IN THIS CHAPTER
Discovering how the digestive and circulatory systems distribute glucose
Examining the role the liver and the muscles play in glucose regulation
Looking at how the body compensates when it’s deprived of carbohydrates
Recognizing the risks of elevated blood glucose
The human body is absolutely amazing. It consists of complex systems that function independently yet are intricately interconnected. The nervous system, respiratory system, and circulatory system work behind the scenes around the clock. When functioning properly, the human body’s systems are more or less on autopilot. It’s easy to take these functions for granted. Our bodies do require some upkeep, though. For starters we need food, fluids, and fitness. For optimal health our bodies require specific amounts of vitamins, minerals, and other key nutrients.
It does sound a little repetitive, but we really do need a balanced and complete diet. Eating is how we provide the critical nutrients that our cells, organs, and tissues require. Our bodies must have carbohydrate, protein, fat, and a wide array of vitamins and minerals. The digestive system breaks food down into its basic building blocks. In the intestine, carbohydrates turn into glucose, proteins are disassembled into individual amino acids, and fats break down into fatty acids. The nutrient building blocks are absorbed from the intestine into the bloodstream and are then are used to build and repair our tissues. The circulatory system makes sure that the nutrients are properly distributed throughout the body.
Chapter 3 mentions that wholesome carbohydrate foods are loaded with nutrients, vitamins, and minerals. This chapter explains the special role carb foods play in terms of fueling our bodies. Glucose is a critical source of energy.
Filling Your Tank and Fueling Your Cells
Have you ever really thought about what happens to the food once you’ve eaten it? It’s more common to let food occupy our attention when grocery shopping and planning meals. Effort and focus goes into food preparation and serving the meal. We savor the flavors and textures while eating. But after we finish a meal and clear the table, we are usually off to thinking about something else and moving on to the next thing on our agenda.
In this section, I pick up where we typically leave off. I want to focus on what happens after the last bite has been enjoyed. Figure 4-1 sets the stage for this discussion by depicting the journey that glucose takes through the body. Carbohydrate foods are emptied from the stomach, digested in the intestine, and absorbed into the bloodstream. Glucose then travels through the bloodstream to fuel the body’s cells and contribute to glycogen stores in the liver and muscles.
Illustration by Kathryn Born, MA
FIGURE 4-1: Glucose movement.
Settling in your stomach
The digestive system starts in your mouth. You’ve likely heard of a delicious meal being described as “mouth-watering.” Seeing or smelling foods and sometimes just thinking of food gets the saliva flowing. Saliva contains an enzyme called amylase that begins the process of digestion. Chewing moistens food and breaks it into smaller particles. Swallowing moves food down through the esophagus and into the stomach.
The stomach serves as a temporary holding tank where food is mixed and churned and prepared for digestion. The stomach secretes hydrochloric acid, which helps to break up the food. It also serves as a defense mechanism as acid kills any bacteria that may have hitched a ride in with the food.
The top of the stomach has a sphincter that is supposed to prevent food from coming back up into the esophagus. We call it heartburn when the acidic stomach contents percolate back up into the sensitive esophagus. Fat and caffeine are known to relax the esophageal sphincter and can make heartburn worse.
The contracting stomach is supposed to move the food downward. The pyloric sphincter at the lower end of the stomach regulates the passage of food from the stomach into the upper intestine. Next stop on the journey is the small intestine where digestion and absorption take place.
Extracting glucose via digestion
Once food has transited the stomach, it enters the duodenum, which is the uppermost part of the small intestine. The pancreas secretes bicarbonate into the duodenum to neutralize the stomach acid and enzymes to digest the slurry of foods. Specific enzymes break down each macronutrient: carbs, proteins, and fats. Digestion is complete when macronutrients are broken down into their separate building blocks: glucose (from carbs), amino acids (from proteins), and fatty acids (from fats).
Individual building blocks are small enough to be transported through the intestinal wall into the network of blood vessels surrounding the intestine. Vitamins and minerals are absorbed into the bloodstream via specific channels. Fiber isn’t digestible, so it continues downward and transits through the entire intestine. Fiber helps keep the intestine clean and healthy. Once glucose has been absorbed into the bloodstream, it begins its journey throughout the body (refer to Figure 4-1 ).
