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CARBS – The Often-Confused Macronutrient Broken Down, Part II: Focus on Digestion and Absorption

on May 13, 2013

digestionLast week, I probably bored you with lots of technical, scientific jargon on carbs and sugars. Hopefully you learned something and found it helpful to at least know the difference between the types of carbohydrates that exist in our foods. I’d like to now focus on how carbohydrates are actually digested and stored in our bodies – an important component to understand the proper carbs to actually be eating and keeping your body healthiest.
The ultimate goal of digestion and absorption of sugars and starches is to break them into small molecules – namely glucose- that the body can absorb and use. The large starch molecules that I explained last week require extensive breakdown; the disaccharides (glucose, fructose, and galactose) need to be broken only once and the monosaccharide not at all (since they are a simple sugar).

When a person eats foods containing starch, enzymes break them down into shorter, smaller parts. Interestingly enough, this process starts right in the mouth with chemical and mechanical digestion. With the teeth grinding away and salivary amylase (the enzyme that breaks down starches….use this enzyme by name and people will be impressed) breaking down the food into simpler sugars, the mouth is an important part to our digestion. Do a cool experiment – chew a piece of starchy food (i.e. a cracker) and hold it in your mouth for a few minutes without swallowing it – you will notice that the cracker begins tasting sweeter as the enzymes act on it. One thing to note – because food is in the mouth for such a short amount of time, very little carbohydrate digestion takes place there (this begins in the small intestine). With that being said, I cannot emphasize enough how important it is that we chew our food slowly and not scarf down our meal like we were never going to see food again. Chewing slowly and mindfully allows better digestion of the starches and sugars, in addition to your body having a chance to feel full. If we eat too fast, our body’s signaling system does not have a chance to send the hormonal signals to our brain to let us feel full. People who chew slower wind up eating less and make note of less digestive pains. This is a great behavior to model for your family and have your children start practicing at a young age.
OK, back to our digestion… Once the chewed ball of food (called a “bolus”) makes its way out of your mouth and down your esophagus into your stomach, it mixes with the stomach’s acid and protein-digesting enzymes (which then shuts off the salivary amylase in your mouth…the body is so smart!). The stomach’s acid breaks down starch, but not as much as one may think because its juices contain no enzymes to digest carbohydrate (think of enzymes as little “keys” that fit perfectly into certain substances to then break them down; without the right enzymes, digestion cannot occur). Fibers hang out in the stomach longer than anything else and help delay emptying (because fibers are not easily broken down), which is important by providing a feeling of being full (called “satiety”). Eating foods high in fiber will help keep you from over-indulging in more of another type of food to feel full (this is why eating fruits and vegetables often make people feel full…due to their fiber content).

The small intestine is what I refer to as the “mecca” of digestion, for this is where all absorption of nutrients occurs. Without your small intestines, pretty much everything you eat would essentially have no meaning or purpose. The small intestine performs most of the work of carbohydrate digestion, thanks in part to a special enzyme called pancreatic amylase (you guessed it…it’s made in the pancreas). This important enzyme breaks down complex sugars into shorter glucose chains and maltose. What happens from here is pretty exemplary: there are three enzymes that hang out in the outer membranes of the intestinal cells, and they are waiting to break down specific disaccharides (sugars).
• Maltase – breaks maltose down (into two glucose molecules)
• Sucrose – breaks sucrose down (into one glucose and one fructose)
• Lactase – breaks lactose down (into one glucose and one galactose)

This marks the point where all complex sugars and starches have been broken down into simple sugars and starches – mostly glucose molecules (with some fructose and galactose lingering around).
Within one to four hours after a meal, all of the sugars and most of the starches have been digested. Only the fibers remain in the digestive tract. These fibers attract water, which softens the stools for passage without straining (think = anti-constipation). Also, bacteria in the GI tract ferment some fibers, and this process generates water, gas, and short-chain fatty acids (which actually leads to some energy being derived from fiber, but not much at all).
Why It’s Important to Eat A Balanced Diet with Enough Healthy Carbohydrates

Ideally, the body needs to take in at least 50 to 100 grams of carbohydrate a day. For kids over 2 years old, a healthy balanced diet should include 50% to 60% of calories consumed coming from carbohydrates. The key is to make sure that the majority of these carbs come from good sources and that added sugar in their diet is limited. To meet the body’s energy needs, a person has to eat dietary carbohydrate frequently. Glucose is the preferred energy source for brain cells, other nerve cells, and developing red blood cells. It’s important to keep a balanced flow of healthy carbs to the body daily, as it is harder for the body to convert energy from fat and protein. When a person does not replenish depleted glycogen stores by eating carbohydrates, body proteins are broken down to make glucose to fuel the brain and other special cells. These body proteins come primarily from the liver and skeletal muscles, which can lead to a breakdown of important body composition. Worse, if the body does not meet the 50-100 grams of carbohydrate a day, an inadequate supply can shift the body’s energy metabolism in a dangerous direction.

What Happens If Too Many Carbohydrates Are Consumed?
After glucose is used to meet energy needs in the body, any extra glucose is broken down into smaller molecules by the liver. These smaller molecules are then bound together to form fat, which is a more permanent energy-storage compound. If the fat is not used within a short period of time, it is then stored in the fatty tissues of the body for future use (BUT this will just keep accumulating if an abundance of carbohydrates are eaten often). Please pay particular caution that fat cells can store seemingly unlimited quantities of fat, so that is why it’s important to fuel your body with healthy carbohydrate options, and not in an abundant quantity.

Next week, we’ll focus on what happens when there are imbalances in blood glucose caused by either too much or too little sugars in the blood. I’ll also cover in general sugar in children’s diets, hidden sources, and why it’s so important we are the sugar police for our families.


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