Know your carbs

By Chris Kresser 

Carbohydrates are broken down into either indigestible fiber, glucose or fructose. Let’s discuss the suitability of each of these as human fuel.

let’s look primarily at how the different types of fat and carbohydrate (our primary sources of energy) affect us, and which of them we should choose as our preferred “human fuel”.

GLUCOSE

Glucose is a simple sugar (monosaccharide) found mostly in plant foods like fruits, vegetables, starchy tubers and grains. It has three main uses in the body:

  • It forms structural molecules call glycoproteins;
  • Like fat, it is a source of energy for cells (especially in the brain); and,
  • it’s a precursor to compounds that play an important role in the immune system.

Glucose preceded fatty acids as a fuel source for living organisms by a very long time, and it is the building block of foods that have the longest evolutionary history of use by mammals like us. The fact that glucose can be produced in the body from protein is often used as an argument that we don’t need to eat it in the diet. But I agree with Dr. Harris’s interpretation that, rather than viewing this as evidence that that glucose isn’t important, we should view it as evidence that glucose is so metabolically essential that we evolved a mechanism to produce it even in its absence in the diet.

One of the few differences between our digestive tract and that of a true carnivore, like a lion, is that we produce an enzyme called amylase. Amylase allows us to digest starch – a long-chain polymer of glucose molecules we can’t absorb – into single molecules of glucose that easily pass through the gut wall into the bloodstream.

Presuming we are metabolically healthy, the glucose and starch we eat is digested and rapidly cleared by the liver and muscle cells. It is only when the metabolism is damaged – usually by years of eating toxins like refined cereal grains, industrial seed oils and fructose – that excess glucose is not properly cleared and leads to insulin resistance and diabetes.

Verdict: the range of glucose that is tolerated varies widely across populations and individuals. Assuming no metabolic problems and an active lifestyle, glucose may be consumed relatively freely. However, many people today do have some form of metabolic dysfunction, and live a sedentary lifestyle. If you fall into this category, glucose should probably be limited to 400 calories (about 100g) of glucose per day.


FRUCTOSE

Fructose is another simple sugar found primarily in fruits and vegetables. While it has the same chemical formula and caloric content as glucose, it has an entirely different effect on the body.

As I pointed out in Step #1: Don’t Eat Toxins, fructose is toxic at high doses. It damages proteins in a process called fructation, which disrupts metabolic function and causes inflammation and oxidative damage. To prevent this, fructose is shunted directly to the liver for conversion into glucose or innocuous fats. But this process damages the liver over time, leading to non-alcoholic fatty liver disease (which one in three Americans now suffer from) and metabolic syndrome.

Another issue is that excess fructose is not well absorbed in the gut, which in turn leads to its rapid fermentation by bacteria in the colon or abnormal overgrowth of bacteria in the small intestine. Small-bowel bacterial overgrowth, or SIBO, is now believed to be the major cause of irritable bowel syndrome (IBS), a common functional bowel disorder that is the second-leading cause of people missing work behind only the common cold.

Most people without metabolic dysfunction can handle small amounts of fructose (as found in a few servings of fruit per day) without problems. But on the scale that fructose is consumed in the U.S. – including 64 pounds of high-fructose corn syrup per person each year on average – fructose wreaks havoc on the body. It should therefore be limited as a source of carbohydrate.

Verdict: 3-4 servings a day of fruit is fine for people without metabolic problems. Those with fatty liver, insulin resistance or other issues should further limit fructose intake, and everyone should avoid high-fructose corn syrup and other concentrated sources like agave syrup.


FIBER

Fiber is plant matter that is indigestible to humans. But although we can’t digest it, some of the 100 trillion bacteria that live in our gut can. In fact, up to 10% of the body’s caloric needs can be met by the conversion of glucose into short-chain fats like butyrate, propionate and acetate by intestinal bacteria. These short-chain fats are the primary energy source for intestinal cells in the colon, and butyrate in particular has been associated with several benefits. These are outlined in The Perfect Health Diet, by Paul & Shou-Ching Jaminet. Butyrate:

  • Prevents obesity.
  • Heals the intestine.
  • Improves gut barrier integrity.
  • Relieves constipation.
  • Improves cardiovascular markers.
  • Reduces inflammation.
  • Stabilizes blood sugar.

The evidence clearly suggests that vegetable fiber is beneficial. However, just as not all fats are created equal, not all fiber is created equal. Grain fiber – which the AHA and other so-called “heart healthy” organizations have been promoting for decades – is toxic for two reasons: it contains toxic proteins like gluten, and it is prone to injure the intestinal wall.

We’ve been bullied into believing that grain fiber prevents heart disease and provides numerous health benefits. But this claim has only been tested in a single clinical trial, and the results were less than spectacular. The Diet and Reinfarction Trial, published in 1989, included 2,033 British men who had suffered a heart attack, and compared a high-fiber group with a control group. The high-fiber group ate whole grains and doubled their grain fiber intake from 9 to 17 grams per day.

How did that work out for them? Not too well. Deaths in the high fiber group were 22% higher over the two year study. 9.9% of the control group died vs. 12.1% of the high fiber group.

There are other reasons to limit all types of fiber. Fiber isn’t essential. Human breast milk doesn’t have any, and traditional people like the Masai – who are free of modern, degenerative disease – eat almost no fiber at all (subsisting on a diet of meat, blood and milk). And while fiber can feed the good bacteria in our gut and increase the production of beneficial short-chain fats like butyrate, it can also feed pathogenic and opportunistic bacteria in the gut.

Verdict: vegetable (but not grain) fiber is beneficial in moderate amounts – about one-half pound of vegetables per day. But think about vegetables and fiber as accompaniments or flavorful condiments to fat and protein, which should form the bulk of calories consumed, rather than the other way around.


SUMMARY OF CARBOHYDRATES

Assuming a healthy metabolism (which isn’t necessarily a safe assumption these days), glucose and starch can be eaten relatively freely, which fructose should be limited to 2-3 servings of fruit per day. Vegetable fiber is beneficial but should also be limited, to about one-half pound of vegetables per day. See the carb pyramid below for a graphic representation.