Pumping through your bloodstream
The circulatory system is like a complex road map. Take California, for example. The state is made up of countless cities, communities, and people that are connected by freeways, highways, boulevards, rural routes, alleys, and streets. Delivery trucks can bring goods from any location and deliver them directly to your doorstep. Similarly, the circulatory system is made up of blood vessels of varying sizes from main arteries to tiny capillaries. Nutrients are picked up from the digestive tract and transported through every blood vessel to be delivered to each cell in the body. The heart pumps tirelessly to keep the blood moving. (Fun fact: The average adult heart beats 115,200 times each day!) Red blood cells also take advantage of the circulatory system’s route to deliver oxygen from the lungs to all tissues. Carbon dioxide waste is carried away and then taken back to the lungs to be exhaled.
Glucose and blood cells share the blood vessels traveling side by side just like cars and trucks share the roadway. Everyone is going about his business. There may have been times when you’ve been at home waiting for an important package to be delivered to your doorstep; likewise, the cells in your body are awaiting delivery of glucose and other key nutrients.
Fueling your cells
To stay with the analogy in the preceding section, sometimes the delivery truck needs you to be home to sign for an important package. There are delivery stipulations for glucose too. In most cases the cells need insulin to be ready and waiting to accept the glucose. The brain is unique in that it can access glucose without insulin. People with diabetes have an insulin issue. Either they don’t make any of their own insulin, as is the case with type 1 diabetes, or they have insulin that doesn’t work very well, as is the case with type 2 diabetes. (See Chapter 2 for more details on what causes diabetes.)
After the insulin transports glucose into the cell, the glucose becomes the fuel that provides energy for cellular functions. Cells need a steady supply of glucose. Because we don’t nibble around the clock, there has to be a mechanism for storing glucose to be used later. The body solves that issue by storing glucose as glycogen, which can then be used later, as discussed next.
Saving Some for Later: Glycogen, the Storage Form of Glucose
It isn’t uncommon for families to do a major weekly shopping trip to load up on groceries and the essentials. Perhaps a nice meal is prepared a few hours later, but most of the groceries are unloaded and put away in the pantry, the refrigerator, and the freezer. We stock up, saving some for later. It’s good to have reserves. We keep our cupboards stocked because we wouldn’t want to run completely out of food. Neither does your body. Carbohydrate foods need to provide enough glucose for immediate use and to have some left over to stash away for later. The storage form of glucose is called glycogen. Glycogen is made when glucose molecules form bonds with other glucose molecules to create a long polymer, a chain of glucose molecules. Only specific tissues can store glycogen: the liver and the muscles. The following details elaborate on the process.
Making a layover in the liver
When foods digest and are absorbed into the bloodstream, the first stop is the liver. The liver takes what it wants. Glucose and other nutrients, including minerals and fat-soluble vitamins, can be stored in the liver and accessed later as needed.
A balanced meal that contains a mixture of carbohydrate, protein, and fat takes about four hours to fully digest. As the meal is digesting, the carbs from the meal turn to glucose. Some glucose is distributed for immediate use by the body, and some goes into storage as glycogen (refer to Figure 4-1 ). After the meal has finished digesting, the liver releases a steady stream of glucose until the next meal is eaten.
A significant amount of time passes between dinner and breakfast. The liver supplies glucose all night long while you sleep. Sometimes glucose levels rise while you sleep. That glucose is coming from your liver. If your fasting glucose levels are typically above target, that calls for a medication adjustment. Speak to your healthcare provider if you face this situation.
About 20 percent of the body’s glycogen reserves are held in the liver. The liver breaks the glycogen back down into individual glucose molecules and releases the glucose into the bloodstream as needed. The liver tries to make sure that blood-glucose levels don’t fall too low. When glycogen stores are depleted, the liver can even be called upon to make new glucose. That process is detailed later in this chapter.
Maintaining the muscles’ glucose reserves
The muscles can’t boast the ability to make new glucose from scratch, but they certainly do their part by storing up glucose to have at the ready for immediate use when muscle power is needed (refer to Figure 4-1 ). Muscle glycogen accounts for about 80 percent of the body’s total glycogen reserves.
The glycogen that is stored in the muscles stays in the muscles and feeds only the muscles. It cannot be mobilized or transported elsewhere in the body. The likely reason is so the muscles will always have immediate fuel for any fight-or-flight situations. Even if blood-glucose levels drop, the glucose stays put inside the muscles to be used by the muscles.
Regular exercise and fitness help to build and condition the muscles, which increases the glycogen storage capacity. People who exercise and stay fit can store more glycogen than those who are sedentary. Glycogen is not stored in body fat. Flip to Chapter 14 for more information on fitness.
