21.1 Nutritional Concepts

Metabolic Requirements

The process by which substances are chemically broken down to meet the body’s nutrient and energy needs is metabolism. The energy needs of the body require the consumption and metabolism of carbohydrates, proteins, and fats in significant quantities. These three energy-rich nutrients provide a consistent amount of energy per gram of nutrient. The calorie is a unit of measure for energy. Both carbohydrates and proteins provide four calories of energy per gram of nutrient, while fats provide nine calories per gram. The rate at which calories burn is called the basal metabolic rate (BMR). A high metabolic rate burns calories faster than a slow metabolic rate. Many factors influence metabolic rates, including age, sex, activity level, genetics, and overall fitness. Underlying medical conditions also significantly affect both metabolic rates and needs. Nutritional wellness occurs when cells burn calories efficiently and excess energy is stored effectively, thus achieving metabolic homeostasis.

The goal of metabolic homeostasis is to balance caloric intake with caloric need. Metabolic imbalance negatively impacts overall wellness and can occur when either too many or too few calories are consumed to meet the body’s immediate needs. When calories consumed exceed caloric need, excess energy is stored as fat, which can lead to weight gain and the development of obesity, metabolic syndrome, and other chronic medical conditions. Caloric deficits can be just as detrimental. When caloric need outweighs caloric intake, a caloric deficit occurs, and the body turns to energy stores to meet energy needs. Carbohydrates, stored as glycogen in the liver, are metabolized first, followed by energy stored in fat cells. If a caloric deficit is prolonged and starvation ensues, the body will metabolize proteins, which can lead to life-threatening medical conditions and death. Metabolic homeostasis is essential to healthy living.

Body Weight Standards

Across a person’s life span, body weight is greatly affected by caloric imbalances. Overconsumption and underconsumption of calories can result in body weight alterations that negatively impact overall health. Thus, it is important for individuals to achieve and maintain an ideal body weight. The measure of ideal body weight (IBW) correlates with positive health benefits. An individual’s IBW is influenced by individual characteristics such as height, weight, body frame, age, and sex. General guidelines have been developed using these characteristics to identify IBW standards. IBW can be a helpful tool when analyzing nutritional health based on generalized standards.

The measure of body mass index (BMI) is a commonly used tool that analyzes height and weight to provide a measurement relative to generalized standards. The Centers for Disease Control and Prevention (CDC) classifies BMI into the following categories: underweight, healthy weight, overweight, and obese (Table 21.1). A calculated BMI is based on an individual’s height, weight, age, and sex and can be a helpful tool in determining an individual’s relative fitness and overall wellness. Since body fat distribution can naturally differ by ethnicity, and other factors (such as exercise) can impact BMI, it should not be used as the only determination of nutritional and related health. There are many BMI calculators available on the web.

Waist circumference is another commonly used measure. Waist circumference is a relative indicator of excess fat storage deposited in the midsection; it is often used as a relative measure to gauge overall caloric balance. Growth charts are used to analyze body height and weight standards across the continuum of growth and development in children through adulthood. The CDC and the World Health Organization (WHO) have developed growth charts for use in clinical settings to track physical growth trends.

Growth charts for infants include measurements of length, weight, and head circumference; charts for children and adolescents include measures of weight and height. Growth charts trend measurements according to percentiles, which compare a measure to an average. Thus, for a child in the fiftieth percentile for weight, 50 percent of children will be above and 50 percent of children will be below that child’s measure. Growth and BMI charts are useful tools for studying trends involving growth, development, and general nutrition.

Caloric Requirements

Energy is measured in calories. The energy-rich nutrients (carbohydrates, fats, and proteins) provide a substantial number of calories per unit of weight: Carbohydrates provide 4 cal/g, proteins provide 4 cal/g, and fats provide 9 cal/g. The USDA provides guidance on daily dietary recommendations including energy requirements. According to the USDA, human energy requirements range between 1,000 and 3,200 calories per day depending on the individual. Individual estimates of energy needs vary based on a variety of characteristics including age, sex, height, weight, health status, and level of physical activity.

Carbohydrates

A macronutrient used to fuel the immediate energy needs of the body is a carbohydrate. Carbohydrates are classified as simple (monosaccharides and disaccharides) or complex (oligosaccharides and polysaccharides). Simple carbohydrates are also known as sugars. Glucose, a monosaccharide, is the body’s primary energy source; it is used to meet immediate energy needs. When glucose is transported into a cell, it undergoes cellular metabolism, which releases ATP and energy. Larger, complex carbohydrates are broken down by enzymes into monosaccharides to be metabolized for energy. Every gram of carbohydrate provides 4 calories of energy. Carbohydrates should make up 45 to 65 percent of a healthy diet.

