Why Calories Matter In Weight Loss (Calories In, Calories Out)

Many of us throw the term “calories” around freely throughout our day, but it turns out that most of us only have a very hazy understanding of what the term really means.

What are calories? Or, what is a calorie? Is it magic? Does it show up at the tip of Harry Potter’s wand as neon blue lightning?

Well, as much as I’d hate to disappoint you, a calorie is not magic. (I hope you’re not surprised) Neither are calories. The calorie is simply a unit of energy. Literally energy — you cannot see them under a microscope, they are not molecules and they don’t have a chemical name. To be succinct, calories are simply a unit of measurement of heat.

To be reaaally anal about the scientific terms, when we refer to calories from a nutritional standpoint, we are really referring to kilocalories (or kcals). When the food label tells you that the Snickers bar contains 488 “calories” per 100 grams, they are really telling you that it’s a delicious, chewy and melty bar of 488 kilocalories (aka 488, 000 calories). Mmm.

A calorie is simply the amount of heat required to heat up 1 gram of water by 1 degrees Celsius. Therefore, a kilocalories is also known as the heat energy required to heat up 1 kilogram of water by 1 degrees Celsius. Not to sweat it though: in this article, I’ll be using “calorie” to refer to kilocalories, as this is simply the most popular/ familiar way of terming them.

Okay, calories are energy, and they heat up water… Great? How does an increase in temperature of water concern you, right? Glad you asked!

See, your body is a templ-, uh, I mean a machine where hundreds of thousands metabolic processes take place at the same time. While some of these processes produce energy (catabolic reactions), many of these processes actually require energy (anabolic reactions).

Much like how we spend money in exchange for material goods and or experiences (hello, Paris!), our body uses ATP (Adenosine Triphosphate) as an energy currency to drive many reactions. Now, the intricate details of how the body “consumes” ATP in metabolic processes are out of the scope of this article but if you’re interested to find out, you can send me a message 😉Or Google, you know, whichever floats your boat.

The “consumption” of ATP can be equated to the “spending” of the energy currency, and it basically is the output of the calories in the whole basis of “input of calories must be smaller than output of calories for weight loss to occur”. And it is also the reason as to why exercise often comes to mind when we talk about “burning calories”.

When we exercise, amongst other things, we move our bodies, our muscles contract and our heart rate increases. All these processes require ATP, aka the consumption of energy.

But wait! How is ATP related to weight gain or weight loss? Is the Dad Bod belly just a bunch of stored ATP getting all cosy with each other? Well, no. ATP is a very high energy molecule, and the body only ever synthesises just the right amount that it needs at a given point of time. It’s like a pay-per-view service, except, you know, in the body.

If ATP cannot be stored, where do humans get their energy from? It turns out that if we consume more energy (calories) than our bodies require for the synthesis of ATP, this extra energy are captured by the body for ATP generation in the future. While energy can be stored in a few different ways, the body’s major repository is actually the adipose tissue (body fat!).

The adipose tissue comprises of adipocytes, which are cells that store MASSIVE amounts of fats in the form of triacylglycerides (TAGs). Other than fat, energy can also be stored in the form of glycogen from carbohydrates. Unlike the storage of fat, however, the storage of glycogen is extremely limited: only 300–400 grams in the liver, and ~400 grams in the skeletal muscles. That’s less than 4000 kcal of stored glycogen (1 gram of carbohydrates contains 4 kcal of energy).

For a better view on the difference between “storage spaces” for fats and glycogen, let’s perform some Math for an average Joe who weighs in at 100 kilograms, and is at 15% body fat level.

Body Fat Mass = 0.15*100 = 15 kilograms
Stored energy from fat = 15 000*9*0.87 = 117, 450 kcals
(1 gram of fat contains 9 kcal of energy, and 1 gram of fat is roughly 13% water)

Given what we currently know: if dietary fat is stored as fat and carbohydrates are stored as glycogen, it MUST be that dietary fat is making us all fat, right? Not so fast. It turns out that glucose (simple carbohydrate) CAN be converted to fat via de novo lipogenesis (DNL).

🤯-> Does this look like you yet? Hang on, there’s more to come!

It turns out that WHILE dietary carbohydrates can be stored as body fat, a recent study which examined overfed women (50% above their maintenance calories) found that the female participants stored a total of 282 grams of fat per day in adipose. Only 4 grams (yes, FOUR) of the 282 grams resulted from DNL (Schwarz, Neese, Turner, Dare, & Hellerstein, 1995), which chalks up to a measly 1.4%.

🙄-> Is this you now? So is it dietary fat or dietary carbohydrates which really affect the storage of body fat?

… It turns out that the amount of dietary fat which is stored as body fat is dependent on the amount of carbohydrates you consume!

Given that total caloric intake remains the same:

If you consume a high-carb, low-fat (HCLF) diet:
👉Glucose will be preferentially oxidised
👉Dietary fats will be spared for storage in adipose

If you consume a high-fat, low-carb (HFLC) diet:
👉Your carbs and insulin will be low
👉The rate of lipolysis and fat oxidation will increase
👉You’re burning a lot of fat BECAUSE you’re eating a lot of fat

Ultimately, your NET fat balance (Fat Storage — Fat Oxidation) is what will determine overall loss of body fat. The rate of fat oxidation versus fat storage is determined by overall energy balance, and this is also know as calories in versus calories out (CICO).

Extremist supporters of the low-carb and low-fat groups will insist that their way of dieting lead to better weight loss results, but numerous metabolic ward studies (extremely tightly controlled studies where researches provide meals to patients) have proven that when total calories and protein are equated, neither provides any significant superiority on weight loss over the other (Hall et al., 2016; Hoffer, Bistrian, Young, Blackburn, & Matthews, 1984; Surwit et al., 1997).

Energy balance regulates weight and fat loss — if you don’t lose weight, you were NOT in a caloric deficit. And ladies and gentlemen, this is why calories matter in weight loss.

Stay scientific — till next time,
Gene ✌️


Hall, K. D., Chen, K. Y., Guo, J., Lam, Y. Y., Leibel, R. L., Mayer, L. E., … Ravussin, E. (2016). Energy expenditure and body composition changes after an isocaloric ketogenic diet in overweight and obese men. The American Journal of Clinical Nutrition, 104(2), 324–333. https://doi.org/10.3945/ajcn.116.133561

Hoffer, L. J., Bistrian, B. R., Young, V. R., Blackburn, G. L., & Matthews, D. E. (1984). Metabolic effects of very low calorie weight reduction diets. The Journal of Clinical Investigation, 73(3), 750–758. https://doi.org/10.1172/JCI111268

Schwarz, J. M., Neese, R. A., Turner, S., Dare, D., & Hellerstein, M. K. (1995). Short-term alterations in carbohydrate energy intake in humans. Striking effects on hepatic glucose production, de novo lipogenesis, lipolysis, and whole-body fuel selection. Journal of Clinical Investigation, 96(6), 2735–2743.

Surwit, R. S., Feinglos, M. N., McCaskill, C. C., Clay, S. L., Babyak, M. A., Brownlow, B. S., … Lin, P. H. (1997). Metabolic and behavioral effects of a high-sucrose diet during weight loss. The American Journal of Clinical Nutrition, 65(4), 908–915. https://doi.org/10.1093/ajcn/65.4.908

Thanks for reading! If you want an evidence-based approach to nutrition, start by learning how to calculate your daily calorie requirements. You’ll like it — I promise!

Fitness & Nutrition | Evidence-Based | I write here: https://byehulla.com/