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Fundamentals of Human Nutrition
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Food Choice

Food choice is a complex product of multiple influences including culture, family and the availability of specific foods. Food choices clearly affect the nutritional value of the diet. While foods are primarily purchased by consumers based on taste, cost and convenience, other factors such as nutritional quality, sensory appearance, and individual routines, habits and associations affect food choices as well. In addition, food, agriculture and trade policies, and food production technology, all have tremendous influence on what foods are available for purchase and selection.

Macronutrients and Micronutrients

Macronutrients and micronutrients make up the nutritional landscape. Micronutrients (including vitamins, minerals, and phytochemicals) derive their name from the fact that they are needed in relatively small amounts in comparison to the macronutrients - carbohydrates, fats and oils, proteins and amino acids, and water. There are also non-nutrient components of the diet such as soluble and insoluble fibers, which play an important role in human nutrition.

The human body stores macronutrients in a very different proportion from those found in the human diet. While human diets may be 30% fat, 50% carbohydrate, and 20% protein on a daily basis, the body stores 160,000 Calories as fat (about 13.5 kg in a typical 70 kg man), and 54,000 Calories (about 13.5 kg) as protein. Surprisingly, only 1200 Calories of carbohydrate are stored as 300 grams of glycogen in the liver and muscle. These storage fractions are correlated with the portability of the energy stored, with fat being most portable and carbohydrate, requiring water of hydration, being the least portable.

Chemical elements essential to life are classified as major, macrominerals, and microminerals. The macrominerals include calcium, chlorine, magnesium, phosphorus, sodium and sulfur. The microminerals include arsenic, boron, chromium, cobalt, copper, fluorine, iodine, iron, manganese, molybdenum, nickel, selenium, silicon, tin, vanadium and zinc. These minerals are derived from the diet or dietary supplements. As the diet becomes refined and processed there is the risk of depletion of some of these elements. In more biodiverse diets, the risk of dietary deficiency is reduced.

A mythical 70 kg man contains 45 liters (or kg) of water, and it is the most prevalent of the macronutrients in the body. This 45 liters is divided among intracellular (30 liters), interstitial (12 liters) and blood plasma (3 liters). A moderately active adult turns over about 2500 ml of water daily, with losses occurring primarily through the urine (50% of daily fluid losses), but also via the skin (25%), lungs (19%) and feces (6%). Body water is derived mainly from beverages (60%), food (30%), and products of metabolism (10%). Excretion is in urine (60%), skin (28%), sweat (8%), and feces (4%).

The metabolism of carbohydrates, fats, and proteins, is interconnected. This interrelationship provides essential flexibility to humans when faced with dietary restriction or starvation. In fact, the body can interconvert to a greater or lesser degree the macronutrients protein, fat and carbohydrate during starvation or overfeeding. Dietary carbohydrates are broken down in the body to glucose, to be used for liver and muscle glycogen stores, and some is taken into the brain and other cells to be broken down to provide energy. Glucose can also converted to body fat and stored when excess calories are consumed, and, if nitrogen is available, can yield non-essential amino acids. The digestion of fat yields fatty acids and glycerol; some are reassembled as triglycerides which are stored in adipose tissue; others are broken down to provide energy. Dietary protein is broken down into amino acids, which are used to manufacture body proteins, but most amino acids can also be converted to glucose when energy is needed, and if taken to excess will be stored as body fat. When there is a surplus of amino acids, or if there is inadequate carbohydrate or fat to meet energy needs, amino acids can also provide energy. Of the energy-containing nutrients, fat provides the most energy by weight.

Protein

Proteins are composed of amino acids. These are classified as essential or non-essential. Essential amino acids must be obtained from the diet. Non-essential amino acids can be synthesized in the body from essential amino acids. The quality of dietary proteins is based on their content of essential and non-essential amino acids. The proteins with highest utilization efficiency are said to have the highest biological value.

Essential Amino Acids

Non-essential Amino Acids

Histidine

Alanine

Isoleucine

Arginine

Leucine

Asparagine

Methionine

Aspartic Acid

Lysine

Cysteine

Phenylalanine

Glutamic Acid

Threonine

Glutamine

Tryptophan

Glycine

Valine

Proline

 

Serine

 

Tyrosine


The protein-digestibility-corrected amino acid score (PDCAAS) method of evaluating the quality of a protein compares the amino acid contents of a protein with human amino acid requirements and corrects for digestibility. Amino acid scoring is a method of evaluating protein quality by comparing a test proteinís amino acid patterns with that of a reference protein. Once the amino acid score is derived, it is compared against the amino acid requirements of preschool-aged children. The rationale behind using the requirements of this age group is that if a protein will effectively support a young childís growth and development, it will meet or exceed the requirements of older children and adults.

Protein Digestibility Corrected Amino Acid Score Values

Casein (milk protein)

1.00

Egg White

1.00

Soybean isolate

0.99

Beef

0.92

Pea flour

0.69

Kidney beans

0.68

Garbanzo beans

0.66

Pinto beans

0.63

Rolled oats

0.57

Lentils

0.52

Peanut meal

0.52

Whole wheat

0.40

 

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