Nutrition

Digestive Systems

Animals use the organs of their digestive systems to extract important nutrients from food they consume, which can later be absorbed.

Learning Objectives

Summarize animal nutrition and the digestive system

Key Takeaways

Key Points

  • Animals obtain lipids, proteins, carbohydrates, essential vitamins, and minerals from the food they consume.
  • The digestive system is composed of a series of organs, each with a specific, yet related function, that work to extract nutrients from food.
  • Organs of the digestive system include the mouth, esophagus, stomach, small intestine, and the large intestine.
  • Accessory organs, such as the liver and pancreas, secrete digestive juices into the gastrointestinal tract to assist with food breakdown.

Key Terms

  • digestion: the process, in the gastrointestinal tract, by which food is converted into substances that can be utilized by the body
  • macromolecule: a very large molecule, especially used in reference to large biological polymers (e.g. nucleic acids and proteins)
  • alimentary canal: the organs of a human or an animal through which food passes; the digestive tract

All living organisms need nutrients to survive. While plants can obtain the molecules required for cellular function through the process of photosynthesis, most animals obtain their nutrients by the consumption of other organisms. At the cellular level, the biological molecules necessary for animal function are amino acids, lipid molecules, nucleotides, and simple sugars. The food consumed consists of protein, fat, and complex carbohydrates, but the requirements of each are different for each animal.

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Balanced human diet: For humans, fruits and vegetables are important in maintaining a balanced diet. Both of these are an important source of vitamins and minerals, as well as carbohydrates, which are broken down through digestion for energy.

Animals must convert these macromolecules into the simple molecules required for maintaining cellular functions, such as assembling new molecules, cells, and tissues. The conversion of the food consumed to the nutrients required is a multi-step process involving digestion and absorption. During digestion, food particles are broken down to smaller components which will later be absorbed by the body.

Digestive System

The digestive system is one of the largest organ systems in the human body. It is responsible for processing ingested food and liquids. The cells of the human body all require a wide array of chemicals to support their metabolic activities, from organic nutrients used as fuel to the water that sustains life at the cellular level. The digestive system not only effectively chemically reduces the compounds in food into their fundamental building blocks, but also acts to retain water and excrete undigested materials. The functions of the digestive system can be summarized as follows: ingestion (eat food), digestion (breakdown of food), absorption (extraction of nutrients from the food), and defecation (removal of waste products).

The digestive system consists of a group of organs that form a closed tube-like structure called the gastrointestinal tract (GI tract) or the alimentary canal. For convenience, the GI tract is divided into upper GI tract and lower GI tract. The organs that make up the GI tract include the mouth, the esophagus, the stomach, the small intestine, and the large intestine. There are also several accessory organs that secrete various enzymes into the GI tract. These include the salivary glands, the liver, and the pancreas.

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Generalized animal digestive system: This diagram shows a generalized animal digestive system, detailing the different organs and their functions.

Challenges to Human Nutrition

One of the challenges in human nutrition is maintaining a balance between food intake, storage, and energy expenditure. Imbalances can have serious health consequences. For example, eating too much food while not expending much energy leads to obesity, which in turn will increase the risk of developing illnesses such as type-2 diabetes and cardiovascular disease. The recent rise in obesity and related diseases means that understanding the role of diet and nutrition in maintaining good health is more important than ever.

Carbohydrates: Sources, Uses in the Body, and Dietary Requirements

Carbohydrates, which break down to glucose, are a major source of energy for humans, but are not an essential nutrient.

Learning Objectives

Evaluate the sources and uses of carbohydrates in the body

Key Takeaways

Key Points

  • Carbohydrates include such items as fruits, grains, beans, and potatoes, along with sugars and sugared foods.
  • While fat is a better source of energy, the brain cannot burn fat and instead requires glucose.
  • Polysaccharides (complex carbs) are difficult for humans to breakdown, but are useful as fiber to enhance the digestive process.
  • Government agencies recommend a dietary intake of 45–65% or 55–75% of carbohydrates to meet daily energy needs.
  • Of daily carbohydrate intake, only 10% should be simple carbs or sugars.

