• Emilina Lomas

A nutritionist debunks the 'calories in vs. calories out' theory

As a nutritionist, one of the most common questions I have received is whether ‘calories in vs. calories out’ is true. Many - if not all - diet plans are based on this theory; you need to eat less and move more to lose weight. But is it the truth? Or is there more to it? In this article we’re going to examine the calories in vs. calories out theory to ascertain whether it’s a good strategy for weight loss.




What is the calories in vs. calories out theory?


The calories in vs. calories out theory is known commonly in the health and fitness industry as IIFYM (if it fits your macros), a way of eating that solely focuses on how many calories are consumed and how many are metabolized through exercise. However, other than IIFYM, the majority of diets you see in the health and fitness industry are based on the same calories in vs. calories out theory.


This theory argues that if you intake more energy (through food) than you expend (through exercise and daily movement), you will gain weight. If you intake less energy than you expend, you will lose weight. It’s all about energy balance.


What’s the science behind the theory?


Your body weight is regulated by energy balance. If you intake more calories than you expend, you will be in a positive energy balance which means your body isn’t able to regulate your caloric intake through expenditure, so you store more calories as fat with the intention of using them later, but rarely get used ‘later’ so just contribute to increasing amounts of adipose (fat) tissue. If you have a negative energy balance you intake less calories than you expend so your body breaks down fat and muscle tissue stores to use for energy. This causes you to lose weight.


So what’s the catch?


In the diet and fitness industry, it’s all about calories and macros. While tracking macros and counting calories can be a helpful indicator as to how much you’re eating, it’s not a perfect science. A calorie isn’t just a calorie. I often see infographics on Instagram with a picture of a McDonalds meal that is 250 calories vs. a healthy meal that is 500 calories with the caption along the lines 0of ‘just because it’s healthy doesn’t mean it’s low calorie’.


First of all - low calorie doesn’t mean healthy, these are not interchangeable terms.

Secondly - despite being higher calorie, the healthier meal is still preferred.

Even if weight loss is your goal, it’s ALWAYS better to choose healthier, nutrient dense, whole foods rather than lower calorie processed alternatives.


In general terms, calories in vs. calories out works. However, if we want to get more specific - it’s definitely not the full picture. Here’s some examples why.


1. Exercise is an inefficient way to create a deficit

Many people assume that their hour long workout is going to put them in a deficit, so they don’t even need to cut their calories. However, exercise is an ineffective way to create an energy deficit. A systematic review of intervention studies using exercise for weight loss, found that exercise energy expenditure had absolutely no correlation with weight loss. Numerous studies conclude that exercise has, at best, a modest impact on weight loss but it’s less than most people believe.


So in terms of calories in vs. calories out - the calories don’t come ‘out’ as easily as you think. The deficit has to be created from your diet.


2. Calories are not all metabolized equally

Fructose and glucose are the two main simple sugars in your diet and gram for gram they provide the same number of calories, but the way they are metabolized in the body is worlds apart.


Firstly, glucose is metabolized by all of your body’s tissues whereas fructose can only be metabolized by the liver. When you consume too much fructose, your liver can’t keep up which leads to a fat accumulation in the liver, due to both increased lipogenesis and impaired fat oxidation. Too much fructose consumption over time causes non-alcoholic fatty liver disease (NAFLD), which is becoming more common in individuals of a normal weight due to the mass consumption of sodas.

This leads us onto our next point…


3. Foods affect our hormones, which can affect how we use or store energy

Ghrelin, the hunger hormone, tells your brain when to eat and when to stop eating. Fructose increases ghrelin levels meaning it stimulates hunger despite satiety, which can not be stimulated by glucose. Glucose produces increases in circulating satiety hormones, making you feel full and to stop eating, whereas fructose does not.


One study that gave one group a fructose sweetened beverage and another group a glucose sweetened beverage and the same caloric intake for 10 weeks and unsurprisingly, the fructose group gained fat and decreased insulin sensitivity, whereas the glucose group did not.


4. Our bodies burn different amounts of energy during digestion of macronutrients

Within the total amount of calories you burn in a day - known as your total daily energy expenditure (TDEE) - 15% accounts for the calories burnt during digestion. This is known as the thermic effect of food (TEF). Protein has the highest thermic effect of any macronutrient, with 20% of it being burnt during metabolism. Carbs have the second highest - 5 to 15% are burnt, largely depending on whether they are complex or refined, and fat has the lowest - with less than 5% being burnt during metabolism.


Thus, if you compare a high protein diet with a lower protein diet - adjusted for calories - the high protein diet will result in less energy storage, which overtime can lead to weight loss if there is a deficit.


5. We're all counting calories and forgetting to count chemicals

Processed foods are filled with chemicals, fillers, trans fat and ingredients that are toxic to the body. FUN FACT: In Europe there are more than 1,300 chemicals that are not legally allowed to be added into food due to their toxic effect on human health. The US has only curbed 11 of these. Note: not made illegal, just recommended against.


Processed foods filled with chemicals are known endocrine disruptors - hormonally active agents - meaning they cause hormonal imbalances, increase disease risk and prevent your body from maintaining homeostasis. As well as this, processed foods are linked to overeating - a recent clinical trial published in Cell Metabolism found that when macronutrients are matched, those on a processed diet eat on average 500 calories MORE when given the chance to snack outside of meals, compared to the group who ate a wholefood diet of the same macros. Processed foods are also associated with obesity, type 2 diabetes, cardiovascular disease, high LDL cholesterol, mental health illness, neurological decline and all cause mortality. Even when researchers adjust for calorie intake and other confounders like smoking and exercise activity.


Conclusion: Your low calorie low fat protein bar is worse than eating salmon and rice, even if the salmon and rice is more calories, because the way you METABOLIZE the processed foods is different and will be stored more readily as fat rather than effectively burnt as fuel.


As you can see, there’s a lot more to food than just their caloric density: the way they are metabolized, the hormonal response they stimulate, their glycemic index and so much more. Moreover, it’s clear that calories in vs. calories out isn’t the whole picture, and can’t be relied upon.


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