Carbohydrates tend to be abundant in the human diet and they can–and should–constitute the major source of calories. So why the bad rep?
As the name suggests, carbohydrates are made up of carbon and hydrogen atoms, but there is much more to them than that. In terms of sheer “bang for the buck”, carbohydrates as a class of nutrient are some of the best sources of fuel for both physical and mental performance, but not all carbohydrates are created–or are processed by the human body–equal.
Carbohydrates in the context of human digestion and metabolism can be divided into two basic categories: simple and complex.
Simple carbohydrates are either a single molecule or are two molecules, known as mono or disaccharides, respectively.
Complex carbohydrates are also known as polysaccharides, or any unit of three or more interconnected molecules. Some starches and glycogen have multiple branches of many saccharide units put together.
In general, the larger the carbohydrate at a molecular level, the longer it takes to be absorbed. This has several ramifications when comes to diet and exercise, including the insulin secreted and effects on blood sugar levels.
Breaking Things Down
Polysaccharide breakdown occurs until until there is a single unit that can be absorbed into the bloodstream. Not all sacharrides are equally good at being broken down by the human body. For example, the chemical bond requires the presence of the enzyme lactase, which some do not have, leading to the term “lactose intolerance”. An insoluble fiber, such as cellulose, will have a beta bond versus an alpha bond. The upshot of this is that human body does produce an enzyme capable of breaking down the molecular bonds of this substance, and instead relies on bacteria in the lower gut to do the job. Methane is a familiar byproduct of these reactions.
Only about 50-65% of total calories should come from carbohydrates. Fats should comprise about 20-30% and protein between 10-20%. The USDA guidelines are 60, 30, 10 for many, but it should be noted that a someone on a low-carbohydrate diet or an someone trying to gain muscle mass may have to make some adjustments within these ranges.
Carbohydrate Metabolism Basics
Glucose, a primary source of fuel for the body, can come from three main sources:
- directly from the digestive tract as carbohydrates are broken down in the small intestine and passed into the bloodstream;
- from the breakdown of glycogen stored either in the liver or muscle (glycogenolysis), and
- from “new synthesis” in the liver from lactate, pyruvate, glycerol, alanine, glutamine, and several other amino acids. This is also known as gluconeogenesis.
The versatile liver can exchange various forms of glucose and make new glucose, as well as regulate glucose. For this latter reason, it is sometimes referred to as the “glucostat” or glucose regulator.
What is so great about Complex Carbs?
Many popular diets recommend lowering carbohydrate intake to lose fat. In general, such diet plans recommend staying away from simple carbohydrates–or mono and disaccharides. These carbohydrates are rapidly absorbed because there is less fiber and less metabolic processing to absorb them than complex carbohydrates. This stimulates a glycemic response, which causes blood sugar level to rise and the subsequent secretion of insulin to compensate. The pancreas can signal various hormones to help in the fuel source storage by means of insulin. One of these metabolic signals is for the liver to keep producing endogenous cholesterol, since the catabolism of fat isn’t happening at the time. This increase in making cholesterol production can lead to increased blood levels, which are then deposited in the lining of the blood vessels to form plaques that eventually can block an artery or vein. So while dietary fat in is often equated to fat in the arteries, arterial plaques can also form from simple sugars.
Yet another reason to avoid high blood sugar levels is that when blood sugar stays elevated for extended periods, the chemical process known as glycation can lead to formation of glycation end-products (ACEs). These can also damage the lining of the blood vessels and contribute to plaque formation and damaged capillaries and even cells membranes. There, in a nutshell, is the basis of the adominition to steer clear of simple carbohydrates in favor of complex carbohydrates when it comes to the circulatory system alone.
The National Federation of Professional Trainers. Sports Nutrition Manual. 2nd Ed. Lafayette, IN: NFPT, 2006.