Introduction to Food and Nutrition Science — Types and Functions of Nutrients

What Is Food and Nutrition Science?

Definition of food and nutrition science:

• The study of a food’s constituents, their role in the body, and their effect on maintaining health and preventing disease

Subfields of food and nutrition science:

• Basic nutrition: the chemical structure and function of nutrients
• Clinical nutrition: nutritional management for disease prevention and treatment
• Food science: a food’s composition, processing, and quality
• Community nutrition: assessing and improving population-level nutritional status

Definition of a nutrient:

• A substance in food that’s essential for maintaining bodily function

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The Six Essential Nutrient Groups

The six essential nutrients:

Energy-yielding nutrients:

1. Carbohydrates: 4 kcal/g
2. Protein: 4 kcal/g
3. Fat (lipids): 9 kcal/g

Regulatory nutrients (no energy):

4. Vitamins
5. Minerals
6. Water

A typical acceptable macronutrient distribution range (AMDR):

• Carbohydrates: 55–65%
• Protein: 7–20%
• Fat: 15–30%

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Carbohydrates

Structure:

• Composed of carbon (C), hydrogen (H), and oxygen (O)
• General formula: Cₙ(H₂O)ₙ

Classification:

Monosaccharides:

• Glucose: the primary energy source for the brain and nervous system
• Fructose: found in fruit and honey
• Galactose: a component of lactose

Disaccharides:

• Sucrose = glucose + fructose
• Lactose = glucose + galactose (milk)
• Maltose = glucose + glucose (germinated grains)

Polysaccharides:

• Starch: digestible, a plant’s energy store
• Glycogen: an animal’s energy store (liver and muscle)
• Dietary fiber: indigestible, supports gut health

Carbohydrate digestion and absorption:

• Salivary amylase → pancreatic amylase → broken down into monosaccharides
• Absorbed in the small intestine → the hepatic portal vein → the liver → the whole body

Blood-sugar regulation:

• Insulin: lowers blood sugar (glucose → glycogen)
• Glucagon: raises blood sugar (glycogen → glucose)

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Protein

Structure:

• A macromolecule of amino acids linked by peptide bonds
• Built from 20 types of amino acids

Essential amino acids:

• Cannot be synthesized in the body → must come from food
• 8–9 of them: valine, leucine, isoleucine, threonine, methionine, phenylalanine, tryptophan, lysine, (histidine)

Functions of protein:

1. Structural function: muscle, skin, hair, nails
2. Enzymes: catalyze chemical reactions (digestive and metabolic enzymes)
3. Hormones: insulin, growth hormone
4. Transport: hemoglobin (carries oxygen)
5. Antibodies: immune function
6. Energy supply (when needed)

Assessing protein quality:

• Complete protein: contains all essential amino acids in sufficient amounts (animal protein)
• Incomplete protein: lacking in essential amino acids (most plant protein)
• Complementary proteins: rice + beans (lysine + tryptophan complement each other)

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Fat (Lipids)

Structure of fat:

• Fatty acid + glycerol (triglycerides)

Classifying fatty acids:

Saturated fatty acids:

• No carbon double bonds
• Solid at room temperature (butter, lard)
• Excess intake raises LDL cholesterol → cardiovascular risk

Unsaturated fatty acids:

• Contain carbon double bonds
• Liquid at room temperature (vegetable oils)
• Monounsaturated: olive oil (oleic acid)
• Polyunsaturated: omega-3, omega-6

Essential fatty acids:

• Linoleic acid (omega-6): vegetable oils
• Alpha-linolenic acid (omega-3): oily fish, perilla oil

Trans fat:

• Produced when vegetable oil is hydrogenated
• Raises LDL and lowers HDL → the worst type of fat
• Now restricted in most countries

Cholesterol:

• A raw material for cell membranes, hormones, and bile acids
• LDL: “bad” cholesterol (deposits in vessel walls)
• HDL: “good” cholesterol (clears cholesterol from vessels)

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Vitamins

Characteristics of vitamins:
→ Regulate bodily function in small amounts
→ Cannot be synthesized in the body → must be obtained from food

Fat-soluble vitamins (A, D, E, K):
→ Dissolve in fat, absorbed alongside fat
→ Can accumulate in the body → toxic in excess

A (retinol): vision, immunity, skin (liver, carrots, eggs)
D (calciferol): calcium absorption, bone health (sunlight, fish)
E (tocopherol): antioxidant, protects cell membranes (vegetable oils, nuts)
K (phylloquinone): blood clotting, bone metabolism (green vegetables)

Water-soluble vitamins (B vitamins, C):
→ Dissolve in water, excreted in urine → low risk of excess
→ Must be consumed daily

B1 (thiamine): carbohydrate metabolism (pork, beans)
B2 (riboflavin): energy metabolism (milk, liver)
B3 (niacin): energy metabolism (meat, fish)
B12 (cobalamin): red blood cells, nervous system (animal foods — a concern for vegans)
C (ascorbic acid): antioxidant, collagen synthesis (fruit, vegetables)
Folate: DNA synthesis, essential during pregnancy (green vegetables)

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Minerals and Water

Key minerals:

Calcium (Ca): bones and teeth, muscle contraction, nerve transmission

• Recommended intake: about 700–800mg/day for adults
• Sources: milk, cheese, small fish eaten with the bones

Iron (Fe): hemoglobin (oxygen transport)

• A deficiency risk for women and growing children
• Sources: red meat, spinach

Potassium (K): blood-pressure regulation, heart function

• Balance with sodium is important

Sodium (Na): fluid balance, nerve transmission

• Excess intake raises the risk of hypertension
• WHO recommendation: under 2g/day (under 5g of salt)

Magnesium (Mg): a cofactor for over 300 enzymes

Zinc (Zn): immunity, protein synthesis, growth

Water:

• 60–70% of body weight
• Transports nutrients, regulates temperature, mediates chemical reactions
• Recommended intake: about 2L/day (including roughly 1L from food)
• With dehydration: thirst → fatigue → headache → in severe cases, death

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Frequently Asked Questions

Q. Do I have to cut out carbs entirely to lose weight? A. Carbohydrates are a primary energy source for the brain and nervous system, so eliminating them entirely isn’t recommended. Replacing refined carbohydrates (white rice, sugar, white bread) with complex carbohydrates (brown rice, whole grains, vegetables) while managing overall calorie intake is a more sustainable approach.

Q. Does eating more protein build more muscle? A. Protein intake up to a certain level supports muscle synthesis, but excess protein is converted into energy or fat. Excessive protein can also burden the kidneys. The recommended intake for adults is 0.8g per kg of body weight, rising to around 1.2–2.0g/kg for those who exercise.