Many Types of Sugar Molecules
A "sugar" molecule has 4 to 7 carbon atoms in a string or ring shape, with hydrogen and oxygen molecules attached to the carbons. Glucose is a 6-carbon sugar that is the source of energy in all animals. It is also "stackable" forming glycogen molecules for easy storage. Fructose is also a 6-carbon sugar, but it is "foreign" to the human body, and cannot form glycogen. When fructose is eaten, it goes into the liver and is converted to triglycerides and free fatty acids, which are harmful.
Major Breakthrough in Metabolism Research
There has been a giant breakthrough in the understanding of obesity and metabolism in the last few years. There is much more chemistry involved than we originally thought! We have discovered that fructose, in the form of high fructose corn syrup, table sugar, and fruit juice, has been been poisoning our bodies by interfering with leptin, insulin and other hormones. These hormones control the amount of food energy that is either used for action or stored as fat. Your metabolic system is what regulates these hormones. When these hormones are blocked, the body stores too much energy, and very little is left over to use for activity. The brain is tricked into thinking you are starving, so it produces other hormones that make you hungry and want to eat more high-energy foods.
As a result, your fat cells are stuffed, but your brain is starving, so you want to eat even more!
In the past we already suspected that eating too much sugar was bad, but we thought it was just the extra calories. Now it is known that the fructose in processed sugar is poisoning our brains by blocking these important hormones. This effect results in increased insulin, which makes the energy from the food you eat go straight into your fat cells and it can't get out. Even if you try to exercise to lose weight, excess insulin blocks the fat from getting out of the fat cells. This is one reason many overweight people can tolerate very little exercise.
Fructose Poisons the Fat and Energy Control Mechanisms
Fructose is quickly absorbed by the intestines and enters the liver where it is converted to free fatty acids and triglycerides. These are then released into the bloodstream. Free fatty acids are known to make muscle cells insulin-resistant. Insulin resistance means the cells do not respond to insulin, making the sugar concentration continue to increase in the blood. This stimulates the pancreas to produce even more insulin, resulting in a "hyper-insulin state" throughout the body.
Excess insulin blocks the effect of the hormone leptin on the hypothalamus (an important part of your brain that controls energy and basic body functions). Leptin is normally produced by your fat cells and tells your brain when you have enough or too much fat. Triglycerides also block leptin from entering the brain. The result is that the brain is tricked into thinking that there is no fat in the fat cells, so it switches to "starvation mode" where it does everything possible to conserve energy and increase energy intake.
energy burning mode
Leptin (released by full fat cells) normally stimulates your hypothalamus to go into "energy burning mode":
1. reduced appetite
2. increased thyroid hormone, which increases energy expenditure
3. increased stimulus to muscles, releasing heat
4. increased fat breakdown, to supply more energy
5. feeling of well-being, desire for activity
energy conserving mode
When there is no leptin (empty fat cells) or if leptin is blocked, the hypothalamus goes into "energy conserving mode":
1. increased hunger
2. slowed heart rate, and decreased energy use in your muscles
3. increased motion of your intestines (hungry growling stomach) and increased food absorption
4. increased insulin production, which causes your body to store energy (lipids) in your fat cells, and prevents fat from leaving the fat cells
5. desire to be sedentary
When overweight kids (or adults) get to this point, they start seeking more high energy foods, like fats and sugars. They eat more fructose, and the whole vicious cycle gets worse! This leads to a hyper-insulin state, where too much insulin is produced, more energy goes into the fat cells, and the brain continues to react as if it were starving. Excess insulin can also result from eating too much fat or too little fiber, or from having a sedentary lifestyle. However, if you eat fiber, exercise and avoid fat, but continue to eat fructose, you will still gain weight, have high blood lipids and remain in a hyper-insulin state.
Is avoiding fructose the whole answer to the obesity epidemic?
Limiting fructose intake is certainly an important key. There are several other ways that a person can sabotage the body's energy control systems, such as eating tons of trans fat, never getting any physical activity or completely avoiding fiber. However, the damage from fructose is so basic, that little or no progress can be made in reversing or preventing obesity and metabolic syndrome, if a person continues to consume even what they perceive as moderate to small amounts of fructose. In 2009, the American Heart Association published guidelines which recommend that people of any weight severely limit the amount of fructose that they eat per day.
Why is there so much fructose in our diets?
Over the last few decades, high fructose corn syrup was discovered to be a cheap source of food enhancement, which also had the marketing advantage of being "addictive" because it makes you want more and more. Today almost all processed foods contain high fructose corn syrup, since it very cheap to add, and increases sales since it starves our brains and makes us want to eat more.
What is high fructose corn syrup?
Natural corn syrup contains glucose, which is the simple sugar used by our bodies. It does not taste very sweet, so food manufacturers add enzymes to corn syrup to convert the glucose to fructose which tastes very sweet. High fructose corn syrup is 55% fructose! Unfortunately, fructose is toxic to our bodies.
Reading labels to look for fructose or high fructose corn syrup can help us spot the foods that should be avoided. Also realize that all table sugar (sucrose) is half fructose, so avoid regular sugar as much as possible. Most of the food we recognize as "junk food" probably is high in fructose or sucrose.
