Rennin and lactase, the enzymes required to digest milk, disappear in the human body by the age of

1. Introduction to the Human Digestive System (basic)

The human digestive system is essentially a highly specialized processing plant that converts the food we eat into energy and building blocks for our body. This journey takes place within a continuous, muscular tube called the alimentary canal, which begins at the mouth and ends at the anus Science-Class VII, NCERT(Revised ed 2025), Life Processes in Animals, p.122. Along this path, food is broken down through two primary methods: mechanical digestion (like the churning of muscle cells in the stomach) and chemical digestion (where digestive juices and acids break down complex molecules) Science, Class VIII, NCERT(Revised ed 2025), The Invisible Living World: Beyond Our Naked Eye, p.14. One of the most fascinating aspects of this system is its sheer scale and adaptation. For instance, while it is called the small intestine, it is actually the longest part of the alimentary canal, stretching nearly 6 metres in an adult—roughly twice the height of an average classroom! Science-Class VII, NCERT(Revised ed 2025), Life Processes in Animals, p.125. Furthermore, our digestive capabilities change as we grow. In infancy, the system is specifically primed for milk digestion with high levels of enzymes like rennin (which curdles milk protein) and lactase. As we transition to a solid diet, the production of these specific enzymes typically declines, a process that is often noted in clinical and academic contexts to be significantly advanced by the age of two. To keep this complex system functioning, our body relies on a partnership with trillions of tiny organisms. The large intestine houses beneficial bacteria that help break down undigested fibers and produce essential nutrients, emphasizing why fermented foods like curd or buttermilk are vital for maintaining a healthy gut microbiome Science-Class VII, NCERT(Revised ed 2025), Life Processes in Animals, p.127.

Component	Primary Function	Key Feature
Alimentary Canal	Transportation and Breakdown	Continuous tube from mouth to anus
Small Intestine	Absorption and Secretion	Longest part (approx. 6m)
Large Intestine	Water Absorption & Waste	Home to healthy bacteria/microbiome

Sources: Science-Class VII . NCERT(Revised ed 2025), Life Processes in Animals, p.122; Science-Class VII . NCERT(Revised ed 2025), Life Processes in Animals, p.125; Science-Class VII . NCERT(Revised ed 2025), Life Processes in Animals, p.127; Science, Class VIII . NCERT(Revised ed 2025), The Invisible Living World: Beyond Our Naked Eye, p.14

2. Enzymes: The Biological Catalysts (basic)

Imagine your body as a high-speed chemical factory. For this factory to run, thousands of reactions must happen every second. **Enzymes** are the biological catalysts that make this possible; they speed up chemical reactions without being consumed in the process. Most enzymes are proteins, and their instructions are coded directly in our DNA Science Class X (NCERT), Heredity, p.131. Without them, the energy-extraction processes of life would be too slow to sustain us. For example, the food we eat is systematically broken down by specific enzymes in the digestive tract, a process that begins in the mouth and continues through the stomach and small intestine Science Class X (NCERT), Our Environment, p.214.

One of the most vital characteristics of enzymes is their specificity. An enzyme is like a specialized key that only fits a specific lock (the substrate). This is why the enzymes that break down proteins cannot break down fats, and why humans cannot derive energy from materials like plastic or coal — we simply don't possess the specific biological "keys" to unlock them Science Class X (NCERT), Our Environment, p.214. Furthermore, enzymes are highly sensitive to their environment. For instance, enzymes in the small intestine require an alkaline (basic) environment to function properly. To facilitate this, the body uses bile juice to neutralize the acidic food coming from the stomach, ensuring the pancreatic enzymes can work efficiently Science Class X (NCERT), Life Processes, p.86.

In the context of human development, certain enzymes are age-dependent. In infants, enzymes like Rennin (which curdles milk for better digestion) and Lactase (which breaks down milk sugar) are at their peak. As a child grows and their diet diversifies, the production of these specific milk-digesting enzymes naturally begins to decline. In many standard clinical and academic observations, a significant reduction in these enzymes is noted by about two years of age, marking the biological transition from infancy toward a more varied diet.