Simple and complex carbohydrates differ in significant ways, though all are made up of carbon, hydrogen, and oxygen. Sugars typically taste sweet and are found in foods like honey and fruits. Complex carbohydrates are longer sugar chains that are digested in the intestines and broken down into monosaccharides that can then be used by the cells. They release energy more slowly than simple sugars. Starches, fiber, and glycogen are complex carbohydrates. Starches are polysaccharides that are digested by enzymes in the intestinal tract. Rice and potatoes are examples of starches. Dietary fiber is broken down in the large intestine by bacteria rather than metabolized by enzymes. Soluble fiber is found in fruits, vegetables, whole grains, and legumes; it helps control blood sugar by reducing absorption of postprandial blood sugar in the gut. Insoluble fiber, such as cellulose, is a bulking agent for stool.

Proteins

A macronutrient made up of amino acids is a protein; they are much larger molecules than carbohydrates. There are twenty different amino acids used by the human body. Nine of these amino acids must be consumed within the diet. These are referred to as essential amino acids. The other amino acids can be synthesized within the body. All proteins are made of strings of amino acids called polypeptides. During digestion, proteins are broken down into smaller units that can then be used to make new proteins. Proteins are essential to human life. They are the main building blocks for muscle, skin, and connective tissue and are involved in the growth, maintenance, and repair of body tissues. Enzymes, hormones, and hemoglobin are proteins. While proteins are not a primary source of energy, they can be broken down for glucose metabolism if cells are starved for energy. Proteins yield 4 calories of energy for every gram of nutrient.

Dietary proteins should make up 10 to 15 percent of an individual’s daily caloric intake. There are situations where protein needs are higher, such as in growing children or when a person is pregnant, lactating, or experiencing a critical illness. However, excessive dietary protein can be harmful to individuals with underlying kidney impairment. Thus, it is important to maintain a protein intake that is healthy and also balanced for individual nutritional needs. Meat, poultry, eggs, and seafood are all good sources of animal protein. Unfortunately, some of these protein sources are associated with high levels of saturated fat, which can negatively impact health and should be limited in a healthy diet. Protein from plants, such as soy products, nuts, seeds, and legumes, can supply adequate amounts of protein as well, though these sources are not metabolized as efficiently as animal protein. The USDA recommends a balanced intake of animal and plant protein. Table 21.2 compares the protein content of animal- and plant-based foods, information that is useful for developing a balanced diet while reducing saturated fat content.

Fats

A large, complex molecule made up of fatty acids and glycerol is a fat. Fats, also known as lipids, are a highly efficient source of stored energy, releasing energy when needed at a very slow, measured rate. Each gram of fat releases 9 calories of energy.

Cholesterol and triglycerides are important lipids within the body. They are associated with essential functions including brain function, hormone regulation, and energy production. Adipose tissue, commonly found in subcutaneous fat, is composed of fat cells that help regulate energy supply within the body. Energy, in the form of excess blood sugar, is converted to lipids and stored as adipose tissue for future use. When glucose stores are depleted, the body turns to these fat stores for energy by breaking down the triglycerides in adipose tissue into simple sugars, which then release energy for immediate use within the body. Cholesterol is a key component of cell membranes; it is transported, along with triglycerides, throughout the body by protein-based compounds called lipoproteins. Lipoproteins, which are classified according to the ratio of lipid to protein, bind to cholesterol and triglycerides for transport through the body. Low-density lipoproteins (LDLs) are essential for healthy cellular function but in excess are associated with increased cardiovascular risk. LDL is often referred to as “bad cholesterol” and can be found in high-fat diets that contain fried foods, red meat, high-fat dairy products, and processed baked goods. High-density lipoproteins are considered “good cholesterol” and are beneficial to cell function. Examples of good cholesterol include nuts, seeds, fish, and legumes.

Lipids are either saturated or unsaturated; unsaturated fats may be monounsaturated or polyunsaturated. Saturated fats are associated with increased cardiovascular risk, leading to atherosclerosis, coronary artery disease, and stroke; they should be limited in a healthy diet. Animal products contain saturated fat, while fat derived from plants typically contains unsaturated fat. Trans fats are human-made fats derived from partially hydrogenated oil. Eliminating trans fats from the diet is recommended because these fats are associated with development of atherosclerosis and adversely affect cholesterol levels. The USDA recommends limiting the consumption of dietary fat to 28 percent or less of daily caloric intake. Saturated fat should be limited to 8 percent or less.

Vitamins

A vitamin is an essential micronutrient, vital to biochemical regulation within the human body. Vitamins are exogenous (not made by the body) and must be ingested as part of a healthy diet. They are classified by solubility. Vitamins are either absorbed and transported in water (water soluble), or they are absorbed and transported in fat (fat soluble). The body’s ability to absorb both fat- and water-soluble vitamins can be affected by various medical conditions. Many drugs can interfere with absorption and metabolism of vitamins. Vitamins can be toxic if taken in excess, but vitamin deficiencies can also have detrimental effects. Therefore, scientists have determined RDA for many vitamins to help reduce associated risks.

