Nutrition - §130.245. Lifetime Nutrition and Wellness

Principles of Digestion and Metabolism

(2) The student understands the principles of digestion and metabolism. The student is expected to:

(A) describe the processes of digestion and metabolism;
(B) calculate and explain basal and activity metabolisms and factors that affect each;
(C) apply knowledge of digestion and metabolism when making decisions related to food intake and physical fitness;
(D) locate community resources that promote physical activity and fitness; and
(E) explain the relationship of activity levels and caloric intake to health and wellness, including weight management.

Describe the Processes of Digestion and Metabolism

Metabolism is the essence of what nutrition is all about. It is the sum of all of the chemical and physiological processes by which our bodies break down and rebuild the foods we eat. Digestion is the mechanical and chemical breaking down of food into smaller components that can be absorbed into a blood stream. Listen to the podcast provided by UC Berkeley which provides an overview of digestion and metabolism of nutrients.

The Process of Digestion

There are three processes involved in digestion:

Look at the diagram below. It is an overview of the gastrointestinal (GI) tract. The GI tract begins at the mouth and ends at the anus and is composed of numerous organs.

Digestion Diagram

The first phase of digestion is called the cephalic phase. Before you take that first bite of food, hunger and appetite work together to prepare the digestive tract for digestion. The nervous system stimulates the release of digestive juices before food even enters our mouth.

Once you take a bite of food, digestion begins in the mouth. Chewing is referred to as mechanical digestion. The salivary glands produce saliva which contains digestive juices including:

Note: only carbohydrate digestion begins in the mouth. The mass of chewed and moistened food is referred to as a bolus.

As the bolus of food is swallowed, rhythmic waves referred to as peristalsis propel the food through the esophagus to the stomach, where food is mixed, digested and absorbed. In general, an empty stomach holds about 6 oz. (3/4 cup), while a full stomach can stretch to hold about 32 oz (4 cups).

The hormone gastrin in the cells lining the stomach signals gastric glands to secrete gastric juice which has several components:

Note: A mucus layer in the stomach protects the lining of the stomach from being digested.

The acidity of the stomach causes carbohydrate digestion to temporarily cease, and protein and fat digestion to begin. The stomach also mechanically mixes food with gastric juice into a semi-liquid called chyme. Chyme remains in the stomach for about 2 hours and is then released in spurts into the small intestine. Most digestion and absorption of nutrients occurs in the small intestine, which has three sections: duodenum, jejunum and ileum. The pancreas, gall bladder and liver all assist with digestion in the small intestine.

The pancreas manufactures, holds, and secretes digestive enzymes. Bicarbonate is secreted to neutralize chyme. The pancreas stores the following enzymes in the inactive form (to be activated in the small intestine):

Insulin and glucagon (hormones) are produced by the pancreas to regulate blood glucose.

The liver is one of the most important organs in the body because it has over 500 functions, including:

A small amount of nutrient absorption takes place in the stomach and large intestine. However, the majority occurs in the small intestine. The villi, microvilli, and brush border which make up the lining of the small intestine provide a tremendous amount of surface area for nutrient absorption. Factors such as going long periods without eating, certain medical conditions including gluten intolerance (celiac disease), and inflammatory bowel disease (Crohn's) can cause the villi to atrophy and create problems of malabsorption.

Any undigested food components in chyme eventually reach the large intestine. The large intestine is also referred to as the colon or bowel. The undigested matter consists of fiber, bacteria, and water. Bacteria assist with final digestion. The main function of the large intestine is to store undigested food material (12 – 24 hours) and absorb water, short chain fatty-acids, and electrolytes. Peristalsis occurs weakly to move feces through the colon. It is important to note that a few stronger urges each day push feces toward the rectum for elimination. This is referred to as mass movement. We all know that fiber and water are important to prevent constipation, but ignoring those stronger urges can result in constipation as well.

The Process of Metabolism

Catabolism begins with digestion and refers to the breaking down of large, complex molecules into smaller molecules. Carbohydrates become monosaccharides, lipids become fatty acids and glycerol, and proteins become amino acids. Catabolism also occurs when old cells are broken down during maintenance and when energy is released during intracellular catabolism.

Anabolism occurs when smaller molecules such as monosaccharides, amino acids and fatty acids are reformed into more complex molecules such as glycogen, hormones, enzymes or whatever the body needs for growth and maintenance of cells and tissues. The opposite of catabolism, anabolism uses energy to build these larger molecules.

ATP (adenosine triphosphate) is referred to as the molecular “currency” of the cells. ATP is formed by the catabolism of glucose, glycerol, fatty acids, and amino acids. It contains high-energy phosphate bonds that release a significant amount of energy when ATP is broken down to ADP (adenosine diphosphate) and AMP (adenosine monophosphate). ATP is regenerated by adding back phosphate groups through the process of phosphorylation.

