Nutrition

1. Nutrition in Living Organisms

Nutrition supplies both the raw materials and energy required for life. Autotrophs synthesise organic food from inorganic substances. Green plants, algae and cyanobacteria generally use sunlight through photosynthesis, incorporating carbon dioxide into organic compounds. Some bacteria and archaea obtain energy by oxidising inorganic substances such as ammonia or hydrogen sulphide; this is chemoautotrophic nutrition. Plants still require mineral nutrients, including nitrogen, phosphorus and potassium, from their environment.

Heterotrophs obtain organic nutrients from other organisms or their products. Holozoic nutrition, characteristic of humans and many animals, involves ingestion, digestion, absorption, assimilation and egestion. Saprotrophic organisms, including many fungi, secrete enzymes onto dead organic matter and absorb soluble products. Parasites derive nutrients from living hosts, often harming them. Nutritional relationships may also be mutualistic: in lichens, the photosynthetic partner supplies organic food while the fungus provides a protective structure and access to water and minerals.

Insectivorous plants such as pitcher plants remain photosynthetic. Captured animals mainly supplement nutrients, particularly nitrogen, in nutrient-poor habitats; they do not replace photosynthesis as the principal source of organic carbon. This distinction is useful in eliminating incorrect statements about plant nutrition.

• Digestion breaks complex food into absorbable substances; assimilation incorporates or uses absorbed nutrients within cells.
• Egestion removes undigested food, whereas excretion removes metabolic wastes.

2. Macronutrients, Micronutrients and a Balanced Diet

Carbohydrates, proteins and fats are energy-yielding macronutrients. Carbohydrates include sugars, starch and dietary fibre. Glucose is an important cellular fuel, while glycogen stores carbohydrate in the liver and muscles. Humans lack enzymes for digesting cellulose. Dietary fibre supports bowel function; some fibre is fermented by intestinal microorganisms, producing short-chain fatty acids. Fibre-rich foods include whole grains, pulses, vegetables and fruits.

Proteins supply amino acids for tissues, enzymes, transport proteins and many hormones. Essential amino acids must be obtained through the diet in adequate amounts. Cereals tend to be relatively low in lysine, while many pulses are relatively low in sulphur-containing amino acids. A varied cereal–pulse diet can therefore improve overall protein quality. Fats provide concentrated energy, essential fatty acids and structural components of cell membranes, and facilitate absorption of fat-soluble vitamins. Industrial trans fats increase cardiovascular risk.

Vitamins and minerals are micronutrients: they are needed in smaller quantities but are indispensable. Calcium and phosphorus contribute to bones and teeth; iron is required for haemoglobin; iodine supports thyroid hormone synthesis; and zinc participates in growth, immunity and enzyme activity. Water provides the medium for biochemical reactions, nutrient transport and temperature regulation. Neither water nor vitamins and minerals directly supply calories.

A balanced diet meets energy and nutrient needs according to age, sex, physical activity and physiological conditions such as pregnancy. Dietary diversity matters more than dependence on a single supposedly superior food. ICMR–National Institute of Nutrition’s Dietary Guidelines for Indians, 2024 emphasise diverse foods and limiting foods high in fat, sugar and salt. Excessive intake of certain supplements, particularly fat-soluble vitamins, can cause toxicity.

3. Human Digestion and Absorption

Digestion begins in the mouth, where chewing increases food’s surface area and salivary amylase starts starch breakdown. Food passes through the oesophagus by peristalsis. In the stomach, hydrochloric acid creates an acidic environment, helps activate pepsin and destroys many ingested microorganisms. Pepsin begins protein digestion. Mucus and other protective mechanisms help prevent damage to the stomach lining.

The liver produces bile, which is stored and concentrated in the gallbladder. Bile salts emulsify fats into smaller droplets and assist lipid absorption. The pancreas releases bicarbonate to neutralise acidic contents entering the small intestine, together with enzymes acting on carbohydrates, proteins, fats and nucleic acids. Pancreatic proteases are generally secreted as inactive precursors, reducing the risk of pancreatic self-digestion.

Enzymes associated with the intestinal lining complete much of digestion. Villi and microvilli greatly increase absorptive surface area. Monosaccharides and amino acids largely enter blood capillaries and reach the liver through the hepatic portal circulation. Most absorbed long-chain dietary fats are packaged into chylomicrons and enter intestinal lymphatic vessels called lacteals before reaching the bloodstream.

Most water absorption occurs in the small intestine; the large intestine absorbs much of the remainder and helps form faeces. Its microbiota ferment undigested substrates and can synthesise certain vitamins. Lactose intolerance results from insufficient lactase activity, whereas milk allergy is an immune reaction to milk proteins.

4. Deficiency Disorders and the Double Burden of Malnutrition

Malnutrition includes undernutrition, micronutrient deficiencies, overweight and obesity. Energy intake can be adequate or excessive while essential micronutrients remain insufficient, a situation often described as hidden hunger. Stunting reflects impaired linear growth associated with prolonged or recurrent adversity; wasting often reflects recent weight loss or failure to gain weight. Underweight can result from wasting, stunting or both.

Vitamin A deficiency can cause night blindness and xerophthalmia. Vitamin D deficiency impairs bone mineralisation, causing rickets in children and osteomalacia in adults. Vitamin C deficiency causes scurvy; thiamine deficiency causes beriberi; and niacin deficiency causes pellagra, classically associated with dermatitis, diarrhoea and dementia. Folate or vitamin B12 deficiency can cause megaloblastic anaemia; vitamin B12 deficiency may also cause neurological damage. Iodine deficiency can cause goitre and impair fetal brain development.

Severe undernutrition may present as marasmus, characterised by marked wasting, or with nutritional oedema, traditionally termed kwashiorkor. The latter should not be reduced to a simple isolated protein deficiency: its causation is complex. Repeated diarrhoea, parasitic infections, unsafe water and poor sanitation can aggravate nutritional deficiencies through reduced intake, absorption and increased losses.

NFHS-5, conducted in 2019–21, reported that 35.5% of Indian children below five years were stunted, 19.3% were wasted and 32.1% were underweight. Anaemia affected 67.1% of children aged 6–59 months and 57.0% of women aged 15–49 years. These are survey estimates for specified groups, not figures for the entire population.

5. Prevention, Fortification and Indian Programmes

Nutrition interventions must combine adequate diets with maternal care, disease prevention, safe water and sanitation. WHO recommends exclusive breastfeeding for the first six months, followed by safe, adequate complementary feeding with continued breastfeeding up to two years or beyond. The first 1,000 days, from conception to a child’s second birthday, are especially important for growth and development.

Fortification adds specified micronutrients during food processing. Under FSSAI standards, fortified rice contains iron, folic acid and vitamin B12; double-fortified salt contains iodine and iron; and fortified milk and edible oil contain vitamins A and D. Biofortification instead improves nutrient content within crops through approaches such as breeding. Supplementation provides nutrients directly, for example iron–folic acid tablets. None substitutes for a diverse diet.

Saksham Anganwadi and POSHAN 2.0 brings together nutrition support, early childhood care and related services. PM POSHAN supports meals for eligible children in government and government-aided schools, including Bal Vatika and Classes I–VIII. Anaemia Mukt Bharat uses a life-cycle approach involving supplementation, deworming, behaviour change, testing and treatment. Effective implementation requires attention to both inadequate intake and non-nutritional causes of anaemia.

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