NOT SO SIMPLE CARBOHYDRATES: GLUCOSE

I believe one of the biggest impediments to proper nutrition is the lack of understanding of basic nutrition principles. When I say basic, I don't mean being to tell me that glycogen is branched because of the alpha-1,6 bonds it contains... I mean the practical knowledge—the knowledge that will actually help us make better decisions in various situations because of our conceptual understanding of nutrition.

For example, most people who follow a "diet" would have some understanding of what a carbohydrate is, right? We all know a carbohydrate is a macromolecule, right? Sure, easy enough. But what about the biochemical difference between a simple carbohydrate and a complex carbohydrate? What about a monosaccharide vs. disaccharide vs. polysaccharide? What about fiber and starch? What's their impact on our nutrition? People hear fiber and think "it helps me poop"—and that's not wrong, but what about in a different context? What if you have regular movements you're just focused on losing weight and looking good via energy regulation? Then fiber can be a completely different tool.

What about protein? How many amino acids are there? 13? 18? 20? What are amino acids? Are they the same thing as protein? How many of the amino acids are "essential" and what exactly makes a protein a "complete" protein? Why would you care about this? What happens when you eat protein? What are the steps to break it down? Does it matter when I eat it? How does eating it after exercise impact digestion and absorption?

**There are 20 amino acids. 9 of which are essential and are required for a food to be a "complete" protein. Dietary proteins are just clusters of many amino acids.**

I have recently put some nutrition resources out into the world. I feel now that I must be sure I take the time to provide the groundwork so individuals, who are so inclined, are able to get the most out of those resources. I will be turning much of my next handful of writings into a course (one that's not boring and irrelevant like many courses are). Maybe I'll call it: Practical Nutrition 1101. And then one day I'll get to teach that course in a university because THAT'S WHAT PEOPLE NEED. So, all of this is going to be made much more simple, but for now, you're getting the granular look at things because I was, and still am, a nerd. Especially for nutrition.

If somebody said to you, "I want to build a house" and you said okay here's a list of what you need and they look at it and confused and said "what's a nail gun?", you'd be like, "okay let's hold off on building the house and start with what all these tools are and what they do." But when it comes to nutrition (because we've been eating food our entire lives) we just say, "eh, fuck it, you'll be fine." And, like the individual building the house, WE NAIL OUR HAND TO THE WALL WITH THE NAIL GUN (metaphorically speaking).

Here's my goal: teach you what the tools of nutrition are and how to use them. I hope to open people's eyes to what it takes to understand nutrition (it's a lot). BUT, at the same time I want to provide straight forward, practical guidance with regard to the items that are relevant so that interested individuals can navigate the field having only to sift through items that pertain to what it is they care about: how to eat well and in a way that's sustainable, helps them feel confident in their eating choices, doesn't set extreme restrictions, and allows them to look good in a bathing suit.

So for my science buffs, you'll enjoy this one. Just know this is leading to something very practical and very helpful (I hope). Enjoy.

Introduction

The current average acceptable macronutrient distribution range (AMDR) according to the National Academies Press is 55% Carbohydrate, 30% Fat, 15% Protein.(1) However, there has been a rampant increase in cases of metabolic syndrome which is a precursor to other life-threatening metabolic diseases such as Type II Diabetes, cancer, CVD, and CHD to name a few (2). All of the aforementioned metabolic disorders appear to have etiology related to excess glucose consumption (3,4,5,6). Glucose plays a role in many metabolic processes in the body, however this paper will focus on its role in energy production and energy storage in relation to various organs and tissues. The goal of this paper is to assess how the current dietary recommendations physiologically impact energy production and energy storage with regard to glucose metabolism both in active individuals as well as sedentary individuals. This paper will provide a basic overview of the biochemistry of glucose and its role in the predominate energy producing pathways to provide the reader with the necessary information for the detailed discussion on glucose metabolism and energy production discussed later on in the paper. To keep the paper specific to the topic at hand, the details of the various pathways and biochemical systems that are presented will be only what is pertinent to the goal of the paper.