Factor	Effect on Enzymes
pH Level	Stomach enzymes prefer acidic conditions; intestinal enzymes need alkaline conditions.
Specificity	Each enzyme only acts on a specific substrate (e.g., lactase only acts on lactose).
Activators	Minerals like Magnesium (Mg) can act as essential activators for enzyme function Environment, Shankar IAS Academy, Agriculture, p.363.

Sources: Science Class X (NCERT), Heredity, p.131; Science Class X (NCERT), Our Environment, p.214; Science Class X (NCERT), Life Processes, p.86; Environment, Shankar IAS Academy, Agriculture, p.363

3. Digestion in the Stomach: Gastric Secretions (intermediate)

When food enters the stomach, it doesn't just sit there; the stomach is a large, muscular organ that expands and vigorously churns the food to mix it with gastric juices Science, Class X (NCERT 2025 ed.), Life Processes, p.85. This chemical cocktail is secreted by specialized gastric glands located in the stomach wall. The primary components of these secretions are Hydrochloric Acid (HCl), the protein-digesting enzyme Pepsin, and Mucus. Each plays a critical, symbiotic role: HCl creates the highly acidic environment (low pH) necessary to activate pepsin, while mucus acts as a sacrificial barrier, protecting the stomach's own inner lining from being eroded by its own acid Science, Class X (NCERT 2025 ed.), Life Processes, p.85.

Beyond protein breakdown, HCl serves a vital defensive function by killing many of the bacteria and pathogens that enter our system through food. However, balance is key. If the stomach produces excess acid—often due to diet or stress—it leads to indigestion, causing pain and irritation. In such cases, we use antacids like Magnesium hydroxide [Mg(OH)₂], also known as Milk of Magnesia, which is a mild base that neutralizes the excess acid to provide relief Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.27.

An interesting aspect of gastric physiology is how it adapts to our life stages. In infants, the stomach secretes Rennin (Chymosin), an enzyme specifically designed to curdle milk. By turning liquid milk into a semi-solid curd, the body ensures the milk stays in the stomach longer, allowing for more thorough digestion. As a child transitions to a solid diet, the production of these milk-specialized enzymes declines. In many standard academic and clinical contexts, it is observed that these specialized milk-digesting capabilities significantly diminish or disappear by about two years of age, reflecting the biological shift from an infant milk-based diet to a more varied adult diet.

Secretion	Primary Function
Hydrochloric Acid (HCl)	Kills bacteria and activates Pepsin by creating an acidic medium.
Pepsin	Breaks down complex proteins into smaller peptides.
Mucus	Protects the stomach wall from acid-induced damage.
Rennin (Infants)	Curdles milk proteins to facilitate slower, better digestion.

Sources: Science, Class X (NCERT 2025 ed.), Life Processes, p.85; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.27

4. Metabolism of Macronutrients (intermediate)

To understand the metabolism of macronutrients, we must first look at how our body breaks down large, complex molecules into units it can actually use for energy and growth. This process begins with digestion. In the stomach, proteins are the primary focus; gastric juices and acid work to break them down into simpler components, while a layer of mucus protects the stomach lining from self-digestion Science-Class VII, Life Processes in Animals, p.125. However, the small intestine is the true hero of this story—it is the site where the complete digestion of carbohydrates, proteins, and fats occurs Science, class X, Life Processes, p.86.

Metabolism is chemically intensive and requires specific environments. For instance, the food leaving the stomach is highly acidic, but pancreatic enzymes require an alkaline (basic) environment to function. The liver provides bile juice to neutralize this acid and emulsify fats—breaking large fat globules into smaller ones so enzymes can work more efficiently Science, class X, Life Processes, p.86. Once broken down into glucose, amino acids, and fatty acids, these nutrients are absorbed into the bloodstream through the thin walls of the small intestine, which are covered in finger-like projections called villi that maximize surface area Science-Class VII, Life Processes in Animals, p.126.

Beyond just breaking things down, metabolism involves utilization. At a cellular level, elements like Nitrogen are essential for building proteins, while Phosphorus is a key component of the enzymes that help cells regulate and fix energy Environment, Agriculture, p.363. Interestingly, our efficiency in processing these nutrients isn't just biological; it's also behavioral. Ancient Indian wisdom from the Charaka Samhita and modern science both agree that mindful eating and proper meal timing are vital for maintaining the digestive health necessary for effective metabolism Science-Class VII, Life Processes in Animals, p.127.