Key Terms

  • glucose: A simple monosaccharide (sugar) with a molecular formula of C6H12O6; it is a principle source of energy for cellular metabolism.
  • carbohydrate: A sugar, starch, or cellulose that is a food source of energy for an animal or plant; a saccharide.
  • saccharide: The unit structure of carbohydrates, of general formula CnH2nOn. Either the simple sugars or polymers such as starch and cellulose. The saccharides exist in either a ring or short chain conformation, and typically contain five or six carbon atoms.

Examples

Daily food intake that includes 8–10 fruit and vegetable servings (not starchy potatoes or grains, such as corn and rice) will not only provide plenty of energy but will also keep glucose levels in balance.

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Carbohydrates are a class of macromolecule: Grain products are rich sources of carbohydrates.

Foods high in carbohydrate include fruits, sweets, soft drinks, breads, pastas, beans, potatoes, bran, rice, and cereals. Carbohydrates are a common source of energy in living organisms, however, a carbohydrate is not an essential nutrient in humans.

Carbohydrates are not necessary building blocks of other molecules, and the body can obtain all its energy from protein and fats. The brain and neurons generally cannot burn fat for energy but use solely glucose or ketones. Humans can synthesize some glucose (in a set of processes known as “gluconeogenesis”) from specific amino acids or from the glycerol backbone in triglycerides and, in some cases, from fatty acids. Carbohydrate and protein contain 4 kilocalories per gram, while fats contain 9 kilocalories per gram. In the case of protein, this is somewhat misleading as only some amino acids are able to undergo conversion into useful energy forms.

Organisms typically cannot metabolize all types of carbohydrate to yield energy. Glucose is a nearly universal and accessible source of calories. Many organisms also have the ability to metabolize other monosaccharides and disaccharides, though glucose is preferred. Polysaccharides are also common sources of energy. Even though these complex carbohydrates are not very digestible, they may comprise important dietary elements for humans. Called “dietary fiber,” these carbohydrates enhance digestion, among other benefits.

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Glucose Molecule: Image of a glucose molecule containing a fixed ratio of carbon, hydrogen and oxygen.

Based on the effects on risk of heart disease and obesity, the Institute of Medicine (IOM) recommends that American and Canadian adults get between 45–65% of dietary energy from carbohydrates. The Food and Agriculture Organization (FAO) and World Health Organization (WHO) jointly recommend that national dietary guidelines set a goal of 55–75% of total energy from carbohydrates, but only 10% directly from sugars (their term for simple carbohydrates).

Lipids: Sources, Uses in the Body, and Dietary Requirements

Fats store energy, facilitate absorption of fat-soluble vitamins, aid brain growth and development, and protect against many diseases.

Learning Objectives

Evaluate the sources and uses of lipids in the body

Key Takeaways

Key Points

  • Vitamins A, D, E, and K should be taken with some dietary fat in order to facilitate their absorption and activity.
  • Humans cannot synthesize omega-6 and omega-3 fatty acids, so these fats must be obtained from the diet.
  • Omega-6 fatty acids are found in many foods, while omega-3 fatty acids are found in walnuts and are especially abundant in fatty fish.
  • Omega-3 fatty acids have many positive health benefits including reduced rates of cancer, cardiovascular disease, mental illness, and dementia.
  • Studies of dietary fat intake have found no link between percentage of calories obtained from fats and risk of cancer, heart disease, or obesity.
  • Trans fats, such as those present in partially hydrogenated vegetable oils, are a risk factor for cardiovascular disease.

Key Terms

  • phospholipids: Phospholipids are a class of lipids that are a major component of all cell membranes as they can form lipid bilayers.
  • trans fats: Trans fats are unsaturated fats generated by physical agents such as heat or pressure that can lead to a variety of health problems.
  • fatty acid: Fatty acids can be saturated or unsaturated and are usually derived from triglycerides or phospholipids.