Noting What Happens When Glucose Levels Remain Elevated
Without diabetes, the normal range for blood glucose is about 70–140 milligrams per deciliter (mg/dl) all the time. Type 1 and type 2 diabetes can both lead to blood-glucose levels that rise far above the normal limits. Elevated blood glucose over time can lead to complications associated with poorly controlled diabetes. The kidneys try to do their part to assist, as I describe in the next section.
Spilling glucose into the urine
Each time the heart beats, blood is pushed throughout the body. Blood is continually making a pass through the kidneys along its journey throughout the body. The kidneys are filled with miniature filters called nephronsthat filter the blood and decide what stays in the blood and what gets taken out. The filtered waste that is removed by the kidneys is sent to the bladder and disposed of in the urine.
When glucose levels rise too high, the kidneys decide to filter out some of the excess glucose. Long before blood-glucose monitors were available, the glucose levels in the urine were checked. The process was woefully inaccurate, and the results were hard to interpret. Urine can accumulate in the bladder over several hours. High glucose in the urine didn’t necessary reflect glucose levels in the blood at any particular time. It just indicated that blood glucose had been high enough in the preceding hours for the kidneys to filter out some of the glucose. Figure 4-2 shows the kidneys removing excess glucose and dumping it into the bladder.
Illustration by Kathryn Born, MA
FIGURE 4-2: Kidneys remove excess glucose.
Keep in mind that glucose is food and has calories. Losing glucose in the urine contributes to the unintended weight loss when diabetes is out of control. When glucose levels are significantly elevated, some of the glucose can attach to tissues in the body as explained in the next section. You sure wouldn’t want to gunk up those little filters in the kidneys, because there is no effective way of repairing damaged nephrons.
Sticking glucose where it doesn’t belong: Glycosylation
Glucose has a real affinity for protein. Glucose can glom onto proteins in the body and is very reluctant to ever let go. Glucose sticking to tissues is called glycosylation. Our bodies are made out of protein, and the bloodstream has the ability to expose all areas of the body to excess glucose when levels are high. Blood vessels are made of protein, so unfortunately, glucose has the opportunity to damage blood vessels. As I mention in the previous section, the tiny filters in the kidneys are vulnerable; so are the retinas of the eyes and the nerves throughout the body. Uncontrolled diabetes even increases the risk of heart attack. The damage isn’t easy to see. You can’t get a camera into the tiny kidney filters to take a look around.
There is a test that can be used to estimate the risk that glycosylation poses to delicate tissues. It’s a lab test called the A1C, which measures the amount of glucose that has attached to hemoglobin A1, a protein on the surface of red blood cells. (See Chapters 2 and 23 for more details on A1C.) Figure 4-3 illustrates glycosylation of red blood cells. Red blood cells travel side by side through the bloodstream along with glucose. They share the road, so to speak. When blood-glucose levels are high, more glucose attaches to the surface of the blood cells.
Illustration by Kathryn Born, MA
FIGURE 4-3: Glycosylation of red blood cells.
Red blood cells live about three months. Every day some new red blood cells are created while some old ones are eliminated. The older cells have more glucose attached to them. It’s possible to estimate the average blood glucose over the previous three months by knowing the A1C; see Table 4-1 . A person without diabetes would have an A1C below 5.7 percent. In general, the target A1C is below 7 percent for adults with diabetes and below 7.5 percent for adolescents with diabetes. Sometimes, such as during pregnancy, the targets are set lower. Conversely there are medically complex situations that may warrant a less-stringent target, such as below 8 percent.
TABLE 4-1 A1C and Estimated Average Glucose (eAG)
|
A1C Percent |
eAG in mg/dl |
|
5 |
97 |
|
6 |
126 |
|
7 |
154 |
|
8 |
183 |
|
9 |
212 |
|
10 |
240 |
|
11 |
269 |
|
12 |
298 |
|
13 |
326 |
|
14 |
355 |
Keep in mind that while A1C can provide information about the average blood-glucose control in the previous three months, it doesn’t show any detail whatsoever in terms of how high or how low the blood glucose has been. The reason the target A1C for an adult is below 7 percent (instead of aiming for an A1C in the normal nondiabetic range of below 5.7 percent) is because the lower you aim, the more significant the risk of causing severe hypoglycemia (for insulin users). See Chapter 15 for more on hypoglycemia.
