Lesson 1, Topic 1
In Progress

02-Macronutrients

March 28, 2021

Dietary sources of nutrients
Put simply, the components of foods that are digested and absorbed by the body are called nutrients. The “big three” nutrients in our diets are carbohydrates (“carbs”), lipids (fats and oils), and proteins. Because they form the bulk of our diet, these three nutrients are sometimes called macronutrients. Vitamins and minerals, by contrast, are called micronutrients because they are needed in only very small quantities in our diet.

Table 19-1 summarizes the three macronutrients, their principal sources in our food, and their main functions in our body. The following sections explore some of these functions more deeply.

 

Carbohydrate metabolism
Carbohydrates are the preferred energy nutrient of the body. The larger carbohydrate molecules are composed of smaller “building blocks,” primarily glucose (see Chapter 2). Human cells catabolize (break down) glucose rather than other substances as long as enough glucose enters them to supply their energy needs.

Glucose catabolism
Glucose catabolism involves three series of chemical reactions called “metabolic pathways,” that occur in a precise sequence:
1. Glycolysis
2. Citric acid cycle (or Krebs cycle)
3. Electron transport system (ETS)
Glycolysis, the first step of glucose catabolism, occurs in the cytoplasm of each cell of the body. As Figure 19-2 shows, glycolysis breaks down glucose (a six-carbon molecule) into two pyruvic acids (three carbon molecules). Glycolysis releases a small amount of energy—enough to generate two adenosine triphosphate (ATP) molecules—but requires no oxygen to

 

FIGURE 19-2 Metabolism of glucose. ​Glucose can be stored as subunits of glycogen in liver and muscle cells until needed to make adenosine triphosphate (ATP). After glycogen is split apart, each individual glucose molecule undergoes glycolysis in the cytoplasm. Glycolysis splits one molecule of glucose (six carbon atoms) into two molecules of pyruvic acid (three carbon atoms each) and produces enough energy to generate two ATPs. Each pyruvic acid is converted to the two-carbon acetyl molecule, which is escorted by coenzyme A (CoA) into the citric acid cycle in the mitochondrion as acetyl CoA. The citric acid cycle breaks apart each pyruvic acid molecule into three carbon dioxide molecules (one carbon atom each) and many high-energy electrons. The electron transport system (also in the mitochondrion) uses energy from these electrons to generate up to 36 ATPs in the presence of oxygen (O2). ADP, Adenosine diphosphate.

 

Each pyruvic acid molecule may then move into a mitochondrion—one of the cell’s tiny “battery chargers” that transfers much more of the nutrient energy to ATP (see Figure 3-2 on p. 45). After pyruvic acid is broken down into the 2-carbon acetyl molecule, a coenzyme A (CoA) then escorts it into the citric acid cycle or Krebs cycle as a molecule called acetyl CoA. The citric acid cycle releases high-energy electrons as it breaks down the acetyl CoA (two carbons) into two carbon dioxides (each having only one carbon) using enzymes located inside the mitochondrion.
The chemical reactions of glycolysis and the citric acid cycle release energy stored in the glucose molecule. More than half of the released energy is in the form of high-energy electrons. The electron transport system, embedded in the inner folds of the mitochondrion, transfers the energy from these electrons to molecules of ATP.
Up to 36 molecules of ATP can be generated in the mitochondrion for every original glucose molecule that enters this metabolic pathway. The rest of the energy originally stored in the glucose molecule is released as heat, which contributes to a person’s body temperature.
The metabolic pathway inside the mitochondrion is, in contrast to glycolysis, an oxygen-using or aerobic process. A cell cannot operate the citric acid cycle or electron transport system—where most of the energy of glucose is released—without oxygen.

To better understand these concepts, use the Active Concept Map Metabolism of Glucose to Generate ATP at evolve.elsevier.com.