Macronutrient	End Product	Key Metabolic Role
Carbohydrates	Simple Sugars (Glucose)	Primary fuel for cellular energy (ATP)
Proteins	Amino Acids	Building blocks for tissues and enzymes; contains Nitrogen
Fats	Fatty acids & Glycerol	Concentrated energy storage and cell membrane integrity

Sources: Science, class X (NCERT 2025 ed.), Life Processes, p.86; Science-Class VII, NCERT (Revised ed 2025), Life Processes in Animals, p.125; Science-Class VII, NCERT (Revised ed 2025), Life Processes in Animals, p.126; Science-Class VII, NCERT (Revised ed 2025), Life Processes in Animals, p.127; Environment, Shankar IAS Academy (ed 10th), Agriculture, p.363

5. Hormonal Regulation of Digestion (intermediate)

To understand the digestion of food, we must look beyond just the mechanical act of chewing. The human digestive system functions like a highly coordinated chemical factory that operates on a 'just-in-time' delivery model. This coordination is largely managed by hormones—chemical messengers secreted by endocrine cells located in the stomach and small intestine. These hormones ensure that digestive juices are produced only when food is present, preventing the body from wasting energy or damaging its own tissues. As noted in general biological principles, animal hormones perform critical regulatory roles in carefully controlled locations throughout the body Science , class X (NCERT 2025 ed.), Control and Coordination, p.109.

The process begins in the stomach. When food enters, the hormone Gastrin is released into the bloodstream. Gastrin loops back to stimulate the gastric glands, prompting them to secrete gastric juice, which contains hydrochloric acid (HCl) and pepsinogen. This acid is vital because it creates the acidic environment necessary for protein-digesting enzymes to function Science-Class VII . NCERT(Revised ed 2025), Life Processes in Animals, p.124. However, once the partially digested, highly acidic food (called chyme) moves into the small intestine, the environment must change rapidly to protect the intestinal lining and allow other enzymes to work.

As chyme enters the duodenum (the first part of the small intestine), it triggers the release of two key hormones: Secretin and Cholecystokinin (CCK). This 'duodenal brake' is a fascinating feedback mechanism:

Hormone	Primary Trigger	Main Function
Secretin	Acidic chyme entering the duodenum.	Stimulates the pancreas to release bicarbonate-rich juice to neutralize stomach acid Science-Class VII . NCERT(Revised ed 2025), Life Processes in Animals, p.126.
CCK	Presence of fats and proteins in the duodenum.	Stimulates the gallbladder to release bile and the pancreas to release digestive enzymes.

By neutralizing the acid, pancreatic juice creates a basic (alkaline) environment, which is the 'sweet spot' for enzymes that break down carbohydrates, proteins, and fats in the small intestine Science-Class VII . NCERT(Revised ed 2025), Life Processes in Animals, p.126. This hormonal relay ensures that each segment of the digestive tract is perfectly prepared for the specific stage of digestion occurring at that moment.

Sources: Science-Class VII . NCERT(Revised ed 2025), Life Processes in Animals, p.124; Science-Class VII . NCERT(Revised ed 2025), Life Processes in Animals, p.126; Science , class X (NCERT 2025 ed.), Control and Coordination, p.109

6. Milk Digestion: Rennin and Lactase (intermediate)

Welcome back! As we dive deeper into the digestive system, it is crucial to understand that nature designs our body to be incredibly efficient based on our age and diet. For an infant, milk is the primary source of nutrition. To process this effectively, the body utilizes two specialized enzymes: Rennin (also known as chymosin) and Lactase. While the stomach generally uses hydrochloric acid and pepsin to digest proteins (Science, Class X, Life Processes, p.85), milk protein (casein) requires a unique first step. Rennin acts by curdling the liquid milk into a semi-solid state. This allows the milk to remain in the stomach longer, giving other enzymes like pepsin enough time to break down the proteins into simpler components (Science-Class VII, Life Processes in Animals, p.125).

While Rennin handles the protein, Lactase is responsible for the sugar found in milk, known as lactose. Enzymes are highly specific; a particular enzyme is required to break down a particular substance (Science, Class X, Our Environment, p.214). Lactase breaks lactose down into simpler sugars (glucose and galactose) that the body can easily absorb. In the digestive journey, the stomach's muscular walls churn the food, mixing it with these vital juices to transform it into a semi-liquid mass (Science-Class VII, Life Processes in Animals, p.124).

