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The Dangers of Fructose

An ever-increasing number of studies show that sugared drinks and foods cause fatty liver, obesity, high blood pressure, diabetes, heart attacks, kidney damage and premature death. A review of recent studies shows that fructose may be more damaging than other sugars (Journal of the American Society of Nephrology, November 29, 2010).

Fructose is found in:
• fruit juices (sucrose, which is glucose and fructose bound together in a single molecule),
• table sugar from sugar cane and beets (sucrose),
• honey (mostly glucose and fructose separate from each other), and
• drinks and foods that contain High Fructose Corn Syrup (glucose and fructose, separate from each other).
• Sucrose is also found in most fruits and many vegetables.

Sucrose contains 50 percent fructose and 50 percent glucose bound together. High Fructose Corn Syrup contains approximately a 55/45 proportion of fructose and glucose. As far as the body is concerned, the ratio of glucose to fructose is not important. The issue is whether fructose is more harmful than glucose.

Glucose, but not fructose, can circulate in your bloodstream
Only single sugars can pass from your intestines into your bloodstream. The double sugars, such as sucrose in fruits or lactose in milk, have to be broken down into single sugars before they can be absorbed. Glucose is the only sugar that is allowed to circulate through your body. Fructose cannot circulate in your bloodstream. Fructose is absorbed from your intestines into the blood vessels that carry blood to your liver, where it is immediately converted to glycogen, the stored sugar in your liver, or to triglycerides, a type of fat.

When the liver's stores of glycogen are needed for energy, the glycogen is converted into glucose and released into the bloodstream. After glucose is absorbed into the bloodstream, it passes into your general circulation and is used for energy for your brain, muscles and other tissues. Glucose in your blood supplies almost 98 percent of the calories necessary for your brain to function, which is why a sudden drop in blood sugar can cause you to pass out.

How fructose causes liver damage, obesity, diabetes and death
Your liver converts excess fructose into triglycerides.
• are building blocks for the bad LDL cholesterol that forms plaques in arteries,
• can be stored in your liver to cause a fatty liver,
• can be stored in your fat cells to make you obese.

Having excess triglycerides in your liver:
• causes a condition called fatty liver which interferes with normal liver function; and
• causes fat to be stored in your belly and decreases insulin sensitivity to cause diabetes (Journal of Clinical Investigation, May 2009).

Triglycerides can pass into your bloodstream to:
• damage your kidneys to cause high blood pressure, and
• in very high amounts, can form clots in your bloodstream.

All of these side effects of excess triglycerides increase your risk for heart attacks and strokes.


Who is harmed by fructose?
Fructose appears to be safe if:
• You don't eat large amounts. You have to take in a lot of fructose to raise your triglycerides and become insulin insensitive. Small amounts will not harm you.

• You are not overweight. The fatter you are, the more likely you are to become diabetic. Full fat cells send out hormones of inflammation that block insulin receptors to increase risk for diabetes.

• You do not store fat primarily in your belly. Storing fat primarily in your belly means that you are already insulin insensitive and have high blood insulin and sugar levels. High levels of insulin cause fat to be stored specifically in the liver and belly.

• You exercise regularly. Contracting muscles draw sugar so rapidly from the bloodstream that there is less fructose available to be turned into triglycerides.

• You get your fructose from fruit, not fruit juice. Fructose in fruit is absorbed far more slowly than fructose in fruit juice. The fiber in fruit can keep fruit in your stomach for up to five hours and markedly slows absorption to reduce blood sugar levels. On the other hand, the sugar in fruit juice passes directly into the intestines to be absorbed immediately and cause a high rise in blood sugar. A high rise in blood sugar causes sugar to stick to cell membranes that causes irreversible damage.


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Fructose helps athletes and exercisers
Fructose can be an athlete's best friend. When you exercise, your muscles and brain constantly draw sugar from your bloodstream as a source of energy. The energy for your brain is supplied by sugar in your bloodstream. There is only enough sugar in your bloodstream to last 3 minutes. So your liver has to constantly release sugar from its cells into your bloodstream. However, there is only enough sugar in your liver to last 12 hours at rest, and far less than that during exercise. Your liver can run out of its stored sugar and your blood sugar level can drop, you feel dizzy, lose all muscle strength and can pass out and suffer seizures. Cyclists call this "bonking." Bonking is common in bicycle racers who do not eat frequently during long races, but it is rare in long distance runners. When you run, your leg muscles are damaged from the constant pounding on the roads and you must slow down. However, you pedal in a smooth rotary motion which does not damage your muscles, so you can continue to pedal at a rapid cadence for many hours, until you run out of sugar.

Fructose is the best and most efficient sugar to keep up liver glycogen during competition and to replenish liver stored after an intense workout (Medicine & Science in Sports & Exercise, March 2011). Bicycle racers who ingest drinks that contain fructose can replace lost liver glycogen four times as fast as those who take drinks containing only glucose.

Glucose plus fructose is better than just fructose
If you exercise for more than a couple hours, you can use up almost all of your (stored liver glycogen. Taking drinks that contain both fructose and glucose will keep you going far more efficiently than if you take only one of these sugars (International Journal of Sport Nutrition and Exercise Metabolism, April 2010; Scandinavian Journal of Medicine & Science in Sports, February 2010). Most bottled sports drinks and sugared soft drinks in North America are sweetened with high-fructose corn syrup, containing glucose and fructose in close to equal portions.

No fructose when you are not exercising
Loading with sugared drinks and foods is safe only for people exercising intensely for more than two hours at a time. Low intensity exercise or exercising for less than two hours will not protect you from the potential damage caused by sugared drinks.

How exercise prevents fatty liver and prolongs life
Any exercise that you do will help to protect you from the ravages of sugared foods and drinks. It is usually safe to take sugared drinks while you exercise because blood sugar levels rarely rise too high during exercise or for up to an hour afterward. Contracting muscles draw sugar so rapidly from the bloodstream that there is no sharp rise in blood sugar.

• Contracting muscles help to prevent the high rise in blood sugar that follows eating refined carbohydrates during rest (American Journal of Clinical Nutrition, July 2008).
• Unlike resting muscles, contracting muscles do not require insulin to move sugar inside their cells (Journal of Applied Physiology, July 2005).
• Contracting muscles remove sugar maximally from the bloodstream, without needing insulin, during & up to one hour after exercise. The effect tapers off to zero at about 17 hours (Journal of Applied Physiology, February 2010).

You should avoid sugared drinks at rest
I recommend that you avoid all sugared drinks (sugared sodas and fruit juices) and foods with added sugars except during vigorous exercise.

What to drink during sports that require intense exercise for more than an hour:
Drink any sugared drinks. Ordinary beverages containing both glucose and fructose are probably best; there is no need to seek out special sugars or sugar combinations.


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May 15th, 2011
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About the Author: Gabe Mirkin, MD

Sports medicine doctor, fitness guru and long-time radio host Gabe Mirkin, M.D., brings you news and tips for your healthful lifestyle. A practicing physician for more than 50 years and a radio talk show host for 25 years, Dr. Mirkin is a graduate of Harvard University and Baylor University College of Medicine. He is board-certified in four specialties: Sports Medicine, Allergy and Immunology, Pediatrics and Pediatric Immunology. The Dr. Mirkin Show, his call-in show on fitness and health, was syndicated in more than 120 cities. Read More
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