To clarify further: An A1C of 6 percent indicates an estimated average blood-glucose level of 126 mg/dl. If a person is not on medications, there is no risk of hypoglycemia. The blood-glucose range may be stable and hover just above or below 126 mg/dl. However, another person could have the same A1C of 6 percent but have erratic blood-glucose levels with values as low as 40 mg/dl and as high as 350 mg/dl. In other words, A1C doesn’t provide enough information. The home blood-glucose monitoring results are crucial for making regimen adjustments to decrease the variability. That’s a good reason to share your blood-glucose records with your healthcare team. If they have only the A1C to work from, they don’t have enough information to safely adjust medications.
Table 4-1 lists A1C in whole numbers, but your result may be a decimal. The American Diabetes Association has an A1C calculator. Just enter your A1C percentage and it converts it to your estimated average glucose. For example, an A1C of 7.4 has an estimated average glucose of 166 mg/dl. Access the calculator at www.diabetes.org and then click the “Living with Diabetes” tab and look under the “Treatment and Care” heading for “A1C.”
Recognizing the Risks of Undereating Carbs
You need enough carb to meet the fuel demands of the day and to keep your glycogen reserves stocked so your liver can dole out carbs overnight while you sleep, as described earlier in this chapter. When glucose levels are too low, you can’t perform or think at your best. The brain can’t store any glucose, so it must have a constant supply. Lack of concentration and trouble completing mental tasks may be a sign of hypoglycemia (glucose deficiency). If you chronically eat fewer carbs than your body needs, you won’t have enough carbs available to keep glycogen stores well-stocked. There are possible consequences to undereating carbs, as explored in this section.
Body size, age, gender, and level of physical activity all determine how much carbohydrate your body requires. It’s not an exact science, but Chapter 5 provides some guidance on how much carb you may want to aim for.
Depleting glucose reserves and dealing with hypoglycemia
If you don’t consume enough carbs at mealtimes, your body has little choice but to tap into the glycogen reserves. The liver can break down glycogen and send it back into circulation for use by other tissues. The glycogen stored within the muscles can be used by the muscles. Muscle glycogen stores prefer to be fully stocked, so when glycogen supplies are low, the muscles pull glucose from the bloodstream to replenish the glycogen reserves. As muscle glycogen is being refilled, blood-glucose levels may drop too low in individuals who inject insulin or take medications that stimulate insulin production. The muscles don’t really care if sucking glucose out of the bloodstream to fill glycogen stores ends up causing hypoglycemia. People without diabetes don’t share the same risks because when the pancreas is working properly, it turns insulin production on and off as necessary to prevent hypoglycemia.
Hypoglycemia is only a risk if you take insulin or certain pills that stimulate your pancreas to produce extra insulin. But if you’re on a medicine that can cause hypoglycemia, then consider this: Depleted glycogen stores draw glucose steadily from the bloodstream until glycogen storage sites are full. That could lead to unexpected hypoglycemia anytime and especially during the night. Hypoglycemia requires carbs to treat the low blood-glucose level, and sometimes you end up with a high blood-glucose level after treating a low-glucose level. It can be quite a roller coaster. It is easier to simply eat the right amount of carb in the first place. (See Chapter 15 for details on handling hypoglycemia.)
If hypoglycemia isn’t a risk for you, it still makes sense to eat a balanced diet with the proper amount of carbohydrate. Many people are cutting the carbs to try to lose weight. It’s better to cut the overall calories and still keep the proper balance between carbs, proteins, and fats. Most people who strive to lose weight want to lose body fat, not muscle. Poorly planned diets can lead to the loss of muscle and ketone production, which I cover at the end of this chapter.
Making glucose but losing lean body mass
The liver is at the hub of metabolic activity. As previously mentioned in this chapter, the liver is able to store glycogen and release it as glucose later when needed. Although the muscles can store glycogen, the glycogen in the muscles can’t be shared or released. Glycogen in the muscles stays in the muscles to be used solely for the muscles. The liver glycogen stores can be depleted, especially if there are long periods of time between meals. The glycogen is broken back down into glucose and released to fuel hungry cells when no glucose is forthcoming from digestion. The liver glycogen stores are tapped into during exercise too.
The bottom line is that the liver stores a finite amount of glucose, and when it’s gone, the liver switches gears and produces brand-new glucose. The process is called gluconeogenesis; gluco (glucose) plus neo (new) plus genesis (beginning) translates to making new glucose. We all know you can’t make something from nothing. You have to have an ingredient to start with that can be converted to glucose.
