Carbohydrate Digestion and Absorption

Digestion 

Act 1: The Mouth

Your mouth is your first line of digestion. The saliva isn’t just to keep your mouth from drying out. The enzyme salivary amylase breaks down some carbohydrate bonds (hydrolyzes, hydro = water) into glucose and fructose. This reinforces the message from the last blog to drink lots of water because it is essential for chemical interactions.

Act 2: Stomach 

By the time the food enters the stomach, the stomach’s acid medium stops this enzyme action.  Act 3 is the small intestine where the bulk of carbohydrate digestion occurs:

  • pancreatic amylase converts starch to maltose units
  • disaccharidases from intestinal cells (the brush borders are storage sites);
  • sucrase to convert sucrose to glucose and fructose;
  • multase to convert maltose to 2 glucose;
  • lactase to convert lactose to galactase and glucose

all ready to be absorbed in the small intestine along with some free fructose and glucose.

Act 4: Large Intestine 

Act 4 is digestion in the large intestine. If a person is lactose intolerant it is because the lactase enzyme is missing. Undigested lactose into the large intestine attracts water molecules producing a bloated feeling. Bacteria ferments the undigested lactose, producing gases creating flatulence.

Absorption

Glucose and galactase pass into cells of intestinal walls mainly by active transport involving simultaneous sodium ion transport. If these two simple sugars are present in very high concentrations they may also transport by diffusion. Fructose absorbed by facilitated diffusion. Very few monosaccharides escape absorption. All of the absorbed monosaccharides go to liver via portal blood. Eventually most diabetics discover the role of their liver in the regulation of blood sugar.

Act 5: The Liver

The liver converts galactose and most fructose to glucose. Some fructose is immediately utilized. About 8% of ingested glucose remains in the liver. After a 12 hour fast, about 50% of an oral glucose load (simple sugar eaten) stays in the liver. The remainder ends up in the other cells of the body.

The liver combines the simple sugar glucose molecules to be stored as the polysaccharide glycogen (called synthesis of glycogen, glycogenesis, for anyone interested). Here the glucose is stuck being part of a glycogen molecule. What’s a glucose to do to get free? The body goes through a removal of glucose units from the glycogen molecules to release glucose into the blood to maintain blood glucose concentrations (a process called glycogenolysis). But the determined glucose doesn’t stop with its liberation from glycogen. It is liberated from lactate, pyruvate, glycerol, and the carbon skeletons of amino acids (called precursors) in a process called gluconeogenesis. This process is very important during fasting when liver glycogen levels are greatly reduced. I personally find it very irritating when the liver doesn’t do this in the middle of the night and my blood sugar goes dangerously low!

Act 6: Blood Glucose Levels 

  • Normal range is 70-100 mg glucose/ dl (100 ml) of blood. The average value is about 90 mg/dl. Glucosuria (glucose in urine) occurs when the blood glucose concentration is above 160 mg/dl.
  • High blood glucose level exceeds the capacity of the kidneys to readsorb glucose back when the blood is filtered.
  • Insulin hormone from pancreas excreated into the blood stream in order to decrease blood glucose concentration by facilitating glucose uptake by muscle and fat tissue.
  • Glucagon hormone from pancreas is secreted into the blood stream increasing blood glucose concentration by the increase release of glucose from the liver. Glucagon acts to increase liver glycogenolysis and/or gluconeogenesis (can you say that 3x fast?).

Act 7: Brain Food 101, Carbohydrates

Glucose is the major fuel for the brain. The brain uses about 125 g glucose/day. Red and white blood cells, a part of the kidney and the retina of the eye also use glucose as the only fuel. Together, these use about 35 g glucose per day. A diet providing 2000 kilocalories of food energy per day where 50% of these calories are from carbohydrates will yield about 250 g glucose each day, well sufficient to satisfy normal glucose needs (1 g glycogen and glucose = approximately 4 kilocalories). Remember to get your sugars from fruits and vegetables and do not restrict yourself to starches.

Act 8: Muscle Fuel

Muscles use fat, carbohydrates and some carbon skeletons of amino acids as fuels. Whereas the glycogen and glucose is stored in the liver, muscle, blood and body fluids, energy is also stored as a triglyceride fat under the skin, around blood vessels and internal organs.

There is much more fat available as a muscle fuel, a good thing since we do not want to use up the carbohydrates needed by the brain and other tissues in muscle activity. Thankfully, there is a biochemical control mechanism which operates in the muscle to preferentially supply fat as fuel.

Carbohydrate use as a fuel depends on the intensity of the work. There is an exponential relationship: carbs provide more energy to the muscle per unit of oxygen consumed, broken down anaerobically to produce lactic acid–something fat cannot do. This allows us to work for short periods of times at an intensity beyond the maximal capacity of the circulatory system to deliver oxygen to the muscles.

Muscles have a capacity to regenerate their ATP by only partially breaking down glucose molecules. This partial breakdown doesn’t require oxygen. If the muscular work is long at a high intensity, it is possible that the carbohydrate stores may become so depleted that the work level must decrease or muscular performance may be compromised. 

Exercised muscles of the body increases its glycogen concentration, influencing improvement in future exercise performance. Diet and previous exercise effects muscle glycogen levels. For each gram of muscle glycogen that is stored, an extra 2 to 3 grams of water is retained by the muscle. “Lean and mean” doesn’t necessarily mean lighter in weight.

Janet Wiebe

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