Anthony L. Komaroff, M.D.
— DEAR DOCTOR K: I keep hearing about how bad fructose and high-fructose corn syrup are for my health. But is fructose really worse than other types of sugar?
DEAR READER: The short answer is yes. I'm not an expert on sugar metabolism, but I'll tell you what the experts at Harvard Medical School tell me.
Fructose and glucose are the most abundant sugars in our diet. Such sugars are a source of energy for our cells.
In the early 1900s, the average American took in only about 15 grams of fructose, also called fruit sugar, a day. Most of it came from eating fruits and vegetables. Today we average four or five times that amount. And almost all of it comes from sweetened foods and beverages such as breakfast cereals, sodas and fruit drinks.
Human beings have been on Earth for tens of thousands of years. In that time, our bodies got used to a certain diet. Then, in the last 100 years, our bodies have been exposed to much higher levels of fructose.
Common sense might say that such a relatively sudden and large dietary change might be hard for the body to adjust to. Common sense isn't always right, but in this case, it is: There is considerable evidence that today's high levels of fructose are harmful.
Virtually every cell in the body can break down glucose for energy. In contrast, about the only cells that can get energy directly from fructose are liver cells. What the liver does with fructose, especially when high levels enter it, has potentially dangerous consequences for our health.
When fructose enters the liver, it goes through a series of changes. One remarkable change is that the liver uses fructose, a carbohydrate, to create fat.
Give the liver enough fructose, and tiny fat droplets begin to accumulate in liver cells. This buildup is called non-alcoholic fatty liver disease. It looks like what happens in the livers of people who drink too much alcohol.
The breakdown of fructose in the liver also:
None of these changes are good for the arteries and the heart.
There is considerable evidence that people who have more fructose in their diets have higher rates of non-alcoholic fatty liver disease, obesity, diabetes and heart disease. However, fructose has not been proven to be a cause of these conditions.
Still, it's worth cutting back on fructose. That's what I tell my patients, and what I have done myself. But don't do it by giving up fruit. Fruit is good for you, and it's only a minor source of fructose for most people.
Instead, cut back on refined sugar and high-fructose corn syrup. For starters, that means limiting your intake of sugar-sweetened drinks, pastries and breakfast cereals.
Dr.Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.
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