ATP
ATP serves as the direct source of energy for doing cellular work in all kinds of living organisms from one-cell plants to trillion-cell animals, including humans. Among biological compounds, therefore, ATP ranks as one of the most important.
The energy transferred to ATP molecules differs in two ways from the energy stored in nutrient molecules: (1) the energy in ATP molecules is not stored but is released almost 536instantaneously, and (2) it can be used directly to do cellular work.
Release of the energy stored in nutrient molecules occurs much more slowly because catabolism of nutrients must occur first. Energy released from nutrient molecules cannot be used directly for doing cellular work. It must first be transferred to ATP molecules and then be suddenly released from them.

As Figure 19-3 shows, ATP is made up of an adenosine group and three phosphate groups. The capacity of ATP to release large amounts of energy is found in the high-energy bonds that hold the phosphate groups (P) together, represented as curvy lines. When a phosphate group breaks off of the molecule, an adenosine diphosphate (ADP) molecule and free phosphate group result. Energy that had been holding the phosphate bond together is freed to do cellular work—muscle fiber contractions, for example.

FIGURE 19-3​Adenosine triphosphate (ATP). ​A, The structure of ATP. A single adenosine group (A) has three attached phosphate groups (P). The high-energy bonds between the phosphate groups can release chemical energy to do cellular work. B, ATP energy cycle. ATP stores energy in its last high-energy phosphate bond. When that bond is later broken, energy is released to do cellular work. The adenosine diphosphate (ADP) and phosphate groups that result can be resynthesized into ATP, capturing additional energy from nutrient catabolism.

HEALTH AND WELL-BEING
CARBOHYDRATE LOADING
A number of athletes and others who must occasionally sustain endurance exercise for a significant period practice carbohydrate loading, or glycogen loading. As with liver cells, some skeletal muscle fibers can take up and store glucose in the form of glycogen. By ceasing intense exercise and switching to a diet high in carbohydrates 2 or 3 days before an endurance event, an athlete can cause the skeletal muscles to store almost twice as much glycogen as usual. This allows the muscles to sustain aerobic exercise for up to 50% longer than usual. The concept of carbohydrate loading has been used to promote the use of “energy bar” sport snacks and some sports or “energy” drinks.

As you can see in Figure 19-3, the ADP and phosphate are reunited by the energy produced by carbohydrate catabolism, making ATP a reusable energy-storage molecule. Only enough ATP for immediate cellular requirements is made at any one time. New ATP is constantly being made to meet changing cellular demands. Glucose that is not needed immediately for ATP production is built up (by anabolic processes) into larger molecules that are stored for later use.

RESEARCH, ISSUES, AND TRENDS
MEASURING ENERGY
Physiologists studying metabolism must be able to express a quantity of energy in mathematical terms. The unit of energy measurement most often used is the calorie (cal). A calorie is the amount of energy needed to raise the temperature of 1 g of water 1° C. Because physiologists often deal with very large amounts of energy, the larger unit, kilocalorie (kcal) or Calorie (notice the uppercase C), is used. There are 1000 cal in 1 kcal or Calorie. Nutritionists in the United States prefer to use Calorie when they express the amount of energy stored in a nutrient.
Most physiologists in the United States—and most nutritionists outside the United States—prefer to use the metric unit joule (J) or kilojoule (kJ) instead of calorie-based units. A simple way to convert kilocalories to kilojoules is kcal × 4.2 = kJ.
To learn more about measuring energy, including examples of the energy content of macronutrients and the energy cost of common activities, review the article Measuring Energy at Connect It! at evolve.elsevier.com.

Anabolism
Glucose anabolism is called glycogenesis. Carried on chiefly by liver and muscle cells, glycogenesis consists of a series of reactions that join glucose molecules together, like many beads in a necklace, to form glycogen, a compound sometimes called animal starch.
Later, when the glucose stored as glycogen is needed to make ATP, a process called glycogenolysis breaks down glycogen in the liver or muscle cells to release individual glucose molecules. Glycogenolysis is an example of catabolism.