Examples

College students require optimal brain function, which is supported by fatty fish and walnuts.

Most of the fats found in food are triglycerides, cholesterol, and phospholipids. Some dietary fat is necessary for the absorption of fat-soluble vitamins (A, D, E, and K) and carotenoids. Humans and other mammals require fatty acids such as linoleic acid (an omega-6 fatty acid) and alpha-linolenic acid (an omega-3 fatty acid), because they cannot be synthesized from simple precursors in the diet.

Both omega-6 and omega-3 are 18-carbon polyunsaturated fatty acids that differ in the number and position of their double bonds. Most vegetable oils (safflower, sunflower, and corn oils) are rich in linoleic acid. Alpha-linolenic acid is found in the green leaves of plants, selected seeds, nuts, and legumes, and particularly in flax, rapeseed, walnut, and soy. Fish oils are especially rich in the longer-chain omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Numerous studies have shown that the consumption of omega-3 fatty acids has positive benefits in terms of infant development, cancer, cardiovascular disease, and mental illnesses such as depression, attention-deficit hyperactivity disorder, and dementia. In contrast, the consumption of trans fats, such as those present in partially hydrogenated vegetable oils, are now known to be a risk factor for cardiovascular disease.

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Omega-3 Fatty Acid: Docosahexaenoic acid (DHA) is an omega-3 fatty acid. It is essential for proper functioning of the brain in both adults and infants. DHA concentrations in breast milk range from 0.07-1.0% of total fatty acids and are influenced by the amount of fatty fish in the mother’s diet. In the U.S., infant formula has been supplemented with DHA since 2001. Research suggests that DHA contributes to numerous nervous system functions such as visual acuity, neurogenesis, and synaptogenesis, and that it lowers the risk for cardiovascular disease. It is highly concentrated in the brain and eye.

Several studies have suggested that total dietary fat intake is linked to obesity and diabetes. However, influential studies like the Women’s Health Initiative Dietary Modification Trial (an eight-year study of 49,000 women), as well as the Nurses’ Health Study and the Health Professionals Follow-up Study, have revealed no such link between percentage of calories from fat and risk of cancer, heart disease, or weight gain. The Nutrition Source, a website maintained by the Department of Nutrition at the Harvard School of Public Health, summarizes the current evidence on the impact of dietary fat as follows: “Detailed research—much of it done at Harvard—shows that the total amount of fat in the diet isn’t really linked with weight or disease.”

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Salmon: Salmon is an excellent source of omega-3 fatty acids.

Proteins: Sources, Uses in the Body, and Dietary Requirements

Proteins are composed of 20 different amino acids, about half of which are essential, meaning they must be obtained from the diet.

Learning Objectives

Evaluate protein sources and uses in the body

Key Takeaways

Key Points

  • Protein -based foods (plant and animal) provide amino acids; however, the best source of essential amino acids is animal.
  • If the diet does not provide adequate protein, the body will obtain what it needs from itself, especially from its own muscles.
  • While adequate protein is required for building skeletal muscle and other tissues, there is ongoing debate regarding the use and necessity of high-protein diets in anaerobic exercise, in particular weight training and bodybuilding.
  • This use of protein as a fuel is particularly important under starvation conditions as it allows the body’s own proteins to be used to support life.

Key Terms

  • amino acid: Any of the twenty naturally occurring α-amino acids (having the amino, and carboxylic acid groups on the same carbon atom), and a variety of side chains, that combine, via peptide bonds, to form proteins.
  • denaturation: Denaturation is a process in which proteins or nucleic acids lose their tertiary and secondary structure which is present in their native state, by application of some external stress or compound such as a strong acid or base, a concentrated inorganic salt, an organic solvent (e.g., alcohol or chloroform), or heat.
  • protein: Any of numerous large, complex naturally-produced molecules composed of one or more long chains of amino acids, in which the amino acid groups are held together by peptide bonds.

