Chemistry Of Nutrition

Nutrition is the process of providing the body with the raw materials it needs to function and grow. There are three major macronutrients (carbohydrates, proteins, and fats) that function as the major building blocks and fuel for physiological processes. Additionally, a balanced diet must contain sufficient micronutrients like vitamins and minerals.

Humans receive their micronutrients and macronutrients from plant and animal sources. The whole structures (leaves of plants, muscles of animals, etc.) are broken down into their chemical components through digestion, then those individual molecules are reassembled to make new cells or are broken down further to provide energy for cellular function.

Improper nutrition (whether it is overnutrition or undernutrition) is associated with chronic diseases and birth defects.

Spina bifida, is a birth defect where the backbone does not close around the spinal cord properly, which can lead to paralysis.^[1] Mothers who do not get enough folic acid (a B vitamin) are far more likely to have an affected infant.

Carbohydrates are neutral compounds made of carbon, hydrogen, and oxygen. Their generic formula is $\ce{C_{n}(H_2O)_{n}}$. Carbohydrates form chains of repeating units, or polymers, and can be classified based on the number of units they contain.

Monosaccharides are the base units of carbohydrates. Glucose $\ce{C_{6}H_{12}O_{6}}$ is the most important monosaccharide in human nutrition, because it is the body's primary source of fuel and the only way the brain can obtain energy. If glucose is not obtained from the diet in sufficient quantities, the body must produce it through gluconeogenesis in order to maintain brain function. Fructose and galactose are other examples of nutritionally important monosaccharides. The ribose and deoxyribose found in DNA are also examples of monosaccharides.

Common monosaccharides^[2]

Disaccharides: two monosaccharides linked together form a disaccharide. Lactose (galactose + glucose) and sucrose (fructose + glucose) are examples.

Oligosaccharides (3-9 sugar molecules): maltodextrin, for example, is a glucose oligosaccharide produced by corn.

Polysaccharides (many sugar molecules): Both starch and cellulose are long chains of glucose molecules $\ce{(C_{6}H_{10}O_{5})_{n}}$. In starch, every glucose molecule is oriented the same way in space (this is called alpha linkage). Cellulose, on the other hand, is a repeating pattern where every other glucose molecule is rotated by 180 degrees (beta linkage). This may seem like a minor difference, but the result is that starch is digestible to humans and a useful source of energy, while cellulose is indigestible for humans, and is an example of dietary fiber. Human bodies have the necessary enzymes to break down starch into glucose, but not cellulose. Some other creatures do, like termites, but cellulose is generally stronger than starch, for instance, it's not dissolvable in water. Starch, a 1,4 alpha-linked glucose polymer. Public Domain Image.

A small change in the structure of the glucose chains--beta linkages--completely changes their function.^[3]

Functions of carbohydrates:

1. Energy homeostasis: sugar (in the form of glucose) is the main energy source for the body and the only energy source the brain can utilize. Problems with carbohydrate homeostasis can result in diabetes mellitus or hypoglycemia.

2. Nucleotide formation: DNA and RNA are made of up nucleic acids, which consist of a sugar (deoxyribose or ribose) bound to phosphate and a nitrogenous base.

3. Glycoprotein formation: Glycoproteins result from a carbohydrate attaching to a amino-acid or polypeptide chain. Mucin (a component of mucus), hemoglobin A1c, antibodies in the immune system, and clotting factors are examples.

Proteins are made of amino acids and account for about 15% of body mass (the second largest component after water).

The generic structure of an amino acid^[4]

The 21 amino acids can combine to make a multitude of different proteins^[5]

About 50% of the protein mass in a human body falls into one of the following categories:

1. Collagen, which is found in skin.

2. Myosin and actin, which are found in muscles.

3. Hemoglobin, which is a component of the blood.

Amino acids are utilized extensively during DNA translation to make new proteins for use throughout the body. Some individual amino acids can be synthesized by the body, as long as the overall quantity of protein is sufficient. Essential amino acids cannot be synthesized endogenously (within the body) and must be obtained directly from the diet. These are also referred to as indispensable amino acids. There are eight:

1. Isoleucine

2. Leucine

3. Phenylalanine

4. Valine

5. Threonine

6. Methionine

7. Tryptophan

8. Lysine

Proteins are classified based on how well they supply essential amino acids. Incomplete proteins are missing one or more of the essential amino acids, while complete proteins contain them all. Generally, animal proteins are the best source of indispensable amino acids, followed by legumes, cereals, and tubers, respectively.

Lipids (fats) have the highest energy content per gram of the macronutrients. They are an excellent way to store energy long term. Lipids are water-insoluble (they do not dissolve in water and, in this case, float in water), so they are transported through the bloodstream by lipoproteins.

Generic triglyceride structure^[6] Triglycerides are an ester found in natural fats and oils. Lipids can be unsaturated, meaning they contain one or more double bonds (olive oil is an example), or saturated (like butter). Naturally occurring unsaturated fats are cis isomers. Trans fats are a manufactured unsaturated fat that behaves like a saturated fat. The inclusion of trans fats in processed foods has been controversial, and their use has been banned in many countries after data showed that eating trans fats increased the risk of coronary heart disease, even compared to saturated fats.

line structure of linoleic acid. Note that the unsaturated bonds are all in the cis conformation. Essential fatty acids cannot be synthesized by the body. Humans must obtain omega-6 (linoleic acid, found in vegetable oils) and omega-3 (alpha-linolenic acid, found in foods like salmon and flax seeds) from their diets. Most other lipids can be manufactured by the liver in a process called lipogenesis.

