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Carbohydrate Metabolism

Updated: Aug 26, 2023

Carbohydrates are our major fuel source. According to the RDA, our diets should be made up of 45-65% carbohydrate sources. This means that 45-65% of the food we eat/the calories we consume should come from carbohydrates!

(This may be different if you have diabetes or other comorbid conditions, so discuss what your specifications should be with your dietitian!) Carbohydrates are classified into 4 groups-

  1. Monosaccharides: made up of one sugar unit, the only form of absorbable sugar.

  2. Disaccharides: made up of 2 sugar units, need to be broken down in order to be absorbed.

  3. Oligosaccharides: made up of 3-10 sugar units (complex/mainly veggies) that also need to be broken down to be absorbed.

  4. Polysaccharides: made up of 11+ sugar units (complex/think starches, plants) that our body tries to breakdown but needs the help of our gut microbes for digestion!

The 3 carbohydrate sources our bodies can absorb are all monosaccharides, because they are in their simplest form. They come from broken down disaccharides, oligosaccharides and polysaccharides which contain glucose, galactose and fructose molecules. Anything other than those molecules awaits digestion in the intestines where the gut microbiome takes over.

The 3 major dietary sources we consume are in the form of disaccharides:

  1. Maltose (and isomaltose found in sweet potatoes, corn, legumes) made up of 2 glucose molecules bonded together.

  2. Lactose (dairy products) made up of a glucose and galactose molecule bonded together.

  3. Sucrose (cane sugar, honey, fruit) made up of a glucose and fructose molecule bonded together.

Our bodies cannot absorb sugar units that are bonded together so we must break these bonds down into their single sugar units (aka. monosaccharides) in order to be absorbed for energy. Each of these sugars have a corresponding enzyme that the body produces in order to break their bonds. The breaking of these bonds starts in the mouth, which is why its so important to chew your food thoroughly!

In the case of lactose intolerance, these people do not have enough (or any at all) of the enzyme lactase to break the bonds between the sugar, lactose. These individuals can experience digestive distress when consuming lactose. Taking digestive enzymes or lactase enzyme specifically, can help some people tolerate foods that contain lactose, but for others, complete elimination is necessary for digestive health. Once food reaches the stomach and the small intestines, digestive enzymes are released from the pancreas to help further breakdown the food into absorbable components. Absorption occurs mainly in the small intestines where nutrients crosses the intestinal lumen into the bloodstream. This is where the cells can now use this fuel for energy! Supplementing with digestive enzymes can be beneficial for someone that has weakened gastric activity and poor gut health. They are not recommended as a daily supplement because we do not want our body relying solely on supplementation to do what it naturally should be doing. Eating a fiber-rich, nutrient dense diet can help eliminate the need and subsequent use of digestive enzyme supplements.

So for digestion of other sugars like polysaccharides and fibrous carbs (think cellulose, pectins, gums, corn fiber, psyllium husk, and dextrin), the body does not have the ability to break these bonds so it relies on the gut microbes to degrade these and allows us to utilize some energy from the breakdown! For this reason, fiber technically has 2 calories per gram unlike carbohydrates which have 4 calories per gram.

Main takeaways:

Make sure you are eating enough carbohydrates! They are so important for overall energy and effective metabolism!

If you're tracking macros: do not worry about the small details of tracking fiber and net carbs. A lot of products will brag about net carbs, but now you know that you cannot neglect the caloric component of fiber and track only net carbs!

If you're trying to eat healthier, have daily bowel movements (so important), and improve your digestion, (because who isn't?!) eat foods high in fiber. I think it's important for both men and women to get between 30-40g of fiber per day, limit processed and refined carbohydrate sources like pre-packaged and white foods, and consider taking a digestive enzyme with larger meals to aid in the digestion process (not a long term treatment).

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