Glucose is so essential to the body that the body can get it in a number of different ways. It can digest glucose from starches and sucrose, which is a chemical combination of glucose and fructose. It can convert excess amino acids into glucose and urea (which the kidneys then have to detoxify). And when too much glucose is consumed, or not enough glucose is burned, it can store glucose as glycogen, the energy store of the liver and also the compound that “pumps up” muscles, or as fat.
Glucose is the carbohydrate the body really wants. The key to healthy carbohydrate consumption is to give the body as much glucose as it wants without giving it so much it has to store the excess as fat.
On low carb diets, which begin to “kick in” about the second day, the body begins to manufacture glucose from protein, through a process called gluconeogenesis. In this process, the body uses more glucose than it takes in.
On high-carb diets—and it's the absolute amount of carbohydrate you consume that makes a diet “high” carb, not the percentage of carbohydrate in the diet that makes it high carb—the body will dispose of excess glucose. It will store it as triglycerides locked up in fat cells, or, if there isn't enough insulin in circulation to accomplish that, spill out a little or a lot of glucose into the urine. It is healthiest neither to consume so little carbohydrate that the body has to break down proteins, or so much that is has to store fat.
Fructose Is the “Bad” Carbohydrate
Fructose, as its name suggests, is the most abundant sugar in fruit. Gram for gram fructose is actually sweeter than other sugars such as sucrose (table sugar, or cane sugar) or glucose (the sugar to which our bodies convert most carbohydrates). The sweetness in fructose is sensed on the tongue more quickly than the sweetness of table sugar, and the sensation of sweetness induced by fructose is more intense than that generated by any other kind of sugar. We humans are pre-programmed to enjoy the taste of fruit.
If fruit was the only place we got fructose, there probably would not be a problem. In the modern world, we don't just get fructose in the fructose we eat in fruit or, if you happen to drink them, soft drinks sweetened with high-fructose corn syrup.
Fructose is also a component of the table sugar sucrose. Chemically, each molecule of sucrose consists of a molecule of fructose joined to a molecule of glucose. Digestive enzymes break down sucrose into fructose + glucose, and then specialized transporter proteins called GLUT2 and GLUT5 carry the fructose into the bloodstream.
If we don't eat fructose on a regular basis, however, our small intestines don't make the carrier proteins that take it into the bloodstream. Depending on how often we consume fructose, our bodies can only absorb 5 to 50 grams (20 to 200 calories) from fructose per day. That's a good thing, because any level of fructose consumption over 25 grams (100 calories) per day is mildly toxic.
How Is Fructose Toxic?
The portal hepatic vein sends fructose from food and drink to the liver, where it is absorbed without the help of insulin. (For this reason, fructose was long considered a safe sweetener for diabetics—but only if it is consumed in amounts of 2 to 3 teaspoons per day. ) Glucose from food gets sent from the liver to other parts of the body, but fructose is quickly converted into dihydroxyacetone phosphate and glyceraldehyde. With the right enzymes, the liver can burn these forms of “sugar” to make energy.
Excesses of these two byproducts of the liver's processing of fructose can form triglycerides, cholesterol, and fat, but most of the resulting fat stays in the liver, sometimes (diabetes expert Mark Hyman says in about 30% of people) accumulating to cause a condition called fatty liver.
But dietary fructose consumed in excess can cause many other problems.
• Fructose may actually increase appetite, by elevating bloodstream concentrations of a hormone called ghrelin.
• Fructose may suppress the brain's appetite control system, by decreasing bloodstream concentrations of a hormone called leptin.
• Fructose increases the permeability of the lining of the small intestine. This increases the likelihood of problem proteins (such as certain proteins in wheat, meat, tomatoes, citrus, chocolate, and dairy products) entering the bloodstream.
• Fructose doesn't activate the brain's satiety centers. There isn't any kind of signal to or from the brain that tells us when we have had enough. That's not a problem when we eat a single piece of fruit or we use a single teaspoon (or maybe two) of fructose to sweeten a beverage, but there is no natural way for the brain to put the brakes on the consumption of fructose. It's natural to eat and eat and eat without feeling full.
• Fructose can enter the same metabolic processes that use glucose. Fructose doesn't require insulin, so cells that use fructose shut down their receptor sites for insulin carrying that glucose. At a later meal, when the energy source is glucose rather than fructose, cells are too insulin-resistant to receive the glucose efficiently. This leaves insulin free to store fat, and elevates blood sugar levels.
• Fructose can undergo the Maillard reaction. In cooking, this would be seen as caramelization. In the human body, fructose accelerates glycation, the process of “sugar coating” red blood cells and nerves.
• Because the body's disposal of fructose generates uric acid and uric acid aggravates gout, which is also aggravated by diets that are high in purines, found in meat and beans.
Small amounts of fructose, however, are actually beneficial:
• Athletes can “pump up” muscles after a workout faster when their post-workout beverages are sweetened with a mixture of about 70% glucose and 30% fructose (or galactose. This would be especially important during an athletic competition or during a race.
• Tiny amounts of fructose, from 3 to 10 grams (about half a teaspoon to two teaspoons) per meal, the equivalent of a single piece of fruit or a single serving of berries, activate the liver to respond more completely to the other sugars released from starch, so that insulin levels are kept down and the body can burn more fat—assuming one doesn't overeat and one gets at least some exercise.