Public Domain Image Phospholipids are a major structural component of cell membranes and help determine how flexible the membrane is. Saturated fatty acids pack more densely, making the overall structure of the membrane more rigid, while the kinks in unsaturated fatty acids take up extra space, making the membrane more fluid.

Sterol, the backbone of cholesterol, is the starting material for many hormones synthesized by the body, including aldosterone, cortisol, and the sex hormones estrogen, progesterone, and testosterone. Sterol Estrogen Testosterone

$\text{Energy in} = \text{Energy out} + \text{Energy stored}$

The $\text{Energy in}$ comes from the macronutrients in the diet. The $\text{Energy out}$ goes to keeping the organism alive (basic metabolic function and physical activity). What's left, $\text{Energy stored}$, becomes fat.

Body mass homeostasis is maintained when $\text{Energy in} = \text{Energy out}$

The $\text{Energy in}$ comes from macronutrients. Every gram of carbohydrates provides 4 kcal (17 kJ) of energy per gram. Protein also provides 4 kcal per gram. Fat provides 9 kcal (17 kJ) per gram, roughly twice the energy provided by a similar mass of protein or carbohydrate.

A balanced diet should include marconutrients in an energy ratio (note: NOT a mass ratio). The standard energy ratio is as follows:

55% carbohydrates : 15% protein : 30% fat

$\text{Energy out}$ consists mainly of the following three categories:

1. Resting metabolic rate (RMR). RMR is the baseline energy the body needs to maintain itself when it's not doing anything. A person in a coma, for example, would still have energy needs equal to their RMR to regulate functions like their internal temperature and heart beat. The RMR for an individual is fairly constant (though it does decrease with age), but the RMR between individuals can vary greatly with age, gender, and mass.

2. Thermic effects of feeding. Roughly 10% of the energy a person consumes is used to digest food. Digestion requires the gut to secrete enzymes, recruit ion channels, and maintain smooth muscle contractions.

3. Physical activity. Physical activity can be measured in a unit called metabolic equivalents (MET), which is a multiple of RMR. For example, performing household chores may burn 2 MET, while sprinting in a footrace may require 16 MET.

Vitamins and minerals are also necessary for biochemical reactions. They are organic molecules that cannot be synthesized by the human body in sufficient quantities to meet its basic metabolic needs. They are either water soluble (vitamins B and C) or fat soluble (vitamins A, D, E, and K).

Minerals of Dietary Importance:

1. Calcium regulates cellular processes.

2. Potassium, sodium, and chloride regulate membrane potentials in nerve and muscle cells.

3. Magnesium acts as a cofactor in hundreds of reactions, also involved in metabolism, protein synthesis, and nucleic acid synthesis.

4. Copper is involved in oxidation reactions.

5. Zinc regulates gene expression.

For most of human history, nutritionally important foods like sugars, salts, and fats have been scarce at least part of the year. As a result, humans developed a taste for foods high in sugar and fat, the ability to store excess calories, and the ability to eat large quantities in a single sitting. (It's pretty nonsensical to fell a woolly mammoth with spears, take three bites of it, and then be full.)

Over the past two centuries, innovations like nitrogen fertilizers and refrigeration have dramatically increased the food supply, transforming the landscape into a land of constant plenty and saving large numbers of people from starvation every year. However, evolution is a relatively slow process, and the human body is still programmed the same way it was when food was scarce. Ancestrally adaptive behaviors sometimes backfire in the 21st century, leading to high rates of obesity, diabetes, hypertension and other chronic diseases. Obesity rates rise as processed foods become more commonplace^[7]

Undernutrition is a major concern for many communities around the world.

Some undernourished individuals suffer from an absolute deficiency of macronutrients, like marasmus, a condition where lack of protein leads to low energy, emaciation, and muscle wasting. Marasmus is caused by inadequate intake, absorption, or utilization of protein. Lack of food is a common cause, but anorexia nervosa, major depression, and malabsorption syndromes can also lead to marasmus.

People suffering from kwashiorkor also have a protein deficiency, even though they have a relatively high energy diet. This condition is common when a diet is based on a low quality diet that lacks some of the essential amino acids, and is referred to as a relative deficiency. People suffering from kwashiorkor are emaciated, but also have a bloated abdomen due to edema.

A child with kwashiorkor. Photo taken during a conflict in Nigeria in the 1960's.^[8]

1. Morse, N. (1907). Postoperative treatment; an epitome of the general management of postoperative care and treatment of surgical cases as practised by prominent American and European surgeons (pp. 354). Philadelphia, PA: Blakiston's Sons and Co..
2. CFCF, . Five Important Monosaccharides. Retrieved from https://commons.wikimedia.org/wiki/File:217_Five_Important_Monosaccharides-01.jpg
3. Laghi, I. Cellulose Strand. Retrieved from https://commons.wikimedia.org/wiki/File:Cellulose_strand.svg
4. Johndoct, . Amino acid structure. Retrieved from https://commons.wikimedia.org/wiki/File:Amino-acid-structure.jpg
5. Cojocari, D. Amino acids. Retrieved from https://commons.wikimedia.org/wiki/File:Amino_acids.png
6. Branton.j263, . triglyceride23. Retrieved from https://commons.wikimedia.org/wiki/File:Triglyceride23.jpg
7. Robin, C. Mr America. Retrieved from https://www.flickr.com/photos/robadob/88894048
8. Centers for Disease Control and Prevention, . Public Health Image Library. Retrieved from http://phil.cdc.gov/phil/details.asp?pid=6901