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Monday, August 30, 2010

Health Hazards of Agave and Fructose

Advertised as a natural sweetener with a low glycemic index, agave nectar has become popular with dieters and diabetics alike. Although agave doesn't immediately increase blood sugar levels, it can still contribute to insulin resistance and increase the risk for diabetes, obesity, premature aging, heart disease and liver problems. Understanding the way it works in the body reveals why everyone should avoid agave and other products full of fructose.

Fructose

Traditional agave sweetener is boiled sap from the succulent desert plant native to Mexico and the southwest United States. But the modern agave nectar found in stores today is an entirely different food. This syrup is made by refining the starchy root bulb through chemical processes using genetically modified enzymes to concentrate fructose. Like high fructose corn syrup, agave contains more fructose than glucose so it scores low on the glycemic index, where foods are ranked according to their effects on glucose levels in the blood.

Fructose occurs naturally in fruits and some root vegetables, but it is present in only small amounts and exists as part of a complex. Bound to fiber, fatty acids, vitamins and minerals, natural fructose in whole foods is digested and absorbed slowly. In contrast, fructose found in simple carbohydrates like agave nectar, agave syrup, high-fructose corn syrup and sugar (which is 50 percent glucose and 50 percent fructose) is free, unbound, quickly digested, and absorbed into the bloodstream in large quantities.

Carbohydrate Metabolism

After carbohydrates are consumed and digested, rising levels of glucose in the blood trigger the pancreas to secrete insulin. This hormone allows glucose to enter cells where it is burned for immediate energy. Insulin also prompts the body to store excess glucose for future energy use, as glycogen in the liver and muscles and as fat in adipose tissue. When insulin levels are high, the body will always accumulate fat.

High levels of insulin send a message to the brain that immediate energy needs have been met and the brain responds with feelings of satiety. Insulin also triggers fat cells to secrete leptin, another hormone that reduces hunger and food intake. But fructose enters cells through transporter proteins that are absent from brain cells and insulin-producing cells in the pancreas, so fructose can never trigger the secretion of insulin or leptin, nor their signals to stop eating.

Large amounts of fructose in the blood inhibit the conversion of glucose to glycogen. As a result, glucose levels remain high and the pancreas secretes even more insulin in an attempt to clear it from the blood. If levels of insulin are chronically high, over time cells will compensate by becoming resistant. When cells aren't sensitive to insulin, they can't take up glucose as effectively, and without enough glucose, cells starve. Starving cells stimulate appetite and the body is motivated by hunger to continue eating. This cycle is a key contributor to weight gain and obesity.

Insulin resistance and elevated glucose levels worsen symptoms of diabetes, and in healthy people, they can cause diabetes. In the short term, high levels of blood sugar can lead to fatigue, dehydration, susceptibility to infections and blurry vision. In the long term, they can cause cardiovascular disease, neurological complications, kidney failure and blindness. Execssive insulin and glucose levels can also stimulate the growth of cancer cells.

Advanced Aging

Other effects of high levels of fructose and glucose in the blood are increased production of reactive oxygen species, like free radicals, and advanced glycation end-products (AGEs). Free radicals are a normal part of metabolism, usually neutralized by antioxidants before they do serious damage to cells. However, too many can overwhelm the body's capacity to control them and increased cellular damage is one cause of aging. Unnaturally high amounts of AGEs can also cause premature aging because they easily bind together in a process called cross-linking and become resistant to natural disposal mechanisms. When AGEs accumulate in tissues, they cause rigidity and interfere with normal function.

Collagen, a fundamental component of connective tissue, is particularly susceptible to cross-linking. Collagen cross-linking in skin causes it to lose elasticity and take on a prematurely old appearance. Collagen cross-linking in bones, cartilage and tendons can cause joints to stiffen. In the cornea, lens and retina of the eye, it can cause browning, opacity and cataract formation. In kidneys: renal failure. In blood vessels: increased pressure and hypertension. In nerve endings: peripheral neuropathy. AGEs have also been found in the brain plaques and tangles associated with Alzheimer’s disease.

Cardiovascular Disease

Because fructose cannot be used as immediate energy like glucose, it is converted into triglycerides in the liver and stored as fat. Triglycerides are attached to lipoprotein molecules that shuttle them through circulation and deposit them in fat cells. Classified as low-density lipoproteins (LDL) and very low density lipoproteins (VLDL), they contain a high percentage of fat and a small percentage (or low density) of protein. After the triglycerides are delivered to fat cells, the proteins become small and dense LDL particles.

Small, dense lipoproteins remain in the blood longer and are more likely to oxidize than the large, fluffy LDL particles produced when triglyceride production is low. Oxidized LDL particles and free radical damage initiate blood vessel injury associated with atherosclerosis and coronary artery disease. Like grains of sand, small, dense lipoproteins get stuck in atherosclerotic plaques and clog blood vessels. Additionally, high levels of insulin stimulate the proliferation of smooth muscle cells that line blood vessels, occluding them even further.

When blood can no longer flow because vessels are too narrow, or because an atherosclerotic plaque ruptures and a piece that breaks off blocks blood flow in a smaller vessel, tissues die. If blood can't circulate through small vessels of the lower leg, it can become gangrenous. If blood flow in the brain is interrupted, a stroke occurs. If arteries around the heart are affected, a heart attack happens. Any organ can be affected.

Liver Disease

Fructose has also been linked to liver problems. A 2008 study found that people who consume it regularly have a two- to three-fold higher risk of developing non-alcoholic fatty liver disease (NAFLD) compared to adults of similar age, gender and body mass index. A 2010 study from Duke University Medical Center found that fructose consumption in people with NAFLD can cause hardening and scarring of the liver which may lead to cirrhosis, liver failure, liver cancer and the need for a liver transplant.

The Bottom Line

Agave nectar is an unhealthy sweetener and it isn't alone. High fructose corn syrup and sugar are also harmful, and artificial sweeteners are poor alternatives. If you're hungry for something sweet, have a piece of fruit. Packed with nutrients and fiber, it's the best sweet treat yet.

References

Abdelmalek MF et al. Increased fructose consumption is associated with fibrosis severity in patients with nonalcoholic fatty liver disease. Hepatology. 2010 Jan 28. DOI 10.1002/hep.23535.

Basciano H et al. Fructose, insulin resistance, and metabolic dyslipidemia. Nutrition and Metabolism. 2005 Feb 21;2(1):5.

Elliott SS et al. Fructose, weight gain, and the insulin resistance syndrome. American Journal of Clinical Nutrition. 2002 Nov;76(5):911-22.

Ouyang X et al. Fructose consumption as a risk factor for non-alcoholic fatty liver disease. Hepatology. 2008 Jun;48(6):993-9.

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