Another source of fructose is fruit juice. Many schools have made the misguided choice to remove soft drinks from their vending machines and replace them with fruit juice. The soft drinks have too much fructose in the form of high fructose corn syrup, but even 100% natural fruit juice has more fructose than full-sugar soft drinks! The American Academy of Pediatrics has been advising children to strictly limit juice intake for several years now.
What is safe for kids to drink?
Water is always the best fluid to drink, so doctors encourage everyone to drink plenty of water. There are many sweet drinks that are made with artificial sweeteners, such as aspartame, saccharin, and sucralose. These do not have fructose, are very low in calories, and have no proven harmful effects on humans. [There are hundreds of reported harms from aspartime and other sweeteners being put on the internet, but so far no scientific evidence has shown any of these to be true.] There have been some concerns raised about sweet drinks, that they may increase the desire to eat or drink other sweet foods or drinks which may contain fructose. There is also a concern that certain sweeteners may have some of the insulin-increasing effects that fructose has, but this has not yet been studied in humans. (There was a study in rats which showed increased insulin from aspartame.) Bottom line: right now most doctors are recommending diet drinks, since these are far less harmful than sugar drinks; as well as lots of water, and a limited amount of low fat milk (2 or 3 cups a day).
[Source: "How Our Western Environment Starves Kids' Brains" by Dr Robert Lustig, Pediatric Annals 35:12, December 2006]
Fructose Q & A
What is the difference between glucose, fructose and sucrose?
All of them are types of sugar. Glucose is the basic energy unit of our bodies, and is a mono-saccharide (single-sugar molecule). Fructose is also a mono-saccharide, but is a "foreign" sugar to the body, and is not used by the body for any beneficial purpose. Sucrose is a di-saccharide (two sugar molecules joined together). It is made up of one glucose molecule and one fructose molecule joined together. When eaten, sucrose is digested easily into glucose and fructose. The glucose can be used by the body for energy, but much of the fructose is converted to harmful fats, such as triglycerides and free fatty acids.
What kind of sugar is in fruit?
Most fruits contain fructose. Sugar cane and sugar beets produce sucrose.
Is eating fruit bad for you?
It's OK to eat fruit, since the fructose is dilute and offset by a high fiber content. But fruit juice and fruit juice concentrate have concentrated fructose with no fiber. It is not healthy to drink fruit juice.
Is sugar a "carb" (carbohydrate)?
Carbohydrates are molecules that contain a specific pattern of carbon and hydrogen, which includes both sugars and starches. Starches are made of thousands of glucose molecules joined together. Our bodies are well suited to eating starches, since it takes longer to digest them, giving us a steady supply of glucose for hours after we eat. On the other hand, simple sugars are absorbed very rapidly, giving a quick blast of glucose, and often a quick blast of fructose as well, depending on the type of sugar eaten.
What is an artificial sweetener?
It is interesting that saccharin, aspartame and sucralose are referred to as "artificial sweeteners" when they are no more processed or artificial than sucrose (refined sugar) or high fructose corn syrup. A better term for these sugar substitutes would be "non-caloric sweeteners". These sweeteners add a sweet flavor to foods and drinks without adding calories, and more importantly, without adding harmful fructose.
What is sucralose?
Sucralose (Splenda ®) is a non-caloric sweetener. It tastes sweet, but is not absorbed by the intestine and has no caloric value. Its name is often confused with sucrose, but it is not related to sucrose. It contains no fructose, and is not harmful to the body. There is some debate about whether using non-caloric sweeteners may still lead to desire for sweet foods, and may make some people eat more sugar in the long run. However, if consumers are aware of the difference between sucralose and sucrose, they can learn to avoid the fructose-containing sucrose.
Why do so many foods have HFCS (high fructose corn syrup) added?
By converting the glucose in corn syrup to fructose, food processing companies have made a liquid form of sweetener that is much cheaper than the non-caloric sweeteners, yet much sweeter than glucose. This can then be added to lots of different foods to make them sweeter, more moist, have a longer shelf life, and even makes it possible to drink fluids with extremely high acid content (like many soft drinks).
Isn't there sugar in milk?
Milk does contain a type of sugar, which is lactose. Lactose is a disccharide (two sugar molecules) like sucrose, but it does not contain fructose. Instead lactose is made of one glucose and one galactose molecule. Both of these sugar molecules are useful to the body. Lactose is produced by the human body in several tissues including mammary glands which make human breastmilk. Cow's milk also contains lactose. Flavored non-fat milk is often used in schools, such as strawberry and chocolate milk. School officials unknowingly believed that the low-fat designation made it healthy. However, the sugar content in 8 ounces of low-fat flavored milk is typically 38-40 gm (8 tsp of sugar!), the same as the sugar content of 12 ounces of soda pop.
Does a "natural" sweetener like honey also have fructose?
Yes, honey has about the same fructose content as refined sugar. Honey is not a healthy food source. Other so-called "natural" sweeteners like brown sugar and others also have high fructose content. However, maple syrup and corn syrup contain glucose, and are not harmful when used in moderation.
Why does it seem that I gain a few pounds if I eat junk food, when the amount of junk food was far less than a pound? Isn't that physically impossible?
Actually it is very possible! The reason you gain weight from eating junk food is NOT just the calories in the food. It is the toxic effect of the fructose on your metabolic system. Your body loses its ability to control fat and energy, too much fat is deposited in your fat cells, and your metabolic system is not able to utilize that stored energy. The fat builds up, and you gain weight.