Physics, Physiology, and Dieting 

By Ryan Ripsman 

Photo credit: MyNetDiary  

In today’s world, people are constantly bombarded by images and ads telling them they need to be skinnier. Dieting has become ubiquitous; as of 2017, 42% of people worldwide were trying to lose weight. It’s no wonder so much energy has gone into finding different dieting “hacks”, fad diets that claim fat can be shed with minimal work. It’s also no surprise, given the number of unsubstantiated claims about dieting, that a certain amount of cynicism has arisen surrounding dieting. In this article I want to target one of the most ubiquitous myths: “A calorie is just a calorie”. It is a common refrain meaning that it doesn’t matter what type of food you eat when dieting if the calorie content is the same. While slogans like that are easy to understand, they are harder to confirm. To understand whether all calories are truly equal, one first needs to delve into the physics of dieting. 

        Dieting at its core is a very simple concept. The body is a machine that takes in food and converts it to energy. When more food is eaten than the body needs for energy production, the extra food is converted to fat and stored for a “rainy day”. When the body does not get enough food to cover all its energy needs, it can convert this stored fat into energy. Therefore, by reducing the amount of food one takes in, over time, one reduces the size of the body’s fat stores. To understand how the calorie content of food relates to dieting, one needs to understand a bit of thermodynamics, the laws of physics governing heat and energy. 

There are two important laws that help form the basis of thermodynamics. The first law states that energy cannot be created or destroyed – the total amount of energy in the universe will always stay constant. The second law states that if you look at one area of the universe, called a “system”, the disorder of that system will always tend to rise over time. 

        A calorie is a measure of energy. In particular, it measures the amount of energy released when something is converted into oxygen gas and water. Since the body produces most of its energy by converting food into oxygen gas and water, the calorie content of a food is a good measure of how much energy the body can derive from that food.  When people say a “calorie is a calorie”, they are referencing the first law of thermodynamics. What they mean is that all foods with the same calorie content release the same amount of energy, so the body should obtain the same amount of energy from each of these foods. While this sounds reasonable, it is patently false because our body is constantly exchanging energy with the environment. It takes in energy in the form of food, and releases it in the form of body heat and movements. As a result, the body does not necessarily absorb all the energy released by the food it consumes. 

        To understand energetics in the body, one must use both the first and the second laws of thermodynamics. All the energy released by food in the body can be broken down into “work”, energy that can be used by the body to perform all its tasks, and “heat”, waste energy that is released into the environment and cannot be used by the body. Heat is a more “disorderly” type of energy than work, and so, by the second law of thermodynamics, it cannot be converted back into work. When the body breaks down different foods with the same calorie content, the same total amount of energy is released. However, only some of that energy can be harnessed by the body as work, with the rest being released as heat. This amount will differ depending on the type of food and the process through which the body breaks it down. Since none of the heat can be converted back into work, the body derives different amounts of usable energy from different calorie sources. 

        Based on this analysis, the most reasonable conclusion is that dieters should try to consume calories that the body processes less efficiently. In fact, there is some evidence that this is a sound dieting strategy in the short term. Several scientific studies have shown that over short lengths of time, more weight is lost in low carbohydrate diets than in low fat diets with the same calorie content. This is hypothesised to be because carbohydrates tend to be broken down using more efficient chemical reactions than fat. However, these results should be taken with a grain of salt until a more formal analysis is completed. 

        But what about the long term? Here is where the science becomes a lot murkier. In the long run, the effectiveness of a diet does not only depend on how it affects the body but also how likely one is to follow it. More restrictive diets may be less effective in the long run, even if they are a more efficient way to burn fat. Furthermore, in the long run, different diets can affect the metabolism changing the way the body processes foods. It seems that after a year, the low carbohydrate diets are no longer better than equivalent calorie low fat diets. However, while carbohydrate diets might not be any more effective in the long-run, certain diets are still associated with better outcomes. The Mediterranean diet, a diet consisting of fruits, vegetables, nuts, olive oil and low amounts of dairy and meat, has been shown to yield increased weight loss over 1–2-year trials in comparison to a low-fat diet. However, these results are still considered preliminary, and other studies show that the mediterranean diet provides no advantage in terms of weight loss.  

        So, is a calorie just a calorie? In a physical sense, no, different types of food with the same calorie content will yield different amounts of energy for the body. In the short-term it seems eating foods that are broken down more efficiently like carbohydrates is a less effective dieting strategy than an equivalent calorie diet of food that is harder to break down. On a long-term basis it is less clear whether different diets with different food content result in different amounts of weight loss. But at the end of the day, it’s important to know that not all calories are created equally. This is true for both dieting and for living an overall healthy lifestyle. It’s important to always look at the full picture of what you are eating and not just the calorie count. 

Disclaimer: I am not a medical professional or dieting specialist and do not endorse any specific diet, nor the act of dieting. Always prioritise your mental/physical health and consult with professionals before making any changes to your diet. 

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