Sugars

Sugar is an informal term for a class of edible carbohydrates, mainly sucrose, lactose, and fructose characterized by a sweet flavor. The term carbohydrates is most common in biochemistry, where it is a synonym of saccharide. In this article we will review all relative carbohydrates and sugars in particular.

Carbohydrates are a superior short-term fuel for organisms because they are simpler to metabolize than fats or those amino acid portions of proteins that are used for fuel.

The carbohydrates (saccharides) are divided into four chemical groupings:

• monosaccharides,
• disaccharides,
• oligosaccharides,
• polysaccharides.

In general, the monosaccharides and disaccharides, which are smaller (lower molecular weight) carbohydrates, are commonly referred to as sugars. 

Carbohydrate contains 3.75 food calories per gram.

Formally speaking, carbohydrate (CHO) is an organic compound with the empirical formula C_m(H₂O)_n, that is, consists only of carbon, hydrogen and oxygen.

Role

Carbohydrates perform numerous roles in living things. They serve for the storage and the most common source of energy  and as structural components in living organisms. Monosaccharide ribose is an important component of coenzymes and the backbone of the genetic molecule RNA. The related deoxyribose is a component of DNA. Saccharides and their derivatives include many other important biomolecules that play key roles in the immune system, fertilization, preventing pathogenesis, blood clotting, and development.

In food science and in many informal contexts, the term carbohydrate often means any food that is particularly rich in starch or sugar.

Carbohydrate is a universal fuel for all cells, the cheapest source of dietary energy, and also the source of plant fiber. The complete absence of dietary CHO entails the breakdown of fat to supply energy (glycerol and ketone bodies as an alternative fuel for the central nervous system), resulting in symptomatic ketosis, which in adults can be prevented by a daily CHO intake of about 50 g. This value appears to approximate the quantity of glucose required to satisfy minimal glucose needs of the CNS and during starvation.

The practical minimal CHO intake set at 150 g. One can get 50-100 g of sugars per 1 kg of fruit, therefore, one should eat minimum 1.5-3 kg of fruit a day regularly, depending on fruit type.

Our body can obtain its energy from protein and fats . The brain and neurons generally cannot burn fat for energy, but can use glucose or ketones. The body can also synthesize some glucose from a few of the amino acids in protein and also from the glycerol backbone in triglycerides.

It is well established that when used to replace dietary fats, carbohydrates can elevate plasma triglyceride levels and lower levels of high-density lipoprotein cholesterol. However, the type of carbohydrate appears to be important. A diet that replaces fat with carbohydrate from fruits, vegetables, whole grains does not increase triglycerides.

Peter Havel, professor of nutrition at the University of California, Davis:

"...The same way that not all fats are the same, not all dietary carbohydrates are the same either."

Sugars in fruits

The table below I build for orientation, choosing most common sweet juicy fruit, their commercial conventional varieties. In orchards of my childhood fruit were less sweet, more flavorful...

The percentage of calories from carbohydrates of total calories I counted by myself, using data on calories (C) and calories from carbohydrates (CC): CC / (C/100).

Eating these common fruits one gets 78-95% of carbohydrates in one's diet. Only few fruit are relatively high on sucrose, and it present in its natural form with fibers present.

Sucrose

What is sucrose and why is it important? Sucrose is the organic compound commonly known as table sugar and sometimes called saccharose.

In mammals, disaccharide sucrose is digested into its component monosaccharides glucose and fructose. 

The following data comes from experiments with isolated refined sucrose from sugarcane (Saccharum spp.) and sugar beets (Beta vulgaris), and may not be relevant to people who consume no table sugar, just fruit or those plant in their natural raw form.

In experiments with rats that were fed a diet one-third of which was sucrose, the sucrose first elevated blood levels of triglycerides, which induced visceral fat and ultimately resulted in insulin resistance, together with hyperglycemia.