Brief Biochemistry of Glucose 

Glucose is a six carbon sugar, or monosaccharide.(9) Mono stemming from the Greek word monos meaning “simple” and saccharide from the Greek word sacchar meaning “sugar.” Monosaccharides are commonly referred to as “simple sugars.” These simple sugars are the most basic form of carbohydrates and cannot be hydrolyzed any further. They are the building blocks of di- and polysaccharides such as sucrose, commonly known as table sugar, which is an equal combination of glucose and fructose molecules (9). The larger or more “complex” the carbohydrate the more mono- and di- saccharides exist in its structure. The molecular skeleton of a carbohydrate is CnH2nOn, the structure of glucose specifically as follows: C6H12O6 (7,9) (Fig. 1a.). 

Figure 1a. Projections of α-D-glucose and β-D-glucose(9)

Glucose is found widely in its α/β-D-glucose form in nature. In the body it can be found in this form of free glucose or as glycogen in the liver, muscle, or brain.(8) Though there are many monosaccharides, the most common found in food are the six carbon (hexose) sugars, Glucose, Fructose, and Galactose(10) (Fig. 1b.).

Figure 1b. Chemical Structures of Glucose, Galactose, and Fructose 

These are all isomers, stereo and constitutional, of one another. Glucose is of particular interest because once inside the enterocyte (intestinal cell), Galactose will be converted to Glucose prior to entering the blood via the glucose transporter, GLUT2, (Fig. 1c.)(11), and many studies have shown that Fructose absorption is dependent on the presence of Glucose.(12,13) For this reason among many other, glucose is the sugar of interest. This will be discussed in detail later on in the paper.

Figure 1c. Absorption and Conversion of Monosaccharides In The Enterocyte

Glucose Metabolism

Glucose metabolism is a very broad term that ultimately refers to the “fate” of glucose once it has entered the body. While carbohydrates are involved in many biological functions in the body, glucose is mainly reserved for energy production and energy storage(8). The major fates of glucose in relation to energy production and energy storage can be seen in the two figures below. 

(Fig 2a)(14) (Fig. 2b)(15) Each of these major pathways will be discussed in detail.

Figure 2a. The Major Fates of Glucose in Energy Production and Energy Storage

Figure 2b. Glycogenesis and Glycogenolysis Pathway

Glycogen Synthesis

The pathway not included in (Figure 2a.), the synthesis and degradation of glycogen, can be seen in (Figure 2b.) This pathway is perhaps the most important role of glucose from an evolutionary standpoint and in relation to “fight or flight.”(16,17)

Glycogen is a branched polymer of glucose with α-l,4 and α-1,6 linkages between glucose units (18). Glycogen resides in the muscle, liver, and to a very small degree in the brain. The “ideal 70-kg man” contains ~350g of glycogen in the muscle, 80g of liver glycogen, and 20g of glucose in extracellular fluids (19). That’s 450g of glycogen in total, which will be an important note later on in the paper. The purpose of glycogen is simply to be a glucose reservoir. Liver glycogen acts as a reserve to maintain the minute 4g of glucose needed in circulation at all times (20). It is important to note that the liver provides the body with glucose when needed while the muscle glycogen is strictly for the muscle. The liver feeds the body, the muscle feeds the muscle. 

The muscle lacks glucose-6-phosphotase, the enzyme necessary to hydrolyze glucose-6-phosphate into free glucose and a phosphate, which would allow the glucose to then exit the cell via GLUT2 and travel to other tissues (22). This is because muscle glycogen sits at the top of the glucose metabolism hierarchy. Glycogen as mentioned earlier appears to have been developed as a survival mechanism to fuel our muscle during a “fight or flight” response (16,17). For this reason when glycogen stores have been depleted to any degree, exogenous glucose will preferentially be used to replenish those stores in both the liver and the muscle (23,24,25).

Closing

I'm gonna stop this one here because we're gonna lose some people—so keep an eye out for glucose pt. 2. Like I said, this will all be used to pull out practical information to help you better understand nutrition at a level that's adequate to allow you to be in control. Many of these principles are in my books, but I'm hoping these supplemental writings can be resources to further elucidate the concepts that I go through a bit quickly in the books.

~ Bonde

References

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