What are they and are they all created equal?

Energy
Why do we need it?
We are made up of many types of energy consuming cells that work together collectively to keep us alive. Keeping the main organs and muscles functioning is an expensive process.
Theoretically, if additional energy wasn't consumed through food, our natural energy stores would eventually become depleted, and we'd ultimately die.
Although we have energy storage sites, predominantly in fat tissue, most of our energy has to come from our nutrition. Even with adequate energy in storage, when food intake is scarce, built in compensatory mechanisms work to preserve it.
During exercise, particularly at higher intensities, energy needs increase. In order to limit fatigue and optimise output, energy from food, particularly carbohydrates, becomes very important.
What is a Calorie?
The "calorie" is not a physical thing, but rather a unit of measurement for the energy we speak of. Just like distance is measured in meters, energy is measured in kilocalories (but we tend to just say "Calorie").
One calorie equals the amount of heat energy needed to raise 1g of water by 1 degree. This wont be in the exam.
Fellow Australians may have seen the Kilojoule on the back of food packages. It's still a measure of energy, but just a different unit. 1Cal = 4.18Kj
Within the food we eat, the calorie containing nutrients are Protein, Carbohydrates and Fats. Both Protein and Carbohydrates provide approximately 4Cal per gram, and Fat provides approximately 9Cal per gram. Alcohol also contains energy, measured at 7Cals per gram.

Energy Density
The amount of energy within a gram of a food is referred to as Energy Density. Foods higher in fat content are more energy dense, therefore providing more energy per bite. Fat isn't inherently "fattening", but may contribute to excess daily energy intake.
Foods high in fibre and water, like veggies, have a lower energy density. This is why switching to a plant based diet, for example, can often result in weight loss. The energy density within the foods has more impact than foods themselves.
Energy Transfer
In a previous post on Energy Balance, we discussed the first law of thermodynamics, which states that energy is neither created or destroyed, but is transferred from one form to another.
When we consume chemical energy in food, it's transferred into the main energy currency of the cell (ATP) for immediate use. When we exercise this energy is transformed in to mechanical (movement) energy, and heat energy (body temperature).

When more energy is consumed than is expended, this energy is transferred into storage sites - Fat cells. This may not matter over the course of 24hours, but over an extended period, if energy balance is in a net positive, weight gain occurs. A net negative energy balance will influence weight loss.
Are all calories created equal?
Speaking in absolute terms, yes. A calorie is a calorie. The concept of energy balance remains central to weight management.
HOWEVER! Food quality matters.
Despite the simplicity of the energy balance equation, the human body is complex, and the foods we eat vary in the way they behave in our body. They are digested, absorbed and metabolised differently, and influence appetite and energy expenditure differently.
Let's talk about the four reasons why a calorie is not necessarily just a calorie:
1: Metabolic pathways

The pathways of metabolism for protein are less efficient than those for carbohydrate and fat. A larger amount of energy is needed to metabolise it.
The increase in energy expenditure to metabolise the macronutrients is what we call Dietary Induced Thermogenesis (DIT). Protein has a DIT of 20-30%, whilst Carbohydrates has a DIT of 5-10% (depending on fibre content), and Fat has a DIT of 1-3%.
Put practically, 100Cals of protein consumed would end up as 70-80Cals of stored energy, whilst 100Cals of fat would end up as 97 to 99 kcal of stored energy.
For this reason, lean protein foods should be prioritised during a weight loss phase. In studies where calories are equal, but different protein amounts are put to the test, the groups given higher protein diets tend to see better results for fat loss. This is also because a higher protein intake better preserves muscle mass .
2: Appetite
Different foods impact appetite differently. The desire to eat, partly controlled by hunger hormones can vary depending on a food/meal's make up.
Once again Protein plays a starring role. Protein rich foods, like chicken, meat or tofu have a greater appetite suppressing effect. Junk foods, despite often being energy dense, tends to stimulate appetite due to their hyper-palatability (damn delicious) and impact on food-reward centres in the brain.
Different sugars, such as glucose and fructose, are also metabolised differently. It's been suggested that fructose may not reduce appetite hormones as much as glucose, and hence may lead to overeating, but is generally seen at very high doses. This should not be taken as "never eat fruit again". Please eat fruit.
Reason 3: Cravings
Highly palatable, energy dense foods, often with highly refined carbohydrates can influence food cravings. Refined carbohydrates lead to rapid blood sugar spikes, followed by high insulin spikes, which are thought to signal centres of the brain responsible for food seeking behaviour.
Although we don't want to demonise individual foods or food groups, we can see why food quality matters when focusing on weight loss. Overall energy balance does dictate weight loss, and knowing this can take the pressure off having to "eat clean" all the time, however, it must be front of mind that food quality strongly determines weight loss success.
Reason 4: Satiety
Satiety is the feeling of fullness. Some foods affect satiety more than others, and in turn influence overall energy intake.
On top of suppressing appetite, Protein makes us feel fuller for longer. Higher breakfast protein intakes can even influence energy intake of following meals, setting the trend for appetite over the whole day. High fibre foods also increase satiety due to its contribution to bulk in the stomach.
Other examples of high satiety foods include: potatoes, eggs, meat, beans/legumes and fruits.
This again supports the notion that higher quality, whole foods and minimally processed foods significantly influence the energy balance equation.
How every diet works
Unfortunately, charismatic endorsement of the myriad diet types can easily lead one to believe that there's a magical fat/weight loss effect that comes from adopting them.
Now that we understand the governing principle of weight loss - Calories - and factors that influence the consumption of calories, we can understand how many diets can be successful in influencing weight loss.

All diets work if they induce a calorie deficit. This doesn't mean that the weight will stay off, though, if it can't be sustained. Typically restrictive diets may work great for a few months, but are often too difficult to sustain on the long run, at which point weight regain is often seen. Without understanding the fundamental principles of weight loss, one might be lead to think that they "failed" the diet, where in reality the diet failed them.
Keep in mind also that even if a diet influences weight loss, it may not necessarily be optimal for health. Yes, losing weight when overweight can reduce disease risk significantly, but in the longterm, deficiency or excess of certain nutrients could influence poor health outcomes.
Summing Up
A calorie is a calorie. Energy balance is at the foundation of weight loss or gain. However, solely focusing on calorie intake and expenditure may lead us astray, because many factors influence the energy balance equation. Food quality is one key factor that influences overall energy intake through its ability to alter appetite, food reward and satiety.
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