories are burned through digesting it.
  

Carbohydrate digestion can potentially burn 5-10% of its calories.

• Ex: 300 calories of pure carbs consumed, around 15–30 calories are burned just through digesting it.
  

What is the Best Fuel Source for the Body?

Carbohydrates and fats both serve as sources of energy for the body, but they are utilized differently depending on the intensity of the activities performed. Carbohydrates are like kindling on a fire, providing a fast source of energy ideal for high-intensity activities like sprinting or intense exercise. However, relying solely on carbohydrates for energy can be like trying to heat your home with just paper, twigs, and lighter fluid, requiring a constant supply to maintain energy levels.

Fats, in contrast, are akin to a slow-burning fire, offering a sustained source of energy suitable for low-intensity activities like walking or light exercise. Although fats provide less energy per gram than carbohydrates, they are a more efficient and longer-lasting energy source. Evolutionarily, it makes sense for the body to utilize both carbohydrates and fats for energy, as our ancestors did not always have a consistent food supply.

While the metabolization of fat produces more ATP per molecule than the metabolization of glucose (129 molecules of ATP compared to 38 molecules of ATP), it's important to consider the overall efficiency and speed of the energy production process. Carbohydrates can be rapidly broken down into glucose, which can then be used for energy production, making them the preferred fuel for high-intensity activities. In contrast, the breakdown of fats into their component fatty acids is a slower and more complex process that requires more oxygen and takes longer to produce ATP.

During high-intensity activities, such as sprinting or intense exercise, the body needs a quick source of energy to meet the energy demands of the muscles. This is why the body relies more on carbohydrates for energy during high-intensity activities. However, during low-intensity activities, the body can rely on stored fats for energy production, which can provide a long-lasting, steady supply of energy.

What are the Hormonal Implications of Food Selection?

By now you should understand that not all calories are created equal, and this is no different when it comes to how they affect hormones. Different types of food can have different effects on the hormones that regulate your appetite, hunger and fullness, and metabolism.

• Insulin helps regulate blood sugar levels in the body. When you consume carbohydrates, your body breaks them down into glucose, at which point they will be used for fuel by the body if needed or stored for later with the help of insulin. The type of carbohydrate, whether simple or complex, will dictate the amount of insulin released, and therefore the amount of energy stored.
  
• Cortisol is released in response to the body being knocked out of homeostasis (or balance). It is indirectly affected by nutrition, and is antagonistic to the hormone Insulin. When blood sugar levels rise, insulin is released, driving levels back down. If levels fall too low, cortisol will be released to liberate energy within the cells to raise blood sugar levels. Alternatively, if blood sugar levels are chronically high this is seen as a stressor on the body and cortisol levels be affected.
  
• Leptin is produced by fat cells. It signals to the brain that the body has enough energy stored and that it's time to stop eating. Leptin signaling plays a large role in the up- or down-regulation of your metabolism (aka your ability to burn fat). Caloric restriction (or dieting) reduces leptin levels and can be one of the reason for stalls in weight loss. Therefore, periodic "diet breaks" can be very useful to bring leptin levels back up, and break weight loss plateaus.
  
• Ghrelin is produced by the stomach and signals to the brain that the body needs food. Ghrelin, sends a message saying, "I'm hungry, feed me." A growling stomach is ghrelin in action. The amount of ghrelin circulating in your body goes up when you haven't eaten in a while and down after you have finished a meal. Ghrelin levels also rise when you haven't had restful sleep or are under stress.