Another reason to avoid too much fructose

UPDATE: April 23, 2019 new podcast on this topic

Fructose is a four letter word.  The bad press from high fructose corn syrup hysteria  led people to conclude that this sugar molecule might be solely responsible for the obesity epidemic.  It can also lead to fat accumulation in the liver and elevated blood lipids.  It turns out we might have other reasons to worry about high fructose intake.  Consider two other four letter words: uric acid.

When you think of fructose, kidney disease probably isn’t the first thing that comes to mind, but according to Dr. Richard Johnson, MD, author of The Sugar Fix and professor in the Division of Renal Diseases at the University of Colorado Anschutz Medical Campus, high fructose intake contributes to kidney disease and hypertension.  When fructose is metabolized in the liver, uric acid is a byproduct of that metabolic process.  The more fructose metabolized, the more uric acid.

Why is this bad for kidneys?  The first clue comes from observations that high fructose intake is linked to elevated blood pressure.  Studies of both humans and rodents have shown this effect.  Elevated uric acid can contribute to hypertension by a direct adverse affect on blood vessel walls.  Uric acid can also cause kidney damage, by direct affect, and by way of damage caused by high blood pressure.

So uric acid generated by fructose metabolism can contribute to hypertension and kidney damage.  Fructose itself may also contribute directly to kidney disease, when high levels are taken up by kidney cells and metabolized, leading to toxic uric acid effects right in the cells.

Dr. Johnson was on a research team that compared the effect of a high fructose vs. glucose diet on kidney function in rats.  The results showed that the high fructose diet accelerated kidney disease progression.  While the amount of fructose fed to the rats was dramatically high compared to what a human would normally eat – 60% of the diet – he noted that it’s necessary to give rodents exaggerated doses, because they’re less sensitive to fructose.

Keep in mind, kidney problems are related to high doses of fructose, from a high intake of added sugars.  According to Dr. Johnson, the small amounts of fructose present in fresh fruit is not likely to cause problems, because the fruits are full of protective nutrients like vitamin C, antioxidants, phenols and potassium.  Additionally, it would be hard to over-eat fructose from fresh fruit, because fruit is full of fiber and is more filling.  Soda pop and other sugary drinks are another matter entirely.  It’s far to easy to guzzle down a big dose of fructose by drinking 2-3 canned soft drinks, or one super-sized drink.  Unfortunately, plenty of people do just that every day.

Can that type of sugar consumption actually cause kidney disease?  Most of the research so far has shown that high fructose consumption makes existing kidney disease worse, but Dr. Johnson is investigating whether high fructose an actually initiate disease.  Certainly high fructose intake from added sugar foods can lead to excess calorie intake and obesity.  There’s really no compelling reason to consume sugary beverages, and a high intake of sugary foods is not compatible with a nutritious diet.  If you needed another reason to cut back on added sugars, the risk of kidney disease is a very good one.

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