Metabolism consists of a series of chemical reactions called metabolic pathways. The various metabolic pathways that occur take place in different parts of the cell, depending on the various enzymes contained in those parts. The mitochondria often referred to as the “powerhouse” of the cell contains the greatest number of metabolic enzymes and is the primary site where ATP is made. In fact, 90% of the energy produced in the body comes from the mitochondria. Different types of cells in the body also contain different enzymes which limit the metabolic pathways that can occur there. For example, a difference of one enzyme means that muscle glycogen cannot be released into the bloodstream for use elsewhere, but liver glycogen can.

Let’s briefly define some of the basic chemical reactions that occur as part of metabolism.

Two chemical reactions involving water are condensation and hydrolysis. Condensation is typically an anabolic process which combines smaller molecules into larger ones and water is released. Hydrolysis on the other hand is usually catabolic and involves breaking apart two molecules by adding a water molecule.

condensation and hydrolysis

Phosphorylation refers to the exchange of phosphate molecules such as when ATP is formed with the addition of a phosphate molecule to ADP. Dephosphorylation is the process that occurs when a phosphate is removed from ATP to make ADP.

Oxidation-reduction reactions involve the exchange of electrons, usually in the form of a hydrogen molecule. Also referred to as redox or exchange reactions, these reactions occur together because the electron received by one molecule must be donated by another molecule. We breathe in oxygen which oxidizes or removes a hydrogen atom from another molecule which then becomes reduced or more negatively charged (e -).

A critical pathway of energy metabolism involves two forms of ribovlavin (vitamin B2). FAD (flavin adeneine dinucleotide), a coenzyme, and FADH2 easily exchange electrons as hydrogen. (See below). Fad and FadH2 are necessary for glycolysis.

Oxidation and reduction of FAD and FADH2
Oxidation and reduction of FAD and FADH2. FADH2 is easily oxidized to FAD, which can easily be reduced back to FADH2.

Coenzymes are the non-protein component of enzymes and provide a functional group that is needed for the enzyme to complete its function. Many of the B vitamins function as coenzymes. Many minerals on the other hand function as cofactors that help bind substances together. Enzymes, coenzymes and cofactors all work together to increase the efficiency of a reaction.

Source: HMS 339 - Nutrition


Calculate and Explain Basal and Activity Metabolisms and Factors that Affect Each

It’s true that physical activity is a big factor in maintaining weight. However, the body uses a great deal of energy just for you to live and breathe. These are considered basal or resting functions. The basal metabolic rate or BMR is what it takes to lie at rest and breathe and accounts for about 60% to 70% of daily energy requirements. In addition to respiration, it includes blood circulation, maintaining body temperature, synthesis of new cells and tissues, secretion of hormones, and nervous system activity. BMR varies from one individual to another and the biggest factor is the amount of lean body mass one has. Lean tissue burns calories even at rest. Tall people have more surface area to lose heat from so their metabolism tends to be higher. Overweight people tend to have more muscle mass to carry their weight so they really don’t have a lower metabolism as some assume. BMR decreases with age due to the loss of muscle mass. The decline is about 3% to 5% per decade after age 30. NOTE: This is mainly due to lack of physical activity with age!

Factors Affecting Basal Metabolic Rate

About 5-10% of Calories taken in each day is used to digest and absorb food – the thermic effect of food (TEF), and the remainder is the energy cost of physical activity at 20-35%. BMR can be decreased by strict dieting. Restricting calories for the purpose of losing weight can lower BMR by 6% to 20%. This is why it becomes harder to lose weight as you go. It’s a protective mechanism built in to survive periods of starvation. It doesn’t have to remain low, however.

The thermic effect of food is the amount of energy required to digest, absorb, transport, store, and metabolize the nutrients consumed. Fat actually requires very little energy when processed compared to carbohydrate and protein. This is where the concept of choosing lower fat foods to help lose weight came from.

The energy cost of physical activity varies with the type of activity chosen. Sitting, standing, and walking requires fewer calories than higher intensity activities such as running, swimming, and many sports. Additionally, the amount of energy used depends on body size, how long the activity is performed, and the level of intensity such as 7 minute miles vs. 12 minute miles for running.

Components of energy expenditure

The components of energy expenditure include basal metabolic rate (BMR), the thermic effect of food (TEF), and the energy cost of physical activity. BMR accounts for 60% to 70% of our total energy output, whereas TEF and physical activity together account for 24% to 45%

Actual energy expenditure can be measured by direct calorimetry, a method that measures how much heat the body releases, or by indirect calorimetry, which measures the amount of oxygen used and carbon dioxide produced. These methods must be performed in a laboratory setting. A third method uses doubly labeled water. Doubly labeled water is water that is labeled with nonradioactive isotopes of hydrogen and oxygen. The water is consumed and carbon dioxide production is measured. None of these methods are practical for every day use.

Source: HMS 339 - Nutrition


Suggested Reading

Apply Knowledge of Digestion and Metabolism When Making Decisions Related to Food Intake and Physical Fitness

An overview of the metabolic pathways that result in ATP production during exercise. Carbohydrate, in the form of glucose, and proteins, in the form of amino acids, can be metabolized via aerobic pathways, whereas fatty acids are predominately metabolized via aerobic pathways. The image to the right is an overview of the metabolic pathways that result in ATP production during exercise. Carbohydrate, in the form of glucose, and proteins, in the form of amino acids, can be metabolized via aerobic pathways, whereas fatty acids are predominately metabolized via aerobic pathways.