Most microorganisms and plants can biosynthesize all 20 standard amino acids, while animals (including humans) must obtain some of the amino acids from the diet. The amino acids that an organism cannot synthesize on its own are referred to as essential amino acids. Key enzymes that synthesize certain amino acids are not present in animals. One example is aspartokinase, which catalyzes the first step in the synthesis of lysine, methionine, and threonine from aspartate.

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Amino Acid: Ball-and-stick model of the cystine molecule, an amino acid formed from two cysteine molecules.

In animals, amino acids are obtained through the consumption of foods containing protein. Ingested proteins are then broken down into amino acids through digestion, which typically involves denaturation of the protein through exposure to acid and hydrolysis by enzymes called proteases. Some ingested amino acids are used for protein biosynthesis, while others are converted to glucose through gluconeogenesis, or fed into the citric acid cycle. This use of protein as a fuel is particularly important under starvation conditions as it allows the body’s own proteins to be used to support life, particularly those found in muscle. Amino acids are also an important dietary source of nitrogen.

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Protein: Muscle meat, such as steak, is an excellent source of the essential amino acids the body needs to create all necessary proteins.

A high-protein diet is often recommended by bodybuilders and nutritionists to help efforts to build muscle and lose fat. It should not be confused with low-carb diets, such as the Atkins Diet, which are not calorie-controlled and which often contain large amounts of fat. While adequate protein is required for building skeletal muscle and other tissues, there is ongoing debate regarding the use and necessity of high-protein diets in anaerobic exercise in particular weight training and bodybuilding. Extreme protein intake (in excess of 200g per day), coupled with inadequate intake of other calorie sources (fat or carbohydrates), can cause a form of metabolic disturbance and death commonly known as rabbit starvation.

Relatively little evidence has been gathered regarding the effect of long-term high intake of protein on the development of chronic diseases. Increased load on the kidney is a result of an increase in reabsorption of NaCl. This causes a decrease in the sensitivity of tubuloglomerular feedback, which, in turn, results in an increased glomerular filtration rate. This increases pressure in glomerular capillaries. When added to any additional renal disease, this may cause permanent glomerular damage.

Food Requirements and Essential Nutrients

Essential nutrients are those that cannot be created by an animal’s metabolism and need to be obtained from the diet.

Learning Objectives

Describe the essential nutrients required for cellular function that cannot be synthesized by the animal body

Key Takeaways

Key Points

  • The animal diet needs to be well-balanced in order to ensure that all necessary vitamins and minerals are being obtained.
  • Vitamins are important for maintaining bodily health, making bones strong, and seeing in the dark.
  • Water-soluble vitamins are not stored by the body and need to be consumed more regularly than fat-soluble vitamins, which build up within body tissues.
  • Essential fatty acids need to be consumed through the diet and are important building blocks of cell membranes.
  • Nine of the 20 amino acids cannot be synthesized by the body and need to be obtained from the diet.

Key Terms

  • nutrient: a source of nourishment, such as food, that can be metabolized by an organism to give energy and build tissue
  • catabolism: destructive metabolism, usually including the release of energy and breakdown of materials
  • vitamin: any of a specific group of organic compounds essential in small quantities for healthy human growth, metabolism, development, and body function

Food Requirements

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A balanced diet: For humans, a balanced diet includes fruits, vegetables, grains, and protein. Each of these food sources provides different nutrients the body cannot make for itself. These include vitamins, omega 3 fatty acids, and some amino acids.

What are the fundamental requirements of the animal diet? The animal diet should be well balanced and provide nutrients required for bodily function along with the minerals and vitamins required for maintaining structure and regulation necessary for good health and reproductive capability.

