Carbohydrate intake and body function

While not nearly as versatile in body functions as fats and proteins, carbohydrates are that main source of energy making it all possible. As their name implies, they are built of carbon and hydrogen. Despite being very simple in their basic structure, carbohydrates do have markedly different properties in each of their four main forms: 

▪ simple and double sugars  
 ▪ starches, and 
 ▪ cellulose.

Simple sugars (or monosaccharides) like glucose and fructose (fruit sugar) are easiest to digest. Double sugars (or disaccharides), the most notable example of being sucrose (table sugar), are built of two simple sugars - glucose and fructose - joined together. Starches (oligosaccharides or polysaccharides) are complex carbohydrates found in grains, potato and so called "starchy" vegetables. Their molecules are built of many - sometimes thousands - simple sugars.

Body converts all carbohydrates - except cellulose - into body's preferred form of energy, glucose (blood sugar). The excess of carbohydrates not needed for the energy body converts into fat (triglycerides) and stores it primarily in so called fat cells. Both, simple and double sugars burn quickly giving near-instant burst of energy, while starches burn more slowly, providing longer lasting, more stable energy source. Every gram of carbohydrates burned for energy releases about 4 calories on average (sugars 3.7 and starches 4.2 calories per gram).

However, carbohydrates play much more complex role in the body than just a plain energy source. On the cellular level, they combine with proteins, forming glycoproteins, and with lipids, forming glycolipids, a large group of compounds needed for many vital body functions (prefix glyco- originates from the Greek word glukus, for "sweet"). For instance, sugar group (specific forms of glucose, galactose, fucose, manose, xylose, etc.) in glycoproteins is needed for proper structuring and stability of cellular proteins.

Glycoproteins themselves have wide variety of functions, from the immune system (antibodies, major histocompatibility complex), to proper platelet function, egg-sperm interaction, or connective tissue health. Some glycoproteins have hormonal functions, such as follicle-stimulating hormone, or TSH (thyroid-stimulating hormone).

Glycolipids are crucial for proper functioning of the cellular membrane in substance exchange, as well as for inter-cellular interactions, such as tissue building.

Another important structural function of sugars is in our very genetic material, namely helicoidal DNA and RNA strands, which are built mainly from monosaccharides deoxyribose (DNA) and ribose (RNA).

The official U.S. adult dietary intake recommendation (DRI, for "Dietary Reference Intakes") for total carbohydrates is 45% to 65% of the caloric intake. A simple math shows that it doesn't fit well into the government's own "acceptable" ranges proposed for macro nutrients, since that for fats is 20% to 35% and for proteins 10% to 35%. In other words, if you are close to the "acceptable" maximum of 35% with both, fats and proteins, you are left with only 30% or little more for carbohydrates. Although it still can be labeled officially "acceptable" according to the DRI, such diet would be very unhealthy for most people.

Since 35% of the calories is, in fact, excessive for either fats and proteins, why not simply make their respective general recommendations max out at 30% and 25%? It is both, healthier and adds up to a proper sum with the minimum recommended carbohydrate intake of 45% of the calories.

Prevailing educated - should we also say impartial - opinion is that the low end of the DRI range for both, fats and proteins - 20% and 10% respectively - is actually closer to their optimum intake level for most healthy adults. Thus carbohydrates optimally should make about 65% of the total caloric intake. Most of them should be complex carbohydrates; any significant intake of simple carbohydrates (sugars) is likely to have negative short- and long-term health consequences.
 

 High intake of fast burning simple and double sugars causes surge in blood glucose level, which stimulates the pancreas to start excreting insulin in an attempt to keep blood glucose at the safe level. Significant amount of sugar will likely provoke enough of the insulin kick to push the glucose level down too much, creating sudden low-blood-sugar condition. Body response to it is feeling of hunger, tiredness, and need for instant energy.

That instant energy is, of course, sugar. The feeling of hunger it produces also stimulates over-consumption of other foods, both leading to obesity.

In addition, excess sugar deprives you of - among other nutrients - chromium, which makes you crave sugars even more. For sugar lovers, it is

welcome to the vicious circle!

If excessive sugar intake comes from consuming nutritionally inferior foods - which is frequently the case - the body is literally starved by the luck of nutrients, particularly vitamin B complex, needed to metabolize sugars. Since the metabolism is body's first priority, it will get needed B vitamins at any price, even if it takes stealing them from where they are most abundant: your brain. It can affect your mood and other items in your "head department" in unpredictable - but likely negative - ways.

Excessive consumption of simple and double sugars puts additional strain on the pancreas and its insulin function, which with time can lead into diabetes type 2. Overeating and obesity, nutritional deficiencies and acidosis (unhealthy high level of blood/body acidity) often accompany sugar over-consumption. Needless to say, any of these make the body vulnerable to further health erosion.

Research indicates that chronically elevated glucose level alters the body's metabolic mode by affecting gene expression. The elevated-glucose mode makes the body less resistant to oxidative damage and to external threats in general. Longer term, it can significantly shorten lifespan.

It is not only sugars that can negatively affect body metabolism, both short and long term, by rising blood glucose level. In general, all carbohydrate-rich foods have that potential, usually commensurate with their sugar content. This is not a reason to avoid all carbohydrates - we do need them to live - just a reminder that

any dietary excess is likely to result in compromised health.

With respect to glucose metabolism, of particular concern are highly processed carbohydrate-rich foods, which can be nearly as bad as sugars, as their glycemic index and glycemic load values show.

There is no official recommendation for simple-to-complex carbohydrates proportion in the diet, although general consensus is that complex carbohydrates are preferable, and ought to make about 90% of the carbohydrate intake. On the average, Americans consume nearly half and half of each, sugars and starches.
 

Cellulose is different from sugars and starches in that its glucose units are joined by different chemical bonds, making it indigestible to humans (and nearly all mammals); therefore, it doesn't contribute caloric value, but it does provide needed bulk for proper functioning of the digestive tract. Cellulose is the main component of dietary fiber, which can be insoluble and soluble.