Proximate principles of nutrition

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Proximate Principles of Nutrition

The term "proximate principles" refers to proteins, fats, and carbohydrates - the three macronutrients that form the main bulk of food. They are called "proximate" because they represent the primary (most immediate) components of diet and are required in relatively large amounts. They are contrasted with micronutrients (vitamins and minerals), which are needed in much smaller quantities.

In the Indian dietary, proximate principles contribute to total energy intake in these proportions:

Proteins: 10-15%
Fats: 15-30%
Carbohydrates: 50-80%

1. Proteins

Definition and Composition

Proteins are complex organic nitrogenous compounds composed of carbon, hydrogen, oxygen, nitrogen (about 16%), and sulphur. Some also contain phosphorus and iron. They constitute about 20% of adult body weight.

Building Blocks: Amino Acids

Proteins are made up of ~20 amino acids. Of these, 9 are "essential" - they cannot be synthesized by the body and must be obtained from diet:

Leucine, Isoleucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine, Histidine

Functions

Structural: Building and repair of body tissues (muscles, enzymes, hormones, antibodies)
Energy: Yield ~4 kcal/gram when oxidized
Regulatory: Enzymes, hormones, immune factors

Protein Quality

Quality depends on the essential amino acid (EAA) content and digestibility. Key measures:

Biological Value (BV): Proportion of absorbed protein retained by the body
Net Protein Utilization (NPU) = Biological Value × Digestibility coefficient / 100 - considered the most practically useful parameter
Protein Efficiency Ratio (PER)

Animal proteins (milk, eggs, meat, fish) are complete proteins with high BV. Plant proteins may be deficient in one or more EAAs:

Cereal proteins: deficient in lysine and threonine
Pulse proteins: deficient in methionine

Supplementary Action of Proteins

When two or more vegetarian foods are combined (e.g., rice + dal), their proteins complement each other - the limiting amino acids of one food are supplied by the other. This is the scientific basis for recommending mixed diets in vegetarian populations.

Protein Metabolism

Proteins are not stored like fat - they must be replaced daily
Body proteins undergo constant turnover; ~1-2% of body protein is replaced daily
Adequate calorie intake is needed for maximum protein utilization (protein-sparing effect of calories)

Dietary Sources

Food	Protein (g per 100 g)
Soyabean	43.2
Pulses	21.0-28.0
Meat	18.0-26.0
Fish	15.0-23.0
Egg	13.0
Cereals	6.0-13.0
Milk	3.2-4.3
Vegetables	1-4
Fruits	1-3

Requirements (ICMR 2020)

0.83 g protein/kg body weight/day as safe intake for Indian adults (assuming median obligatory nitrogen loss of 48 mg/kg body weight)

2. Fats (Lipids)

Definition and Classification

Fats are concentrated sources of energy (9 kcal/gram), solid at 20°C; liquids are called oils. They are classified as:

Simple lipids - triglycerides (99% of body fat stored in adipose tissue)
Compound lipids - phospholipids (structural membranes)
Derived lipids - cholesterol (steroid hormones, bile acids)

Adipose tissue constitutes 10-15% of body weight in normal adults; 1 kg of adipose tissue = 7,700 kcal.

Fatty Acids

On hydrolysis, fats yield fatty acids and glycerol. Fatty acids are classified as:

Type	Examples	Primary Source
Saturated fatty acids (SFAs)	Palmitic, Stearic, Lauric	Animal fats
Monounsaturated (MUFAs)	Oleic acid	Olive oil, groundnut
Polyunsaturated (PUFAs)	Linoleic, Linolenic acid	Vegetable oils

Note: Coconut and palm oils are vegetable oils but are high in saturated fatty acids. Fish oils are animal-derived but high in MUFAs and PUFAs.