ATP or adenosine triphosphate is the most basic form of energy used by the cells. There is only enough ATP stored in a muscle cell to keep it active for 1-3 seconds. The figure shows the various pathways that generate ATP from glucose, amino acids, and fatty acids. The adenosine triphosphate – creatine phosphate (ATP-CP) cycle and the anaerobic and aerobic breakdown of carbohydrates are the primary energy systems used to provide energy. The type and intensity of exercise determine which pathway is used.

For high-intensity activities lasting 3 – 15 seconds, creatine phosphate can be broken down in an anaerobic pathway to provide energy and regenerate ATP. This would include activities such as weight lifting or sprinting. Any activity lasting longer than 2 minutes, requires the breakdown of carbohydrates, proteins, and fats.

Glycolysis provides energy from the breakdown of glucose. The breakdown of one molecule of glucose provides two ATP molecules and 2 molecules of pyruvate. Muscle glycogen and circulating glucose are used for this process. When the availability of oxygen is limited, pyruvate is converted to lactic acid. Lactic acid was once thought to be a waste product and actually toxic to the muscles. Excess lactic acid is processed in the liver through the Cori cycle into glucose and recirculated back to the muscles. Glycolysis provides energy for activities of 30 seconds to 3 minutes in duration. Glycolysis is the fastest way to regenerate ATP for exercise, after the ATP-CP system.

The Cori cycle is the metabolic pathway by which excess lactic acid can be converted into glucose in the liver.
The Cori cycle is the metabolic pathway by which excess lactic acid can be converted into glucose in the liver.

For activities lasting longer than 3 minutes, the aerobic energy system is required. This system involves the TCA cycle and the electron transport chain and requires oxygen. This process is slower than glycolysis, but one molecule of glucose yields 36 to 38 ATP molecules verses 2 ATP molecules through the anaerobic process. A huge difference! A more important advantage of aerobic energy metabolism is that there is not a buildup of products such as lactic acid that lead to muscle fatigue, so lower-intensity activities can be performed for up to 3 hours.

We actually use some combination of carbohydrate and fat for energy most of the time. When we are at rest, we rely mainly on fat which spares carbohydrate. When we push ourselves to the max during exercise, we use mostly carbohydrate and very little fat to fuel our efforts.

The relative contribution of ATP-CP, carbohydrate, and fat to activities of various durations and intensities.

The advantages of using fat as fuel include its abundance, even in lean people, and at 9 kcal /gram it provides more than twice the amount of energy as carbohydrates. Triglycerides are made of one glycerol molecule + 3 fatty acids. Fatty acids vary in the number of carbons. The length of the fatty acid chain determines how much energy is provided. A fatty acid such as palmitic acid which contains 16 carbons yields 129 ATP molecules. The amount of glycogen in muscle varies from about 200 to 500 g and yields 800 to 2000 kcal of energy, whereas 15 lbs of body fat provides more than 50,000 kcal! Endurance exercise training enhances our ability to use both glycogen and fat for energy.

It is possible to use amino acids directly for energy, but they are mostly used to maintain blood glucose levels during exercise. Depending on the intensity and duration of the activity, amino acids can contribute about 3% to 6 % of the energy needed during exercise. Amino acids can be converted into pyruvate or acetyl coA, or directly into the TCA cycle. Amino acids are more important in the building and repair of tissues than as an energy source.

Source: HMS 339 - Nutrition


Locate Community Resources that Promote Physical Activity and Fitness


Explain the Relationship of Activity Levels and Caloric Intake to Health and Wellness, Including Weight Management

Energy BalanceEnergy balance is the state of maintaining a given weight. Fluctuations in weight occur when energy intake (eating) and energy expenditure (metabolism and physical activity) don’t match.

Energy balance describes the relationship between the food we eat and the energy we expend each day. Look at the graphic to the right: (a) Weight loss occurs when food intake is less than energy output. (b) Weight gain occurs when food intake is greater than energy output. (c) We maintain our body weight when food intake equals energy output.

Energy intake is the total energy (kilocalories) in all the foods and beverages consumed in one day (kcal/day). Remember that we get Calories (kilocalories) from carbohydrates, fats, and proteins only, and that we need vitamins and minerals to process those calories.

Multiply the number of grams of CHO, PRO, and FAT in each serving of food by the appropriate factor to determine total calories from a given food and then add it up for the total number of foods in a day. You can go to to enter the foods you eat and the program will calculate this for you. Several types of computer software are also available to use for this purpose.

One pound = 3500 kcal. If energy intake is 3500 kcal more than energy expenditure by, then one pound is gained, most likely in the form of fat.

Eat too much and exercise too little, and “calories in” will go up, and “calories out” will go down. This positive caloric balance drives weight gain. Eat less and exercise more, and this will lead to a negative caloric balance and, thus, to weight loss.


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