Regulation of carbohydrate metabolism
Something worth noting is that the amount of glucose and other nutrients in the blood normally does not change very much, not even when we go without food for many hours, when we exercise and use a lot of nutrients for energy, or when we sleep and use few nutrients for energy. The amount of glucose in our blood, for example, usually stays at about 80 to 110 mg in 100 mL of blood when we are “fasting” between meals.
Several hormones help regulate carbohydrate metabolism to keep blood glucose at a normal level. Insulin is one of the most 537important of these. Although the exact details of its mechanism of action are still being worked out, insulin is known to accelerate glucose transport through cell membranes. As insulin secretion increases, more glucose leaves the blood and enters the cells—particularly the liver cells (see Figure 12-4 on p. 324 and Figure 19-4).

FIGURE 19-4Role of insulin. ​Insulin operates in a negative feedback loop that prevents blood glucose concentration from increasing too far above the normal range. After a meal, intestinal absorption rises and hepatic portal blood glucose concentration increases—as shown by the blue line in the graph. Insulin secretion by the pancreatic islets increases in response (orange line). Insulin promotes uptake of glucose (out of the blood) by liver cells. As blood glucose decreases to its setpoint level, feedback to the pancreatic islets reduces insulin secretion—thus maintaining normal blood glucose concentration. One expects to see a sharp rise in blood insulin levels shortly after a meal high in carbohydrates.

Too little insulin secretion or resistance to insulin effects, such as occurs in people with various forms of diabetes mellitus (DM), produces the opposite effects. Less glucose leaves the blood and enters cells. More glucose therefore remains in the blood, and less glucose is metabolized by cells. In other words, high blood glucose (hyperglycemia) and a low rate of glucose metabolism characterize insulin deficiency or resistance.
Insulin is the only hormone that significantly lowers the blood glucose level. Several other hormones, on the other hand, can increase it. Growth hormone secreted by the anterior pituitary gland, cortisone secreted by the adrenal cortex, epinephrine secreted by the adrenal medulla, and glucagon secreted by the pancreatic islets are four of the most important hormones that increase blood glucose. More information about these hormones can be found in Chapter 12.

To learn more about the citric acid cycle, go to AnimationDirect online at evolve.elsevier.com.

HEALTH AND WELL-BEING
LOW-CARB DIETS
Low-carbohydrate diets have become increasingly popular among those attempting weight loss. When carbohydrate catabolism equals energy needs, fats are not taken out of storage and catabolized. Low-carbohydrate diets are based on the rationale that when the body is not supplied with excess amounts of carbohydrates to meet its energy needs, it will not convert the surplus carbohydrates to fat and store it. Instead, the body relies on fat metabolism to supply energy needs between meals. This eventually reduces overall triglyceride stores in the body, and as a result, the person loses their excess weight. In addition, some research studies on these diets have demonstrated an improved plasma lipid profile.
However, there is still much controversy regarding the many types of low-carbohydrate diets and which are most safe and effective for those struggling with obesity, diabetes, and other disorders. Ultimately, the most successful weight-reducing diet may be the one that each person can stick to for the longest duration and produces the best long-term health effects.

Fat metabolism
Lipids, like carbohydrates, are primarily energy nutrients. As cells begin to run low on adequate amounts of glucose to catabolize a few hours after a meal, they immediately shift to the catabolism of triglycerides—fat—for energy.
Fats are first broken down into fatty acids and glycerol in adipose tissue and released into the blood stream. In cells, fatty acids are broken down to form acetyl CoA, which then proceeds through the citric acid cycle (see Figure 19-2). Glycerol is converted to a compound that can enter the glycolysis pathway in a process is known as gluconeogenesis.
Gluconeogenesis, discussed in Chapter 12, is a process that is performed mainly by liver cells. By converting the 538components of fat into molecules that enter the pathway for glucose catabolism, this process allows energy to be transferred from a nutrient to ATP—which can be used directly for work in the cells of the body.
Fat catabolism happens normally when a person goes without carbohydrates for a few hours. It happens abnormally in individuals with untreated DM. Because of an insulin deficiency, too little glucose enters the cells of a diabetic person to supply all energy needs. The result is that the cells catabolize fats to make up the difference (Figure 19-5).