Isocaloric substitution of sucrose for starch results in hyperinsulinemia and deterioration of glucose tolerance, suggesting a loss of insulin sensitivity.

Increased insulin levels in turn elevate the blood calcium levels that then lead to an increased concentration of calcium in the urine. It has been experimentally shown that stone-formers lose up to five times the normal amount of calcium in the urine after ingesting 100 g of sucrose. In one study about 60% of habitual stone formers had an excessive insulin response to sugar. Another demonstrated effect of high sucrose and fructose consumption is a rise in blood levels of uric acid.

When large quantities of sugar are consumed, the body loses the ability to pump in enough insulin to handle it over a long period. This may result in severe damage to liver and pancreas.

The adrenal glands, subjected to stress, raise the level of adrenalin. As a result hyper activity, head ache and mood swing are caused. Too much of sugar changes to saturated fats increasing triglyceride levels in the blood stream. It triggers high cholesterol levels also leading to cardiovascular disease.

To improve the insulin situation dramatically you must be very physically active, what is actually recommended on the site, but you must move a lot and forget about prolonged sedentary activities, and here is why. One study has shown that if rats, that were "running spontaneously in exercise wheel cages", eat 32% it prevents the loss of insulin sensitivity seen in other sucrose-fed rats, and actually improves glucose uptake beyond that seen in the control group. 

glucose

"Glucose is the form of sugar that travels in your bloodstream to fuel the mitochondrial furnaces responsible for your brain power. Glucose is the only fuel normally used by brain cells. Because neurons cannot store glucose, they depend on the bloodstream to deliver a constant supply of this precious fuel.
... The new findings suggest that glucose is not always present in ample amounts to optimally support learning and memory functions."

Glucose is the obligatory energy substrate for brain and it is almost entirely oxidized to CO₂ and H₂O. Although the brain represents only 2% of the body weight in the average adult human, it receives 15% of the cardiac output, 20% of total body oxygen consumption, and 25% of total body glucose utilization.

When glucose is consumed, it can be easily used as energy, and production of a hormone leptin is increased, that helps control appetite and fat storage, and, a stomach hormone ghrelin is reduced, which is thought to help hunger go away.

Glucose is the most important carbohydrate, a simple sugar (monosaccharide) that is metabolized by nearly all known organisms. Energy obtained from metabolism (e.g. oxidation of glucose) is usually stored temporarily within cells in the form of ATP. Organisms capable of aerobic respiration metabolize glucose and oxygen to release energy with carbon dioxide and water as byproducts.

The level of glucose is used as the main control for the central metabolic hormone, insulin.

Glucose is C₆H₁₂O₆.

Fructose

Fructose is a natural form of carbohydrate. For thousands of years it has been an important source of energy for the body. Tree fruits, berries, melons contain significant amounts of molecular fructose, usually in combination with glucose. Fructose exists in foods either as a free monosaccharide, or bound to glucose as sucrose, a disaccharide. Foods with fructose usually contain about an equal amount of free glucose.

Fructose has a very low glycemic index of ~19, compared with 100 for glucose and ~68 for sucrose. Studies show that fructose consumed before a meal may lessen the glycemic response of the meal.

Some fruits have larger proportions of fructose to glucose compared to others. For example, apples and pears contain more than twice as much free fructose as glucose (ratios 2 - 2.1), while for apricots the proportion is less than half as much fructose as glucose (0.7). Peaches, red peppers, grapes and pineapples contain approximately equal amounts of free fructose and glucose (ratios 0.9 - 1.2).

It is important to have an idea of comparative amounts of fructose from fruits and other sources. For example, in watermelons and apples there is 3 to 5 gram of fructose per 100 g (~4%), in dried fruit as raisins - already around 30 g (30%), in sweeteners like sucrose and high fructose corn syrup - 50-90%.

