Let’s now look at what happens when we eat from a metabolic point of view. Metabolism is a series of chemical reactions that take place inside our body to maintain life. The requirements of life for us humans are things such as reproducing, growing, maintaining a constant body temperature, and so on. All of this can be broken down to the body’s ability to utilise four different biomolecules – fats, proteins, carbohydrates and nucleic acids. As we already know, we are designed to eat a certain diet, and the evolution of our species hasn’t yet progressed to consume the manufactured and lab created stuff that we all too often put into our body. We were designed to eat free roaming-animals and naturally-growing plants. It’s that simple. After all, fresh plants and wild animals were the only food available to our caveman forefathers.
As we eat food, our body digests each meal and breaks it all down to the component parts of these four biomolecules. For example, the smallest com- ponent of fat is known as fatty acids, the smallest component of proteins is amino acids (the good stuff that builds muscles), and for carbohydrates they simply become sugar. The forth is nucleic acids that break down into nucleotides. I haven’t yet mentioned these, but they are found in shellfish and offal (organs such as the liver and heart).
The body processes different nutrients in a particuar order, based on how easy the task is. Alcohol is absorbed very quickly and around a quarter of what we drink can be transferred into the bloodstream and hit the brain in less than one minute! Next come carbohydrates, which easily break down into sugar, followed by the conversion of protein into amino ac- ids. Fats are left until last, as it is quite a complex task for the body to convert them to fatty acids.
Once the small intestine has finished its work metabolising all of the macronutrients, they follow two different routes. Glucose, amino acids and minerals such as magnesium, calcium and iron, along with soluble vitamins B and C, are transported via the bloodstream to the liver for processing before being dispatched throughout the body. Fatty acids and vitamins A, D, E and K take a different route and enter the lymphatic system where they are passed directly into the blood and sent to cells around the body. We will go over this in more detail shortly.
Once our small intestine has finished its metabolic act, the indigestible remains are sent to the large intestine for processing. Here is where most of the body’s bacteria live and they also have a go at consuming extra energy from the waste. As we will learn throughout this book, their ability to complete this task can have a massive effect on our body weight. The main purpose of the large intestine (also known as the colon) is to extract water. Once everything is wrapped up, the colon – with the aid of its legions of bacteria – bundle up all of the leftovers and all being well create a quality parcel of faeces.
With the exception of alcohol, we might ask why the digestion system breaks down these four biomolecules when we have read elsewhere that the body then reconstructs them back to their original form? The act of breaking down our food and then reassembling it is the cycle of metabolism. The breaking down of food is known as catabolism and the reconstruction of the biomolecules is known as anabolism. It’s the breaking down (catabolism) of the nutrients that creates energy for the body to survive, and it’s the reas- sembling (anabolism) that builds and repairs the body.
The energy currency of our body is a high-energy molecule called adenosine triphosphate (let’s call it by its abbreviation ATP as its complicated to say, let alone memorise). This gets broken down to release a chemical energy (it loses a phosphates group to do this and is then renamed from ATP to ADP, short for adenosine diphosphate). This too is a cycle and eventually ADP must get back to being ATP - and to do this it requires food. This cycle has a name too and it’s called cellular respiration. All of this is regulated by hormones that basically make the decision whether we should be breaking down food into molecules (catabolism) or reassembling them (anabolism).