Essential Fatty Acids (EFAs)

EFAs cannot be synthesized by the body and must come from food:

Linoleic acid (omega-6) - the most important EFA; precursor to arachidonic acid
Alpha-linolenic acid (omega-3) - precursor to EPA and DHA
Arachidonic acid - found in meat, eggs, milk fat
Eicosapentaenoic acid (EPA) - found in fish oil

Functions of Fats

Concentrated energy store (9 kcal/g)
Carrier of fat-soluble vitamins (A, D, E, K)
Provide EFAs for cell membrane structure
Insulation and protection of organs
Palatability and satiety

Visible vs. Invisible Fats

Visible fats: Ghee, butter, cooking oils - easily quantified
Invisible fats: Present in cereals, pulses, nuts, milk, eggs - difficult to estimate; actually contribute more to total fat intake in Indian diets than visible fats (e.g., rice 3%, wheat 3%, bajra 6.5% fat)

Fat Requirements (ICMR)

Minimum visible fat recommended for Indian adults (g/day):

Activity Level	Men	Women
Sedentary	25	20
Moderate	30	25
Heavy	40	30

Minimum fat should provide 20% of total energy; 30 g/day minimum for pregnant/lactating women.

Fats and Disease

Obesity: High fat diet encourages adipose accumulation
Phrenoderma ("toad skin"): EFA deficiency - horny papular eruptions on limbs, back, buttocks; treated with linseed/safflower oil + B-complex
Coronary Heart Disease (CHD): High saturated fat intake is a major risk factor; LDL/VLDL are atherogenic, HDL is protective; inverse relationship between EFA intake and CHD mortality
Cancer: High fat diets linked to colon and breast cancer

WHO/FAO Population Goals (% of Energy):

Total fat: 15-30%
SFAs: <10%
PUFAs: 6-10%
Trans-fatty acids: <1%
Cholesterol: <300 mg/day

3. Carbohydrates

Definition and Classification

Carbohydrates are the main source of energy (4 kcal/gram) and the most economical fuel for the body. They are also essential for:

Oxidation of fats ("fat burns in the flame of carbohydrate")
Synthesis of certain non-essential amino acids

Three main forms:

Type	Examples
Starches (polysaccharides)	Cereals, roots, tubers - the bulk of human diet
Sugars (mono + disaccharides)	Glucose, fructose, galactose; sucrose, lactose, maltose
Cellulose (Dietary Fiber)	Plant cell walls - not digestible but vital for gut function

Functions

Primary energy source - particularly for the brain (only uses glucose) and RBCs
Spares protein from being used for energy (protein-sparing effect)
Required for complete fat oxidation (prevents ketosis)
Dietary fiber promotes bowel health, reduces cholesterol absorption, stabilizes blood sugar
Glycogen stores in liver and muscle provide quick energy reserve

Glycemic Index (GI)

Different carbohydrates raise blood glucose at different rates. Low-GI foods (e.g., legumes, most vegetables) are preferred for sustained energy and metabolic health.

Dietary Fiber

Indigestible plant polysaccharides (cellulose, hemicellulose, pectin, gums). Benefits:

Reduces constipation
Lowers serum cholesterol
Helps control blood sugar
Protective against colon cancer
Promotes satiety

Dietary Requirements

Should provide 50-80% of total caloric intake in Indian diets
WHO recommends free sugars should be <10% of total energy (<5% for additional benefits)

Summary Comparison of Proximate Principles

Feature	Proteins	Fats	Carbohydrates
Energy yield	4 kcal/g	9 kcal/g	4 kcal/g
% in Indian diet	10-15%	15-30%	50-80%
Elemental composition	C, H, O, N, S	C, H, O	C, H, O
Key function	Body building	Energy store, fat-soluble vitamin carrier	Primary energy fuel
Storage in body	Not stored	Yes (adipose)	Limited (glycogen)
Essential components	9 EAAs	2-3 EFAs	Dietary fiber
Deficiency disease	Kwashiorkor, Marasmus	Phrenoderma (EFA deficiency)	Hypoglycemia, energy deficiency

Source: Park's Textbook of Preventive and Social Medicine, Chapter on Nutrition (Nutrients section)

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