Fructose is the sweetest of all naturally occurring carbohydrates. Fructose is generally regarded as being 1.73 times as sweet as sucrose (73%). The sweetness of fructose is perceived earlier than that of sucrose or dextrose, and the taste sensation reaches a peak (higher than sucrose) and diminishes more quickly than sucrose. Fructose can also enhance other flavors in the system.

Absorption

Free fructose is absorbed directly by the intestine. When fructose is consumed in the form of sucrose, digestion occurs entirely in the upper small intestine. As sucrose comes into contact with the membrane of the small intestine, the enzyme sucrase catalyzes the cleavage of sucrose to yield one glucose unit and one fructose unit. The mechanism of fructose absorption in the small intestine is not completely understood. Some studies show the greatest absorption rate occurs when glucose and fructose are administered in equal quantities.

Fructose transfer activity increases with dietary fructose intake. High-fructose diets have been shown to increase abundance of transport proteins within 3 days.

When fructose is not absorbed in the small intestine, it is transported into the large intestine, where it is fermented by the colonic flora. Hydrogen is produced during the fermentation process and dissolves into the blood. This hydrogen is transported to the lungs. The colonic flora also produces carbon dioxide, short-chain fatty acids, organic acids, and trace gases in the presence of unabsorbed fructose.

Unabsorbed fructose creates higher osmolarity in the small intestine, which draws water into the gastrointestinal tract, resulting in osmotic diarrhea. Exercise immediately after consumption can increase bloating, diarrhea, flatulence by decreasing transit time in the small intestine, resulting in a greater amount of fructose being emptied into the large intestine.

Uptake of fructose by the liver is not regulated by insulin.

When fructose is consumed it appears to behave more like fat with respect to the hormones involved in body weight regulation. Fructose does not stimulate insulin secretion. It does not increase leptin  production or suppress production of ghrelin (recently identified 28 amino acid peptide found in hypothalamus and stomach).

Leptin (a protein hormone that plays a key role in regulating energy intake and energy expenditure, including appetite and metabolism) acts on receptors in the hypothalamus of the brain where it inhibits appetite.

Fructose and HFCS are not the same

The chemically-processed High-Fructose Corn Syrup (HFCS) and natural form of fructose, such as in fruit, are different.

In HFCS glucose chains are broken down into long chains of glucose molecules, that are less sweet, but offer easier viscosity and functionality than raw glucose. Producers dry the genetically-modified corn and mix it with water and sulfar dioxide. The starches are separated from the kernel, fiber and protein, and  then processed at very high temperatures, mixed with acid, neutralized, and then treated with an enzyme to create a 42% fructose and 55% glucose syrup.

Fructan

Chains of fructose molecules known as fructans occur naturally in many foods. 

The most common structural forms of fructan are inulin, levanare and geraminan. The human body absorbs directly only ~5% of fructan.

Fructans may not be well tolerated by some subjects, especially when higher doses are ingested. Unlike glucose, fructans are not efficiently digested or absorbed by the small intestine.

The following foods have a high in fructan: artichokes, leeks, wheat flour.

Inulin

Inulins are mainly comprised of fructose units and typically have a terminal glucose. Plant inulins generally contain between 2 to 140 fructose units. Inulin is indigestible by human enzymes ptyalin and amylase, which are designed to digest starch. As a result, inulin passes through much of the digestive system intact. Inulin is considered to be probiotic, it stimulates the growth of beneficial bacteria in the gut. The activation of these bacteria is thought to reduce the risk of bowel cancer. Inulin has been clinically proven to increase calcium absorption.

Inulin is not chemically related to insulin, the similarities in name do not relate to any similarity in form or function. Inulin has a mildly sweet taste (10% sweetness of sucrose), but does not affect blood sugar levels. People have used plants containing inulin to help relieve diabetes mellitus. Inulin contains a quarter to a third of the food energy of sugar or other carbohydrates.

It is estimated that approximately one-third of the earth's vegetation contains this substance.

Best sources of quality carbs by TheHealthRanger: