Glucose in the Body

The primary role of the available carbohydrates in human nutrition is to supply the body's cells with glucose to deliver the indispensable commodity, energy. Starch contributes most to the body's glucose supply, but as explained earlier, any of the sugars can also provide glucose.

Glucose plays the central role in carbohydrate metabolism. The next two sections provide an overview first of the pathways glucose can follow in the body and then of the ways the body regulates those pathways.

A Preview of Carbohydrate Metabolism

This brief discussion provides just enough information about carbohydrate metabolism to illustrate that the body needs and uses glucose as a chief energy nutrient. Chapter 7 provides a full description of energy metabolism.

• Storing Glucose as Glycogen • The liver stores one-third of the body's total glycogen and releases glucose as needed. During times of plenty, blood glucose rises, and liver cells link the excess glucose molecules into long, branching chains of glycogen. When blood glucose falls, the liver cells dismantle the glycogen into single molecules of glucose and release them into the bloodstream. Thus glucose can supply energy to the central nervous system and other organs regardless of whether the person has eaten recently. Muscle cells can also store glucose as glyco-gen (the other two-thirds), but they hoard most of their own supply, using it just for themselves during exercise.

Glycogen holds water and therefore is rather bulky. The body can store only enough glycogen to provide energy for relatively short periods of time—during exercise, a few hours' worth at most. For its long-term energy reserves, for use over days or weeks of food deprivation, the body uses its abundant, water-free fuel, fat, as Chapter 5 describes.

• Using Glucose for Energy • Glucose fuels the work of most of the body's cells. Inside a cell, enzymes break glucose in half. These halves can be put back together to make glucose, or they can be further broken down into smaller fragments (never again to be reassembled to form glucose). The small fragments can yield energy when broken down completely to carbon dioxide and water, or they can be reassembled, but only into units of body fat.

As mentioned, glycogen stores last only for hours, not for days. To keep providing glucose to meet the body's energy needs, a person has to eat dietary carbohydrate frequently. Yet people who do not always attend faithfully to their bodies' carbohydrate needs still survive. How do they manage without glucose from dietary carbohydrate? Do they simply draw energy from the other two energy-yielding nutrients, fat and protein? They do draw energy, but not simply.

• Making Glucose from Protein • Body protein can be converted to glucose to some extent, but protein has jobs of its own that no other nutrient can do. Body fat cannot be converted to glucose to any significant extent, and although fat breakdown can yield energy for many of the body's cells, only glucose can provide energy for brain cells, other nerve cells, and developing red blood cells.

Thus, when a person does not replenish depleted glycogen stores by eating carbohydrate, body proteins are dismantled to make glucose to fuel these special cells. The conversion of protein to glucose is called gluconeogenesis—literally, the making of new glucose. Only adequate dietary carbohydrate can prevent this use of protein for energy, and this role of carbohydrate is known as its protein-sparing action.

• Making Ketone Bodies from Fat Fragments • An inadequate supply of carbohydrate combined with an accelerated breakdown of fat can shift the body's energy metabolism in a precarious direction. With less carbohydrate available for energy, more fat may be broken down, but not all the way to energy. Instead, the fat fragments combine with each other, forming ketone bodies. Muscles and other tissues can use ketone bodies for energy, but when their production exceeds their use, they accumulate in the blood, causing ketosis, a condition that disturbs the body's normal acid-base balance, as Chapter 7 describes.

To ensure complete sparing of body protein and to prevent ketosis requires 50 to 100 grams of carbohydrate a day. Dietary recommendations urge people to select abundantly from carbohydrate-rich foods to provide for this allowance and considerably more.

• Converting Glucose to Fat • Given more carbohydrate than it needs, the body uses glucose to meet its energy needs, fills its glycogen stores to capacity, and may still have some leftover. To store the extra glucose, the liver breaks it (and energy-containing fragments from protein or fat, too) into smaller molecules and puts them together into the more permanent energy-storage compound—fat. Then the

Making Glucose From Protein
The carbohydrates of grains, vegetables, fruits, and legumes supply most of the energy in a healthful diet.

gluconeogenesis (gloo-co-nee-oh-GEN-ih-sis): the making of glucose from a noncarbo-hydrate source (described in more detail in Chapter 7).

  • gluco = glucose
  • neo = new
  • genesis = making protein-sparing action: the action of carbohydrate (and fat) in providing energy that allows protein to be used for other purposes.

ketone (KEE-tone) bodies: the product of the incomplete breakdown of fat when glucose is not available in the cells.

ketosis (kee-TOE-sis): an undesirably high concentration of ketone bodies in the blood and urine.

acid-base balance: the equilibrium in the body between acid and base concentrations; see Chapter 12.

Reminder: Homeostasis is the maintenance of constant internal conditions by the body's control systems.

