Obligatory disclaimer: I am not a medical doctor, and the content of this website was created for informational purposes only. Such content is not intended as a substitute for medical advice, treatment or diagnosis.
In previous posts we’ve talked about the starvation response (aka starvation mode), metabolic adaptation (aka metabolic damage), the thermic effect of food, and thermogenesis. In this week’s post we’ll deal with macronutrients, we’ll learn what the are, how they are classified, and the basics of their metabolism. Due to the large amount of content that we’ll cover, this will be a four-part series dealing with every single one of the macronutrients. Furthermore, given the sort of content that we’ll be dealing with in this post, rather than rely on scientific papers, we’ll use the following references:
- Gropper, S. S., & Smith, J. L. (2012). Advanced nutrition and human metabolism. Cengage Learning.
- Wildman, R. E., & Medeiros, D. M. (2014). Advanced human nutrition. CRC press.
- Hankey, C., & Whelan, K. (Eds.). (2018). Advanced Nutrition and Dietetics in Obesity. John Wiley & Sons.
Let’s get down to business, what are macronutrients? Macronutrients are the dietary nutrients that supply energy to an organism, and, in the interest of being thorough, nutrients are substances that are required by organisms in order to remain alive, grow, and reproduce. We’ve all heard examples of macronutrients before, as they come up any time discussion about diet and food happens. Namely they are carbohydrates, proteins, fats (lipids), and alcohol. Lets deal with each macronutrient, one at a time, starting with carbohydrates.
What are carbohydrates?
Carbohydrates, also known as carbs, are substances made up of carbon, hydrogen and oxygen atoms in a proportion that resembles “hydrates of carbon”, Cx(H2O)y, where x and y may be different numbers. Carbohydrates can be classified into two main groups: complex and simple carbohydrates. Simple carbs are made up of one or two saccharid molecules, and are known respectively as monosaccharides and disaccharides. Complex carbs are further divided into oligosaccarides, between three to ten saccharide molecules, and polysaccharides, anything beyond ten saccharide molecules.
As may be apparent from the classification of carbohydrates, monosaccharides are the simplest form carbohydrates come in and polysaccharides are the most complex. However, we’ve previously met an example of each one of them: glucose is a monosaccharide and glycogen is a polysaccharide. As we also learned from our posts on the starvation response / starvation mode, glycogen stored in the liver is broken down into glucose through glycogenolysis, and that means that complex and simple carbohydrates are metabolized in different ways by the organism.
Unless our eating habits are based mainly on processed food, free monosaccharides are not present in large quantities in our diet. This means that polysaccharides and disaccharides are the most important dietary carbohydrates. Although monosaccharides are absorbed into the bloodstream from the gastrointestinal tract, polysaccharides and disaccharides must first be broken into their individual monosaccharide components.
Polysaccharide digestion first begins in the mouth, as enzymes contained in saliva begin the breaking down process. As food moves into the stomach and intestines, enzymes continue this digestion until maltose (a disaccharid), isomaltose (another disaccharid) and glucose (remember, monosaccharid) are the main products.
Disaccharides, on the other hand, are not broken down either in the mouth or the stomach, and their digestion takes place in the upper small intestine. As was the case with polysaccharides, enzymes are responsible for disaccharides breaking down into their constituent monosaccharides.
Whether it’s monosaccharides, disaccharides, or polysaccharides, virtually all carbohydrates are broken down into monosaccharides and absorbed into the bloodstream by the time they reach the jejunum, which is in the upper part of the small intestine. After this, they are transported to the liver, where they are processed into, and stored, as glycogen. Some of that glycogen is then transported to the cells of different tissues to be used as a fuel source. Alternatively, the glycogen may be catabolized on the spot and used to provide energy and maintain blood level homeostasis.
Just from this very simple description of carbohydrate digestion, you may get the feeling that simple carbohydrates are easier and quicker to process than complex ones, and you would be correct. However, I think it would be best to tackle the glycemic index / the glycemic response in a later blog post, where we will take a look at the relationship between glycemic response and blood levels of sugar. In that post we we’ll also take a look at the basics of carbohydrate metabolism.
On the issue of fiber
Fiber is a kind of polysaccharide that cannot be completely broken down by digestive enzymes, and that makes up the structure of fruit skins, seeds, leaves, stems, and roots. Fiber can be classified into two main categories: water-soluble fiber and water insoluble-fiber. The first kind of fiber can be fermented in the colon and delays gastric emptying, resulting in feeling full for a longer time. The second kind of fiber absorbs water as it moves through the digestive system, easing the process of defecation.
Sources of carbohydrates
Carbohydrates can be found in whole grains (grains that haven’t been refined to remove their endosperm, grain, and bran), beans, fruits, mushrooms, legumes and vegetables. Of course, they can also be found in bread, soda, popcorn and pastries. As we will see in a later blog post, there is a reason we intuitively see some sources of carbohydrates as healthier than others. For now, though, this is as far as we’ll cover the basics of carbohydrates.
So now we know a little more about one of the macronutrients: carbohydrates. We have an idea of what they are, how they are digested, and where we can find them. Next week we’ll deal with proteins, and the week after that with fats / lipids. Hope you enjoyed this week’s post, and see you next week!
If you liked this post and would like to see similar material, please visit the Rebuttals to Fatlogic section of the blog.