This article was written by one of our contributors; medical student with a BSc in human nutrition + MSc in clinical and public health nutrition – Rebecca Fox.
Calories. Everyone’s talking about them. But what is a calorie and why does our body need them?
What is a calorie?
In short, a calorie is a unit of energy. While we often use the term “calorie” in day-to-day life, when we talk about food, we’re actually talking about kilocalories or “kcal”. One kcal is the amount of heat energy needed to raise 1 kilogram of water by 1℃. While this seems like a very technical definition, when we think about how our bodies use energy it all starts to make sense.
Humans need some form of fuel to function. In our case, fuel is found in four different food components called macronutrients: carbohydrates, protein, fat, (and alcohol). These four components carry different amounts of energy (kcal) within them based on their chemical structure. In general:
- Carbohydrates = 4 kcals/g
- Protein = 4 kcals/g
- Fat = 9 kcals/g
- Alcohol = 7 kcals/g
While still vital for our health, other food components such as vitamins and antioxidants don’t provide any kcals themselves. Instead, these compounds help our bodies metabolise energy and perform other functions.
What is energy balance?
Our bodies get energy from food using a series of digestive processes and chemical reactions. Protein, carbohydrates, fat and alcohol all have individual chemical properties and therefore have different amounts of kcals stored within their bonds that get released as the body’s energy currency: ATP. So, after being digested and absorbed through our gastrointestinal tract, our body works to use those food components in one of two ways depending on our energy needs:
1) Providing usable energy for the body
2) Store energy in the form of fat or glycogen (a storage form of glucose)
Our bodies try their best to maintain a stable environment for optimal function (i.e. homeostasis). In theory, if we are in a state of energy balance (i.e. kcals in = kcals out) we will neither gain nor lose weight. However, if we are in an energy deficit, take dieting for example, there isn’t enough immediate energy from food for our bodies to maintain proper function. Therefore, our bodies have to start breaking down its stored forms in the form of fat and glycogen and we lose weight. Although in reality, weight loss is much more complex than just kcals in versus kcals out, as many factors play a role in this, including our environment, psychological factors, our metabolism, our hormones and much more (11).
How much energy do we need?
Essentially it comes down to the fact that we are all different, and therefore need different amounts of energy. Our basal metabolic rate (BMR) is the amount of kcals needed to keep your body functioning in a resting state (1).
There are a number of factors that contribute to our individual BMR including:
- Body composition (i.e. our relative amounts of fat and muscle): Muscle is a highly metabolically active tissue, meaning it uses a lot of kcals in order to function. Whether or not you are actively thinking about using your muscles, at least some of them are always working. For example, even standing still requires activation of multiple postural muscles – and these muscles all need energy. However, adipose (fat) tissue doesn’t use very much energy and isn’t considered very metabolically active. So, someone with more muscle mass will burn more kcals at rest than someone with less muscle – even if those two people weigh the same.
- Genetics: A number of different genetic elements have been identified that may contribute to energy balance. Therefore, there is likely a sizeable hereditary component to how much energy someone requires (11).
- Age: BMR decreases with age, so younger people generally have a faster basal metabolism than older people. Different life stages also impact BMR, for example BMR changes in response to pregnancy needs.
- Gender: Various factors contribute to differences between male and female BMR. For example, hormonal changes occurring throughout the menstrual cycle are associated with increased BMR during the luteal phase (day 14-28) compared to the follicular phase (day 1-14) (2). Further, on average, males have a greater proportion of muscle mass compared to females. Therefore, on average, they are more likely to have a faster basal metabolic rate (3, 4).
- Physical Activity: Strength exercise has been shown to increase basal metabolic rate both in the short and long term . Strength exercises such as weight lifting or resistance training for example, will increase muscle mass. Therefore, in the long run, increasing your overall muscle mass will increase your metabolic rate – meaning your body will utilise more kcals at a basal level (5).
- Illness: BMR increases due to the extra energy the body needs to fight an illness or an infection.
Many of these factors are incorporated into equations that are commonly used both clinically and in many fitness and nutrition apps. However, these equations only provide an estimate of BMR and kcal requirements.
What happens during fasting/dieting?
When we don’t eat for long periods of time (> 18 hours or so), or we are continuously eating less than our energy balance requires, our bodies go into a state of breakdown (catabolism). Without food energy, our blood glucose drops, meaning less glucose circulating for use by the body. Glucose is the body’s primary energy source and will be used preferentially to other nutrients. If there isn’t enough glucose available, we will produce glucose by converting other macronutrients (like protein or fat) as a back-up option.
In a state of fasting or a low energy intake, a stored form of glucose found in the liver and skeletal muscle called glycogen is broken down and is released into the bloodstream as free glucose where it can be used in other tissues (like the brain). Interestingly, the structure of glycogen in the liver and muscle is highly branched. This branched structure allows it to hold onto a lot of water (~4g water per 1g glycogen). Thus, losing glycogen will also cause the loss of a considerable amount of water. This is a common factor in many fad diets such as the Atkins or ketogenic diet where little to no carbohydrates are consumed (and therefore glycogen is not replenished). So, in the early stages of these diets, weight loss may likely be just fluid loss rather than fat loss (6).
Then, once all of this stored liver glycogen is used up, lactate and amino acids found in muscle are converted in a reaction that makes more glucose (called gluconeogenesis) for use throughout the body. This process is tightly regulated.
Finally, fat tissue (adipose) will be broken down and converted in the form of ketone bodies which can be used for energy by the brain. This concept of ketone production from fat is the basis of the ketogenic diet, a fad diet whereby very little or no carbohydrates are consumed (6). In this case, since no carbohydrates are available, no glucose is readily available either. Therefore, the body starts to break down fat as an energy source. (This diet is problematic for a number of health reasons but is beyond the scope of this article! You can read more about the keto diet here).
Limitations with focusing on calories
While calories are certainly an important aspect of measuring a food’s nutritional content, they aren’t the whole story. Nutrition and overall health involves so much more than calories alone, and focusing solely on a food’s energy content can be unhelpful. For those looking to lose weight, while energy balance is important, weight loss is more complex than simply calories in vs calories out. For example, various apps and websites are available to estimate individual kcal requirements. However, many of these calculated requirements are an estimate based on large populations and aren’t necessarily the best predictors of individual kcal needs (7). This is largely due to the fact that energy requirements vary between individuals due to factors mentioned above, such as hormonal changes and body composition (8).
Furthermore, for some people, tracking kcal intake can become triggering, obsessive, or potentially lead to disordered eating patterns (9, 10).
So, while it’s important to understand the kcal content of food, the overall nutrient quality is much more important for overall health and well-being.
- “Kcal” (kilocalorie) is the unit used to describe how much energy a food contains.
- Carbohydrates, protein, fat and alcohol are the components of food that contain kcal. Vitamins and minerals do not provide kcal, but are vital for other body functions.
- Energy balance refers to the relative amount of kcals we take in vs kcals we expend with physical activity. This is the basis of weight loss and weight gain, though these processes are also more complex than kcals alone.
- Kcal needs are based on body composition, age, gender, genetics, physical activity, and illness. Many equations are available to predict kcal needs of an individual, taking these factors into account.
- Fasting and dieting changes how our bodies utilise energy.
- Calories are just one small aspect of nutrition, and focusing on the overall composition of foods is more important for overall health and well-being.
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