Normal blood glucose: 80 to 120 mg/dL.

insulin (IN-suh-lin): a hormone secreted by special cells in the pancreas in response to (among other things) increased blood glucose concentration. The primary role of insulin is to control the transport of glucose from the bloodstream into the cells.

glucagon (GLOO-ka-gon): a hormone that is secreted by special cells in the pancreas in response to low blood glucose concentration and elicits release of glucose from storage.

epinephrine (EP-ih-NEFF-rin): a hormone of the adrenal gland that modulates the stress response; formerly called adrenaline.

fat travels to the fatty tissues of the body for storage. Unlike the liver cells, which can store only about half a day's worth of glycogen, fat cells can store unlimited quantities of fat.

Even though excess carbohydrate can be converted to fat and stored, this is a minor pathway.13 Storing carbohydrate as body fat is energetically expensive. Quite simply, the body uses more energy to convert dietary carbohydrate to body fat than it does to convert dietary fat to body fat. Consequently, body fat comes mainly from dietary fat.14 A balanced diet high in complex carbohydrates actually helps control body weight. Most carbohydrate-rich foods are so bulky and naturally so low in fat that when large quantities are eaten, they tend to crowd fat out of the diet. Since carbohydrate is less energy dense than fat (with only 4 kcalories to the gram compared with fat's 9), eating a diet high in carbohydrate usually reduces energy intake and supports weight control.

The Constancy of Blood Glucose

Every body cell depends on glucose for its fuel to some extent, and ordinarily, the cells of the brain and the rest of the nervous system depend primarily on glucose for their energy. The activities of these cells never cease, and they do not have the ability to store glucose. Day and night they continually draw on the supply of glucose in the fluid surrounding them. To maintain the supply, a steady stream of blood moves past these cells bringing more glucose from either the intestines (food) or the liver (glycogen).

  • Maintaining Glucose Homeostasis • To function optimally, the body must maintain blood glucose within limits that permit the cells to nourish themselves. If blood glucose falls below normal, the person may become dizzy and weak; if it rises above normal, the person may become confused and have difficulty breathing. Left untreated, fluctuations to the extremes—either high or low—can be fatal.
  • The Regulating Hormones • Blood glucose homeostasis is regulated primarily by two hormones: insulin, which moves glucose from the blood into the cells, and glucagon, which brings glucose out of storage when necessary. Figure 4-12 depicts these hormonal regulators at work.

After a meal, as blood glucose rises, special cells of the pancreas respond by secreting insulin into the blood.* As the circulating insulin contacts the receptors on the body's other cells, the receptors respond by ushering glucose from the blood into the cells. Most of the cells take only the glucose they can use for energy right away, but the liver and muscle cells can assemble the small glucose units into long, branching chains of glycogen for storage. The liver cells can also convert glucose to fat for export to other cells. Thus high blood glucose returns to normal as excess glucose is stored as glycogen (which can be converted back to glucose) and fat (which cannot be).

When blood glucose falls (as occurs between meals), other special cells of the pancreas respond by secreting glucagon into the blood.** Glucagon raises blood glucose by signaling the liver to dismantle its glycogen stores and release glucose into the blood for use by all the other body cells.

Another hormone that calls glucose from the liver cells is the "fight-or-flight" hormone, epinephrine. When a person experiences stress, epinephrine acts quickly, ensuring that all the body cells have energy fuel in emergencies. Like glucagon, epinephrine works to return glucose to the blood from liver glycogen.

  • The beta (BAY-tuh) cells, one of several types of cells in the pancreas, secrete insulin in response to elevated blood glucose concentration.
  • The alpha cells of the pancreas secrete glucagon in response to low blood glucose.

When a person eats, blood glucose rises.

2 High blood glucose stimulates the pancreas to release insulin.

4 As the body's cells use glucose, blood levels decline

6 Glucagon stimulates liver cells to break down glycogen and release glucose into the blood.a

Uptake Glucose Cells

3 Insulin stimulates the uptake of glucose into cells and storage as glycogen in the liver and muscle. Insulin also stimulates the conversion of excess glucose into fat for storage.

5 Low blood glucose stimulates the pancreas to release glucagon into the bloodstream.

aThe stress hormone epinephrine and other hormones also bring glucose out of storage

7 Blood glucose begins to rise.

aThe stress hormone epinephrine and other hormones also bring glucose out of storage

When a person eats, blood glucose rises.

2 High blood glucose stimulates the pancreas to release insulin.

3 Insulin stimulates the uptake of glucose into cells and storage as glycogen in the liver and muscle. Insulin also stimulates the conversion of excess glucose into fat for storage.

