Calories are an energy unit and the first law of thermodynamics states that within a closed system energy cannot be created or destroyed, only converted from one form to another. That means that the calories you consume and the calories you expend are a reliable predictor of the total amount of energy you will have stored within your body and as such, your weight. This is true and undeniable, but as we all know there is a lot more to consider than simply how much someone weighs.
The calories we consume are packaged into macronutrients which we refer to as protein, carbohydrate, fat and alcohol. The macronutrients are named macronutrients to differentiate them from micronutrients (which are the vitamins and minerals). From a molecular standpoint they are physically larger, and we consume them in gram amounts per day (as opposed to micronutrients which we may consume in mg amounts or less).
Each macronutrient has a given calorie value, usually expressed as the amount of calories that it provides per gram. The values are:
These are approximate measures because, as you will see, each macronutrient (especially fat and protein) is not a single thing, but rather a heterogeneous group of smaller things which may differ in their energy value. Some grams of protein that you eat may have slightly more than 4kcal and some might have slightly less, but these values are workable averages which are used in almost all calculations for practical purposes, and are more than close and accurate enough to base your food choices from.
As a form of explanation, you will learn that protein is made up of amino acids. Each amino acid can have as many as 7 and as few as 3kcal per gram, but 4 is the average.
For example, if a portion of Greek yoghurt contains 8g fat, 7g carbohydrate and 15g protein, we can deduce that it provides (8×9) + (7×4) + (15×4) = 160kcal.
While all of these provide energy, this is not necessarily the prime role that they play nutritionally. Each can also be considered a substrate which has a given function within our body, for example:
One problem with focusing only upon calories is that we then run the risk of skewing our macronutrient intake too far in any one direction. Having more of one nutrient within a given calorie amount necessarily means having less of another, and consuming insufficient amounts of any one of the macronutrients (granted, carbohydrates less so) will have negative consequences eventually, either in terms of your health, your energy levels or your adherence to your approach.
When you eat a meal, it will likely contain all of the macronutrients in some ratio or other, packaged into a different form. Carbohydrate may be consumed as starch or sucrose (table sugar) amongst others, fats covers a huge range of different molecules which may come as plant oils or animal fat, and protein will typically come in the form of either animal contractile tissue, albumen, the proteins found in milk such as beta-lactoglobulin, alpha-lactalbumin, bovine serum albumin and alpha s1 and s2 caseins, or the proteins found within plant cells. These raw materials all have corresponding enzymes which are secreted at some point in the digestive system, and are therefore broken down into smaller parts for use. In their larger forms, we cannot use them.
Note: Please bear in mind that from here we will talk quite a lot about biology and biochemistry. Though some of the content in this module may seem complex, understanding it will allow you to truly understand the topics at hand. Please don’t be intimidated by the more in-depth looking content, as we will fully explain everything in the simplest terms possible.
Enzymes are proteins found in almost all life forms. They are catalysts, which means they speed up certain reactions by reducing the amount of energy that is required for the reaction to take place. As you will discover later, proteins are comparatively large molecules which adopt a specific 3D shape that in turn gives the protein it’s function, much like the shape of a Lego brick or cog determines how well it works in the environment in which it is found.
Each enzyme has a certain shape which can be considered like a lock, into which specific keys will fit. The key is the specific thing that the enzyme interacts with – for example, the enzymes collectively known as DNA Helicase are produced within your cells, and act only upon your DNA, where they function to break it up to allow your cells to read the code. Without DNA helicase, your cell couldn’t work properly because DNA couldn’t be temporarily broken up, but if you applied these enzymes to any other structure, nothing would happen.
It’s worth remembering that after an enzyme has performed its function by making a reaction happen it dissociates and often that enzyme can be used again. What’s also really important here is that enzymes are proteins made within the body, so we don’t need to consume them and we never ‘run out’. Enzymes are made by almost every life form known right down to the bacterial level which shows just how critical they are.
When it comes to digestion, enzymes have a very simple function: they facilitate the breaking down of macronutrients into smaller molecules so that they can be absorbed from your digestive system into your body. It may seem strange to think of things in your digestive system as being ‘not in your body’ but it’s a useful perspective to adopt. In effect, the digestive system acts like a hose which separates the things inside it from the surrounding environment, selectively allowing certain things to pass through its walls. This is why food you consume can make its way to your bloodstream, but a metal pellet for example, would pass out unchanged rather than making its way to your organs. This function is vital because we consume things that could kill us every day if they ended up in our bloodstream – your digestive system provides a barrier which keeps you alive.
Proteins are enzymatically broken down into their constituent parts, amino acids, via enzymes called proteases which are found mostly in the stomach. Starches become glucose when they come into contact with amylase which is found in the mouth and intestines (this is why bread will start to taste sweet if you chew it for a long time as the starch is being broken down into glucose which is a sugar) and fats are digested from the release of bile from the gall bladder into the intestines, becoming fatty acids and glycerol. Don’t worry, you don’t need to remember all of this just yet.
As you can see, enzymes play a crucial role. If food particles were not broken down into their simplest forms via digestion, we could not survive and that’s to say nothing of all the other functions enzymes play in more or less everything that goes on in your body on a chemical level! Now, back to the topic.
We will now detail the three macronutrients, their digestive fate, some roles they play in your body and the key reasons that you should pay attention to them nutritionally, before giving some guidelines for determining your ideal intake. Note that we will be giving guidelines which are somewhat specific and numerical, but that is not to say that weighing and measuring food is the only way to go. These recommendations are there because it is, according to the best information we can gather, what you should aim for in an ideal world, but that doesn’t mean you can’t be healthy unless you now the precise protein and fat gram content of your steak.
In the final section of this module, the recommendations are summarised and a flowchart is provided to outline the steps simply, so don’t worry if you don’t quite see how it all fits together as we proceed.