Paleo Diet Dilemma: The Facts About Fructose

One of the bugaboos of modern nutrition is the excessive consumption of fructose. Once a relatively hard-to-get sweetener actually prescribed by doctors for diabetics, fructose has become the bane of modern dieters due to wildly excessive use in the form of high-fructose corn syrup.

Fructose in small amounts is actually a health food. It is only when fructose is consumed in excessive amounts that it is detrimental to health. Small amounts of fructose, generally up to about 25 grams (100 calories) per day, not only aren't toxic, they can enhance health. Here are the facts about fructose and its safe inclusion in modern diets.

Our taste buds are pre-programmed to respond to the taste of fructose.

Fructose is the predominant sugar in fruit. Pound for pound, gram for gram, fructose is sweeter than other common sugars such as sucrose (cane sugar) or glucose(the predominant sugar in grains and syrups, including unrefined corn syrup). Our tongues sense sweetness from fructose more quickly than from other kinds of sugar, and the taste response induced by fructose is more intense that that generated by most other common kinds of sugars.

In the modern world, fruit is not the only (or even primary) source of fructose.

The main source of fructose for most people in North America is high-fructose corn syrup, which is actually a mixture of the super-sweet sugar fructose, concentrated by a refining process, and the so-so sweet sugar glucose. High-fructose corn syrup is found in soft drinks and baked goods and even in many other foods labeled as "naturally sweetened."

Chemically, table sugar, or sucrose, consists of a molecule of fructose chemically bound to a molecule of glucose. The digestive process releases fructose from table sugar, too, and specialized transporter proteins carry it from the gut to the liver.

Only the liver can use fructose as fuel.

Glucose can be used as fuel by cells all over the body, but fructose can only be "burned" by the liver. The liver has to produce enzymes to transform fructose into two chemicals called dihydroxyacetone phosphate and glyceraldehyde.

Read More: The Truth About Paleo Diets

If the liver isn't up to the task, however, the fructose becomes fat (more precisely, triglycerides) in the liver itself. When fructose is consumed in excess, the liver gets sicker and sicker.

More Facts About Fructose

But dietary fructose consumed in excess can cause many other problems.

• Fructose aggravates leaky gut syndrome. It increases the permeability of the lining of the small intestine to problem protein, such as certain proteins in wheat, meat, tomatoes, citrus, chocolate, and dairy products, which can cause inflammation all over the body.

• 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. It isn't the fructose that elevates blood sugar levels, it's glucose from other foods, but fructose consumed in excess interferes with the body's ability to regulate glucose.

• As it fructose enters cells, it can undergo the Maillard reaction. In cooking, this would be seen as browing or caramelization. In the human body, fructose accelerates glycation, the process of “sugar coating” red blood cells and nerves.

• Fructose increases appetite, by elevating bloodstream concentrations of a hormone called ghrelin.

• Fructose suppresses the brain's abilities to control excessive appetite, by decreasing bloodstream concentrations of a hormone called leptin.

• Fructose doesn't activate the brain's satiety centers. There isn't any kind of signal generated by the consumption of fructose that travels 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 fruit or fructose or products made high-fructose corn syrup without ever feeling full.

The fiber in fruit will help us feel full, but there is nothing in cookies or cakes or colas that has a similar effect

• The body's disposal of fructose generates uric acid. Uric acid aggravates gout. Fruit, especially cherries, may contain other compounds that relieve gout, but high-fructose corn syrup is to be strictly avoided.

Read More: Using The Paleo Diet To Beat Common Digestive Problems

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. Since muscles add bulk with glycogen, their back-up fuel supply, fructose-sweetened beverages are be especially important during an athletic competition or 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. This keeps insulin levels down, and helps the body burn more fat.

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.