Vitamin C and the complex of B vitamins are examples of water-soluble vitamins: that is, they dissolve in water. Water-soluble vitamins are eliminated in the urine fairly quickly, so toxicities are rare. Deficiencies are often due to lack of access to quality food. Vitamin C is found in citrus fruits, vegetables, and potatoes. It helps regulate collagen, hormone, and amino acid production and assists with iron absorption and wound healing. The complex of B vitamins helps regulate a variety of physiological and neurological processes. Common B vitamins include B1 (thiamine), B2 (riboflavin), B3 (niacin), and B12 (cyanocobalamin). Foods rich in B-complex vitamins include meat, fish, eggs, and legumes.

Fat-soluble vitamins dissolve in lipids. Common fat-soluble vitamins include vitamin A, vitamin D, and vitamin K. Vitamin A is essential to maintain vision and healthy skin and is found in fish liver oils, egg yolks, and vegetables. Vitamin D helps build and maintain healthy bones and is not found naturally in food. Instead, vitamin D is absorbed through direct sunlight, though many fortified foods now contain vitamin D. Vitamin K promotes blood clotting and reduces bleeding risk and is found in dark green, leafy vegetables. Vitamin E is a fat-soluble vitamin with antioxidant properties, protecting cells against the effects of free radicals. Foods rich in vitamin E include vegetable oils, peanuts, and leafy greens. Many fat-soluble vitamins are stored in the liver and can have toxic effects on the body if taken in excess.

Minerals

Another essential component of physiological processing is the mineral. Some minerals are macronutrients, and some are micronutrients. Electrolytes are macronutrient minerals that are essential to cardiac, musculoskeletal, and neurological functions. They help maintain water balance and acid-base balance. Important electrolytes include sodium, potassium, calcium, phosphorous, and magnesium. Each of these electrolytes must be balanced within a tight therapeutic range to maintain homeostasis and avoid harmful effects.

Some minerals are micronutrients and are found in minute quantities within the body. These minerals, also known as trace minerals, have RDA to avoid toxicities. Important trace minerals include iodine, iron, and zinc. Iodine is essential to thyroid hormone production and is mainly found in sea salt. Iron is an important component of blood, specifically hemoglobin. It is found in animal products. Zinc is essential to immune and metabolic processes and can be found in many meat products.

Water

Water is an essential macronutrient that must be maintained in homeostatic balance to avoid the detrimental effects of both over- and underhydration. Two-thirds of the volume of total body water (TBW) are found inside body cells (intracellular). The other third is extracellular and found in the vascular system and interstitial fluids (Figure 21.2a). TBW is roughly 50 to 60 percent of overall body weight, but the amount is age dependent (Figure 21.2b). Water is ingested and absorbed in the digestive tract and regulated by various mechanisms: the thirst mechanism, kidney function, pituitary function, and osmosis. The body loses about a liter of fluid a day through urine, sweat, and so on. Thus, the average recommended daily intake of water is 2.5 L to maintain adequate kidney function and vital capacity.

Figure 21.2 (a) Total body water is a critical component of a person’s health. Many structures of the human body are mostly water. (b) As people age, the percentage of their bodies that consists of water decreases. (credit a: modification of work from Anatomy and Physiology. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license; credit b: data source: Lu, H., Ayers, E., Patel, P., & Mattoo, T. K. (2023). Body water percentage from childhood to old age. Kidney Research and Clinical Practice, 42(3), 340–348. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Malnutrition

An imbalance between the nutrients consumed and nutrients needed leads to malnutrition. These nutritional imbalances can result in both undernutrition and overnutrition and can involve both macronutrients and micronutrients. The RDA are a helpful tool to identify individuals at risk (see Table 21.3 and Table 21.4). However, malnutrition is multifactorial, and careful clinical follow-up is necessary to minimize long-term negative impacts of poor nutrition.

Hydration

As shown in Figure 21.3, the human body maintains water homeostasis through a balance of ingestion and elimination. Most water is ingested by drinking fluids, such as plain water, juices, and caffeinated beverages. Ingested water also comes from food, especially fruits and vegetables. Water that is visible and measurable as it is lost is called sensible fluid loss. Urination is an example of sensible fluid loss. Insensible fluid loss cannot be measured. An example of insensible loss is water vapor lost through breathing. Adequate hydration will balance the water that enters and exits the human body. An imbalance in hydration can lead to either a fluid volume deficit or fluid volume overload. Both of these conditions present problems, but for very different reasons.

Figure 21.3 The human body maintains water homeostasis by ingesting and producing water and eliminating it through a number of processes. Water loss can be increased in dry climates or when a patient has diarrhea. (attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)