Just thinking about eating causes your body to start secreting insulin, a hormone that helps keep blood sugar (glucose) under control. Insulin is made by the pancreas. As you eat, more insulin is released, in response to the carbohydrates in the meal. Insulin is released when you eat protein-rich foods, but at a slower rate. If your pancreas is functioning properly, the amount of carbohydrates in what you’re eating usually determines how much insulin is released.
As you digest carbohydrates, they go into the blood stream as glucose. To keep blood sugar levels under control, insulin signals the cells in your body to take in glucose from the blood stream. The cells use some of glucose for energy and store some for later use. The way glucose is stored depends on the type of cell doing the storing. Muscle cells store glucose as glycogen. Liver cells store some glucose as glycogen and convert some to fat. Fat cells store glucose as fat.
A special note about high-fructose corn syrup: High-fructose corn syrup was introduced in 1978 and replaced the sugar in most soft drinks by about 1985. Total yearly sugar consumption (which excludes artificial sweeteners) immediately increased from 120 pounds per person to 150 pounds per person. High-fructose corn syrup is 55% fructose, 42% glucose, and 3% other carbohydrates. One important fact about fructose: it’s the carbohydrate your body converts to fat most easily. When you digest high-fructose corn syrup, much of the glucose in it ends up in the blood stream, raising your blood sugar levels. But the fructose in high-fructose corn syrup is processed almost totally in the liver, which has the proper enzymes to do the job. So fructose has no immediate effect on blood sugar and insulin levels, but there are lots of long-term effects.
Your liver isn’t designed to process the amount of fructose most people eat today. Fruit has fairly small amounts of fructose – a cup of blueberries has about 30 calories of fructose in it. But soft drinks or juices sweetened with high fructose corn syrup have much higher amounts – 12 ounces of Pepsi or Coke has 80 calories of fructose; 12 ounces of apple juice has 85 calories of fructose.
Your liver’s answer to this flood of fructose is to turn most of it into fat and ship it to your fat tissue. At the same time, the glucose that comes with the fructose in high-fructose corn syrup raises your blood sugar levels and makes your body secrete insulin, which tells your fat cells to store whatever comes their way, including the fructose processed into fat coming from your liver.
The more high-fructose corn syrup you eat and the more years you spend eating it, the more your body adapts by converting high-fructose corn syrup to fat. Over time, you accumulate fat in your liver (a condition called “fatty liver disease”). So while fructose has no immediate effect on your blood sugar and insulin, after a few years it will likely cause you to store calories as fat.
As glucose is removed from the blood stream, insulin levels go down and your cells start using fat for fuel instead of glucose. This is why you can go for long stretches – overnight, for example, when you’re sleeping, without eating. Your cells rely on fat for fuel.
There are two types of body fat: fatty acids and triglycerides. Fatty acids are small enough to move in and out of cells and be used as fuel for cells. Fat is stored inside fat cells as triglycerides, three fatty acids bound together. Triglycerides are too big to flow through cell membranes and so are stored for future use.
Insulin also plays a major role in telling your body when to store and use fat and protein. It does this by affecting the actions of two enzymes, lipoprotein lipase (LPL) and hormone-sensitive lipase (HSL).
LPL sits on the surface of cells and pulls fat out of the bloodstream and into the cell. If LPL is on a muscle cell, it pulls fat into the cell where it’s used for fuel. If LPL is on a fat cell, it pulls fat into the cell and makes it fatter.
It’s important to know that the hormone estrogen suppresses LPL activity on fat cells. This could be one reason why some women gain weight after menopause or after breast cancer treatment that dramatically decreases estrogen levels. With less estrogen in the body, LPL can pull more fat into fat cells and store it there.
The HSL enzyme works to make fat cells leaner by breaking down triglycerides into fatty acids that then can leave the fat cell and be used as fuel by other cells. So the higher HSL levels, the more fat we break down and burn.
Insulin reduces HSL enzyme levels, which stops triglycerides from being broken down and means more fat is stored in fat cells. When insulin levels are up even a little bit, fat accumulates in fat cells.
Some research suggests that keeping insulin levels steady can help some people lose weight. Eating healthy sources of protein and fat (lean meat, fish, poultry and nuts and seeds), as well as complex carbohydrates that are good sources of vitamins and minerals and (vegetables, fruit, whole grains) instead of refined carbohydrates (candy, sugar, cookies, cakes, white bread, pie) can help prevent insulin spikes.
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