When there is no food digesting and no more glycogen in the liver, the liver can convert amino acids into glucose. Muscles are made of protein, and protein is made out of amino acids. Amino acids are made out of carbon, hydrogen, oxygen, and nitrogen. If you remove the nitrogen and rearrange the other elements, you can create glucose. The scientific abbreviation for carbohydrate is CHO, which represents carbon, hydrogen, and oxygen. See Figure 4-4 for a look at the starvation mode of metabolism.
Illustration by Kathryn Born, MA
FIGURE 4-4: Gluconeogenesis.
People are able to survive under extreme food shortages by breaking down their own muscles to produce glucose. It comes at a cost though. You lose healthy muscle tissue. (Unfortunately, fat cannot turn into glucose. During the starvation mode of metabolism, fat turns into ketones, as explained in the next section.)
Producing ketones
Burning fuels always creates some sort of byproduct. Cars have exhaust fumes, for example. Dietary fats can be burned (metabolized) to form very safe byproducts, including carbon dioxide (which you exhale) and water (which turns to sweat or urine).
Cells need insulin to transport glucose. Cells also require the proper fuel mix of fat and glucose in order to produce safe byproducts. When the cells lack insulin or have an extreme shortage of glucose, the fat is incompletely metabolized, and the byproducts are ketones.
In Figure 4-5 , notice what happens when there is no insulin available, which can happen in the case of type 1 diabetes and missed insulin delivery. Without insulin, the glucose can’t enter the cell. The fat can still get in, but it is only partially utilized. Unfortunately, without insulin the fat turns into a different byproduct called ketones. Ketones have a low pH, which means they are acidic like vinegar. They produce a distinctive fruity smell to the breath. The lungs remove some ketones in the exhaled breath. Someone with high levels of ketones may have labored breathing and sweet-smelling breath.
Illustration by Kathryn Born, MA
FIGURE 4-5: Making ketones.
The kidneys filter some of the ketones out of the blood and dispose of them in the urine. Ketone dipsticks are available from the pharmacy and can be dipped into a urine sample to detect ketone levels. In addition, several meters on the market can measure blood-ketone levels. A drop of blood is applied to a special strip in a manner that mimics blood-glucose monitoring. Blood-ketone testing is more accurate because urine-ketone results may show a delayed picture. Keep in mind that urine can continue to show ketones even when blood-ketone levels have resolved. That’s because urine accumulates in the bladder over time, and it takes time to completely wash out of the bladder.
Someone without diabetes may produce ketones when following a very-low-carb diet or fasting for extended periods of time. Ketone levels won’t be high enough to cause diabetic ketoacidosis (DKA) because nondiabetic people produce insulin. On a low-carb diet, a person’s liver can still crank out glucose by turning amino acids from protein into glucose (gluconeogenesis). Body fat can be turned into ketones. Without diabetes, there will still be insulin and enough glucose to burn alongside the fat such that DKA is not a concern. However, if the body has to resort to digesting itself due to drastic dieting, metabolism may slow down and the body will become more efficient at burning calories; the net effect can make losing weight even harder. (See the nearby sidebar for more about DKA.)
DANGER! KETONES CAN LEAD TO DKA
If you have type 1 diabetes and you have ketones, it means you need insulin. Check your blood-glucose levels.
· If you are on an insulin pump and you have ketones, you shouldn’t use your pump to give a correction dose. You should assume that there may be an issue with the pump. Instead, use an insulin syringe or insulin pen to give your dose of insulin. Next you can problem-solve what may be wrong with the pump.
· If you are on injections and have ketones, you may have forgotten a dose, or maybe something is wrong with your insulin and it isn’t working properly. Insulin that is past its expiration date loses potency. Your insulin pen or vial can be left at room temperature, but insulin is destroyed by exposure to high heat, so don’t leave your insulin in sunlight, near the heater, or in a hot car. Once you start using a pen or vial, you usually have to discard it in 28 days even if it isn’t gone. Some types of insulin have a shorter shelf life, so ask your pharmacist about your insulin storage.
If large amounts of ketones accumulate, the pH of the blood can drop. If uncorrected, this can lead to a very dangerous metabolic disturbance called diabetic ketoacidosis (DKA). DKA can lead to complications including electrolyte abnormalities, coma, and death, which is why type 1 diabetes was a fatal disease prior to the discovery and use of insulin. DKA is corrected by administering insulin and fluids for rehydration. DKA requires medical management; speak to your healthcare provide if you have questions about DKA.