Muscle

Muscle

4 As the body's cells use glucose, blood levels decline

5 Low blood glucose stimulates the pancreas to release glucagon into the bloodstream.

6 Glucagon stimulates liver cells to break down glycogen and release glucose into the blood.a

The Carbohydrates: Sugars, Starches, and Fibers Figure 4-12

Maintaining Blood Glucose Homeostasis

  • Glucose ^ Insulin ^ Glucagon
  • Glycogen

7 Blood glucose begins to rise.

  • Balancing within the Normal Range • The maintenance of normal blood glucose ordinarily depends on two processes. When blood glucose falls too low, food can replenish it, or in the absence of food, glucagon can signal the liver to break down glycogen stores. When blood glucose rises too high, insulin can signal the cells to take in glucose for energy. Eating balanced meals helps the body maintain a happy medium between the extremes. Balanced meals provide abundant complex carbohydrates, including fibers, some protein, and a little fat. The fibers and fat slow down the digestion and absorption of carbohydrate, so glucose enters the blood gradually, providing a steady, ongoing supply. Dietary protein elicits the secretion of glucagon, whose effects oppose those of insulin, helping to maintain blood glucose within the normal range.
  • Falling outside the Normal Range • This influence of foods on blood glucose has given rise to the oversimplification that foods govern blood glucose concentrations. Foods do not; the body does. In some people, however, blood glucose regulation fails. When this happens, either of two conditions can result: diabetes or hypoglycemia. People with these conditions can often use special diet patterns to help maintain their blood glucose within a normal range.

diabetes (DYE-ah-BEE-teez): a disorder of carbohydrate metabolism resulting from inadequate or ineffective insulin.

hypoglycemia (HIGH-po-gligh-SEE-me-ah): an abnormally low blood glucose concentration.

type 1 diabetes: the less common type of diabetes in which the person produces no insulin at all; also known as insulin-dependent diabetes mellitus (IDDM) or juvenile-onset diabetes (because it frequently develops in childhood), although some cases arise in adulthood.

type 2 diabetes: the more common type of diabetes in which the fat cells resist insulin; also called noninsulin-dependent diabetes mellitus (NIDDM) or adult-onset diabetes.

Type 2 usually progresses more slowly than type 1.

glycemic (gligh-SEEM-ic) effect: a measure of the extent to which a food, as compared with pure glucose, raises the blood glucose concentration and elicits an insulin response.

Popular articles sometimes describe eating many small meals and snacks throughout the day as grazing.

In diabetes, blood glucose remains high after a meal because insulin is either inadequate or ineffective. Thus, while blood glucose is central to diabetes, dietary carbohydrates do not cause diabetes.

There are two main types of diabetes.15 In type 1 diabetes, which is the less common type of diabetes, the pancreas fails to make insulin; researchers hold genetics, toxins, a virus, and a disordered immune system responsible. In type 2 diabetes, which is the more common type of diabetes, the cells fail to respond to insulin; this condition tends to occur as a consequence of obesity. Because obesity can precipitate type 2 diabetes, the best preventive measure is to maintain a healthy body weight. Recommendations for those who have diabetes encourage a diet low in fat and rich in complex carbohydrates and fibers. Concentrated sweets are not strictly excluded from the diabetic diet as they once were, but can be eaten in limited amounts with meals as part of a healthy diet.16 Diabetes and its associated problems receive full attention in Chapter 18.

• The Glycemic Effect • The term glycemic effect describes the effect of food on blood glucose: how quickly glucose is absorbed after a person eats, how high blood glucose rises, and how quickly it returns to normal. Slow absorption, a modest rise in blood glucose, and a smooth return to normal are considered desirable; fast absorption, a surge in blood glucose, and an overreaction that plunges glucose below normal are undesirable. Different foods have different effects on blood glucose depending on a number of factors working together, and the effect is not always what one might expect.17 Ice cream, for example, is a high-sugar food, but it produces less of a response than baked potatoes, a high-starch food.18

Most relevant to real life, a food's glycemic effect differs depending on whether it is eaten alone or as part of a mixed meal. In addition, eating small meals frequently spreads glucose absorption across the day and thus offers the same metabolic advantages as do foods with a low glycemic effect.19

The rate of glucose absorption is particularly important to people with diabetes, who may benefit from avoiding foods that produce too great a rise, or too sudden a fall, in blood glucose. Indeed, some studies have shown that taking the glycemic effect into account in meal planning is a practical way to improve glucose con-trol.20 Overall, though, planning should focus on total carbohydrate intake rather than the source of carbohydrate.21

IN SUMMARY

Dietary carbohydrates provide glucose that can be used by the cells for ^ energy, stored by the liver and muscle as glycogen, or converted into fat if intakes exceed needs. All of the body's cells depend on glucose; those of the central nervous system are especially dependent on it. Without glucose, the body is forced to break down its protein tissues to make glucose and to alter energy metabolism to make ketone bodies from fats. Blood glucose regulation depends primarily on two pancreatic hormones: insulin to remove glucose from the blood into the cells when levels are high and glucagon to free glucose from glycogen stores and release it into the blood when levels are low.

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Responses

  • hagos
    How glucose,insulin,glucogon,glycogen are related?
    4 years ago

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