The bottom line about fructose is that a little not only is OK, it's healthy. A “little” is up to the equivalent of a single serving of fruit at each meal, or no fruit at meals and two or three fruit snacks. Diabetics actually get slightly better control over their blood sugars when they consume small amounts of fruit.
What Is the Right Amount of Glucose for Our Bodies?
Many diet gurus talk about the percentage of the diet that should come from carbohydrates. Typically a moderate-carb diet is about 40% carbohydrate, a high-carb diet is more than 50% (and most people eat about this amount of carbohydrate whether they are dieting or not), and a low-carb diet may approach 10% or less. Small amounts of carbohydrate even appear in meat, since muscle stores sugar in the form of glycogen.
The reality is a percentage approach isn’t particularly helpful. It is more productive to analyze the body's carbohydrate needs in terms of the minimum amount of carbohydrate it must have from food, and the maximum amount of carbohydrate it can process without converting carb into fat.
Glucose is one of the relatively few major nutrients the body can store. The body can convert protein into glucose when there is a deficiency. It can transform glucose into fat when there is an excess. The best amount of carbohydrate in the diet, however, is the level at which the body is neither creating glucose from proteins nor transforming glucose into fat. (There are important exceptions for people who have diabetes, seizure disorders, or certain other metabolic conditions. We'll address the special needs of people who have these health problems in our section on ketogenic diets. )
A typical adult body burns between 120 and 160 grams (480 to 640) calories of glucose per day at rest. Vigorous exercise increases the body's need for glucose. When we work out hard, our muscles burn glucose without oxygen, creating lactic acid. This is the chemical that gives you the feeling of a “burn. ” Vigorous workouts burn off sugar more than 30 times faster than just sitting around. The body can create energy for slow exercise from fat.
The body also uses glucose to make certain kinds of “sticky” compounds:
• Mucin, which is largely made of carbohydrate, is the key component of mucus. Our respiratory tracts and digestive tracts use mucus to trap pathogens and to protect the lining of the stomach from highly acidic stomach juices. Mucin is also a key component of saliva and tears, helping them adhere to surfaces to keep them moist.
• Hyalouran, along with glucosamine and chondroitin, form the framework that shapes cells into tissues. All three of these compounds contain large amounts of glucose.
Glucose fuels the brain. An adult human brain consumes about 5 grams (20 calories) of glucose every hour, 24 hours a day. That's about 480 calories per day. Those 480 essential calories for brain fuel can be provided by glucose alone, or some of the brain's fuel can come from ketone bodies, which are formed from fat or protein.
Glucose also “pumps up” muscle. The muscles store glucose in long chemical chains to make glycogen, an energy-rich compound that they can use to make quick energy during heavy exertion. During mild exercise, the muscles prefer to burn fat. When we work out so hard that we get out of breath, however, the muscles can make their energy out of glycogen (with lactic acid, causing a “burn,” as a byproduct). Heavy exercise can use up to 1000 calories from glycogen every hour.
On the other hand, it is extremely important to avoid consuming more glucose than our bodies can burn or store. We have to avoid high blood sugar levels, which are the hallmark of diabetes, as well as bouts of relatively high but non-diabetic blood sugar levels which interfere with the body's normal storage and use of fat.
It would seem like a good idea to avoid high blood sugar levels by eating no carbohydrate at all. Some of the world's best known diabetes experts, such as Dr. Richard Bernstein, counsel eating no more than 30 calories (6 grams, or about 1/5 of an ounce) of carbohydrate, preferably in the form of leafy greens, at most meals. But as a long-term strategy, eating less starch to keep blood sugar levels down is a questionable strategy.
How Much Carbohydrate Is Enough?
The chemistry-inclined can understand that the human body consumes around 8 micromoles of glucose per kilogram of body weight per minute. Translated into plain English, that means that most adults need to get 120 to 170 grams, or 480 to 680 calories, of glucose every day.
All of that glucose, however, isn't burned as the body's fuel. About 20 of those calories go into making a structural component of connective tissue we’ve previously mentioned called hyalouran.
About 200 of those calories go into the making of other glycoproteins, and about 200 of those calories are burned by the brain as its preferred fuel. If we don’t get carbohydrate at all, then our bodies have trouble making mucus and connective tissue, and our brains get a little foggy. Even Arctic explorers would get some carb calories from the seaweed and lichens and moss and, even though it sounds gross, fecal matter they would consume along with their raw meat.
Muscles actually prefer to burn fat. They can, of course, burn glucose, especially when oxygen is limited. How long a muscle can burn glucose during intense exercise is limited by the accumulation of a toxic byproduct, lactic acid, which causes the “burn” from an intense workout. The red blood cells have to use glucose as their fuel, and the neurons in the brain don't have ready access to fatty acids. They have to burn glucose or ketones made from protein in the liver.
It doesn't take a lot of carbohydrate to keep the body going, but it does take some. The problem almost universally is that we get more carbs of all kinds than our bodies really need. That's particularly true of the carbohydrate called fructose, but even when people eliminate fructose they often continue to consume too much of the carbohydrates that become glucose.
Photo credit: Jacques Torres chocolates...mmmm. By John Hritz. Wikimedia.