The fifth leading killer of Americans, with 73,000 deaths a year, diabetes is a disease in which the body’s failure to regulate glucose, or blood sugar, can lead to serious and even fatal complications. Until very recently, the regulation of glucose — how much sugar is present in a person’s blood, how much is taken up by cells for fuel, and how much is released from energy stores — was regarded as a conversation between a few key players: the pancreas, the liver, muscle and fat.
Now, however, the party is proving to be much louder and more complex than anyone had shown before.
New research suggests that a hormone from the skeleton, of all places, may influence how the body handles sugar. Mounting evidence also demonstrates that signals from the immune system, the brain and the gut play critical roles in controlling glucose and lipid metabolism. (The findings are mainly relevant to Type 2 diabetes, the more common kind, which comes on in adulthood.)
Focusing on the cross-talk between more different organs, cells and molecules represents a “very important change in our paradigm” for understanding how the body handles glucose, said Dr. C. Ronald Kahn, a diabetes researcher and professor at Harvard Medical School.
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The defining feature of diabetes is elevated blood sugar. But the reasons for abnormal sugar seem to “differ tremendously from person to person,” said Dr. Robert A. Rizza, a professor at the Mayo Clinic College of Medicine. Understanding exactly what signals are involved, he said, raises the hope of “providing the right care for each person each day, rather than giving everyone the same drug.”
Last summer, researchers at Columbia University Medical Center published startling results showing that a hormone released from bone may help regulate blood glucose.
When the lead researcher, Dr. Gerard Karsenty, first described the findings at a conference, the assembled scientists “were overwhelmed by the potential implications,” said Dr. Saul Malozowski, senior adviser for endocrine physiology research at the National Institute of Diabetes and Digestive and Kidney Diseases, who was not involved in the research. “It was coming from left field. People thought, ‘Oof, this is really new.’
“For the first time,” he went on, “we see that the skeleton is actually an endocrine organ,” producing hormones that act outside of bone.
In previous work, Dr. Karsenty had shown that leptin, a hormone produced by fat, is an important regulator of bone metabolism. In this work, he tested the idea that the conversation was a two-way street. “We hypothesized that if fat regulates bone, bone in essence must regulate fat,” he said.
Working with mice, he found that a previously known substance called osteocalcin, which is produced by bone, acted by signaling fat cells as well as the pancreas. The net effect is to improve how mice secrete and handle insulin, the hormone that helps the body move glucose from the bloodstream into cells of the muscle and liver, where it can be used for energy or stored for future use. Insulin is also important in regulating lipids.
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In Type 2 diabetes, patients’ bodies no longer heed the hormone’s directives. Their cells are insulin-resistant, and blood glucose levels surge. Eventually, production of insulin in the pancreas declines as well.
Dr. Karsenty found that in mice prone to Type 2 diabetes, an increase in osteocalcin addressed the twin problems of insulin resistance and low insulin production. That is, it made the mice more sensitive to insulin and it increased their insulin production, thus bringing their blood sugar down. As a bonus, it also made obese mice less fat.
If osteocalcin works similarly in humans, it could turn out to be a “unique new treatment” for Type 2 diabetes, Dr. Malozowski said. (Most current diabetes drugs either raise insulin production or improve insulin sensitivity, but not both. Drugs that increase production tend to make insulin resistance worse.)
A deficiency in osteocalcin could also turn out to be a cause of Type 2 diabetes, Dr. Karsenty said. Another recent suspect in glucose regulation is the immune system. In 2003, researchers from two laboratories found that fat tissue from obese mice contained an abnormally large number of macrophages, immune cells that contribute to inflammation. The finding piqued the curiosity of researchers. “I remember reading the paper and thinking: ‘Wow, look at all those macrophages. What are they doing?’” said Dr. Jerrold M. Olefsky of the University of California, San Diego, School of Medicine.
Scientists have long suspected that inflammation was somehow related to insulin resistance, which precedes nearly all cases of Type 2 diabetes. In the early 1900s, diabetics were sometimes given high doses of aspirin, which is an anti-inflammatory, Dr. Olefsky said.
Only in the past few years has research into the relationship of obesity, inflammation and insulin resistance become “really hot,” said Dr. Alan R. Saltiel, director of the Life Sciences Institute at the University of Michigan.
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