Organic Precursors

The organic molecules required for building cellular material and tissues must come from food. Carbohydrates or sugars are the primary source of organic carbons in the animal body. During digestion, digestible carbohydrates are ultimately broken down into glucose and used to provide energy through metabolic pathways. The excess sugars in the body are converted into glycogen and stored in the liver and muscles for later use. Glycogen stores are used to fuel prolonged exertions, such as long-distance running, and to provide energy during food shortage. Excess digestible carbohydrates are stored by mammals in order to survive famine and aid in mobility.

Another important requirement is that of nitrogen. Protein catabolism provides a source of organic nitrogen. Amino acids are the building blocks of proteins and protein breakdown provides amino acids that are used for cellular function. The carbon and nitrogen derived from these become the building block for nucleotides, nucleic acids, proteins, cells, and tissues. Excess nitrogen must be excreted, as it is toxic. Fats add flavor to food and promote a sense of satiety or fullness. Fatty foods are also significant sources of energy because one gram of fat contains nine calories. Fats are required in the diet to aid the absorption of fat-soluble vitamins and the production of fat-soluble hormones.

Essential Nutrients

While the animal body can synthesize many of the molecules required for function from the organic precursors, there are some nutrients that need to be consumed from food. These nutrients are termed essential nutrients: they must be eaten as the body cannot produce them.

Vitamins and minerals are substances found in the food we eat. Your body needs them to be able to work properly and for growth and development. Each vitamin has its own special role to play. For example, vitamin D (added to whole milk or naturally-occurring in sardines), helps make bones strong, while vitamin A (found in carrots) helps with night vision. Vitamins fall into two categories: fat soluble and water soluble. The fat-soluble vitamins dissolve in fat and can be stored in your body, whereas the water-soluble vitamins need to dissolve in water before your body can absorb them; therefore, the body cannot store them.

Fat-soluble vitamins are found primarily in foods that contain fat and oil, such as animal fats, vegetable oils, dairy foods, liver, and fatty fish. Your body needs these vitamins every day to enable it to work properly. However, you do not need to eat foods containing these every day. If your body does not need these vitamins immediately, they will be stored in the liver and fat tissues for future use. This means that stores can build up; if you have more than you need, fat soluble vitamins can become harmful. Some fat-soluble vitamins include vitamin A, vitamin K, vitamin D, and vitamin E. Unlike the other fat-soluble vitamins, vitamin D is difficult to obtain in adequate quantities in a normal diet; therefore, supplementation may be necessary.

Water-soluble vitamins are not stored in the body; therefore, you need to have them more frequently. If you have more then you need, the body rids itself of the extra vitamins during urination. Because the body does not store these vitamins, they are generally not harmful. Water-soluble vitamins are found in foods that include fruits, vegetables, and grains. Unlike fat-soluble vitamins, they can be destroyed by heat. This means that sometimes these vitamins can often be lost during cooking. This is why it is better to steam or grill these foods rather then boil them. Some water-soluble vitamins include vitamin B6, vitamin B12, vitamin C, biotin, folic acid, niacin, and riboflavin.

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Sea buckthorn seed oil: Sea buckthorn seed oil contains many vital nutrients.

The omega-3 alpha-linolenic acid and the omega-6 linoleic acid are essential fatty acids needed to synthesize some membrane phospholipids. Many people take supplements to ensure they are obtaining all the essential fatty acids they need. Sea buckthorn contains many of these fatty acids and is also high in vitamins. Sea buckthorn can be used to treat acne and promote weight loss and wound healing.

Minerals are inorganic essential nutrients that must also be obtained from food. Among their many functions, minerals help in cell structure and regulation; they are also considered co-factors. In addition to vitamins and minerals, certain amino acids must also be procured from food and cannot be synthesized by the body. These amino acids are the “essential” amino acids. The human body can synthesize only 11 of the 20 required amino acids. The rest must be obtained from food.

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Amino Acids: There are 20 known amino acids. Animals can make only 11, so the others must be obtained through the diet. Meats are the best source of amino acids, although some amino acids can also be obtained from vegetables and grains.