From a UPSC perspective, the most critical detail is the temporal nature of these enzymes. These milk-digesting powerhouses are at their peak during infancy. As a child transitions to solid foods—a process known as weaning—the demand for these enzymes drops. In standard physiological models and common competitive exam frameworks, it is noted that Rennin and Lactase activity significantly declines or disappears by about two years of age. In adults, Rennin is virtually absent, and the task of curdling milk is taken over by the acidic environment created by HCl in the stomach.

Enzyme	Target Substance	Primary Role
Rennin	Casein (Milk Protein)	Curdles milk to delay its passage from the stomach.
Lactase	Lactose (Milk Sugar)	Breaks down complex milk sugar into simple sugars.

Sources: Science, Class X (NCERT 2025 ed.), Life Processes, p.85; Science-Class VII, NCERT (Revised ed 2025), Life Processes in Animals, p.124; Science-Class VII, NCERT (Revised ed 2025), Life Processes in Animals, p.125; Science, Class X (NCERT 2025 ed.), Our Environment, p.214

7. Age-related Physiological Changes in Digestion (exam-level)

In the human lifecycle, the digestive system undergoes remarkable physiological adaptations to suit our changing nutritional needs. During infancy, the system is specialized for the digestion of mother's milk. Two critical enzymes dominate this stage: Rennin (also called chymosin) and Lactase. Rennin acts by curdling milk proteins (casein), allowing them to stay in the stomach longer for better absorption, while lactase breaks down the milk sugar, lactose, into simpler sugars. As the child transitions to a solid diet, usually around the age of two years, the production of these enzymes begins to decline significantly. In many individuals, rennin disappears entirely, and lactase levels may drop, leading to what we commonly recognize as varying degrees of lactose intolerance in adulthood.

As we move into adolescence, the body's physiological demands shift from simple milk digestion to more complex nutrient management. This is a critical stage where the digestive system must efficiently absorb minerals like iron and vitamins like B12 to support rapid growth and blood formation Science-Class VII, Adolescence: A Stage of Growth and Change, p.80. Deficiency during this time can lead to health problems like anemia. Furthermore, our gut health becomes increasingly dependent on beneficial microorganisms. For instance, bacteria like Lactobacillus play a vital role in our digestive tract by fermenting sugars and maintaining a healthy environment, similar to how they convert milk into curd in controlled settings Science, Class VIII, The Invisible Living World: Beyond Our Naked Eye, p.22.

In later stages of life, the digestive system experiences a general decline in efficiency. The secretion of digestive juices like hydrochloric acid (HCl) and pepsin in the stomach decreases, and the motility of the gastrointestinal tract slows down. This can impact the absorption of essential nutrients, emphasizing why the high enzyme activity of infancy is a specific, time-bound physiological adaptation. While growth is regulated by the pituitary gland and thyroid gland through hormones like growth hormone and thyroxin Science, Class X, Control and Coordination, p.110, the digestive system provides the raw fuel necessary for these hormones to execute their developmental roles.

Sources: Science-Class VII, Adolescence: A Stage of Growth and Change, p.80; Science, Class VIII, The Invisible Living World: Beyond Our Naked Eye, p.22; Science, Class X, Control and Coordination, p.110

8. Solving the Original PYQ (exam-level)

This question bridges your knowledge of digestive enzymes and human physiological development. You previously learned that Rennin (also known as chymosin) is essential for curdling milk to prolong its stay in the stomach, while Lactase breaks down lactose into glucose and galactose. In the context of the human life cycle, these enzymes are evolutionary adaptations for infancy. As the body transitions from a purely liquid milk diet to solid foods—a process known as weaning—the biological necessity for high concentrations of these enzymes diminishes. This question asks you to identify the specific developmental milestone where this transition is traditionally considered complete.

To arrive at the correct answer, you must think about the standard weaning period in human biology. While modern clinical research, such as that found in StatPearls (NCBI), suggests that the decline of lactase can be gradual and varies across populations (lactase persistence), competitive exams like the UPSC often rely on traditional physiological benchmarks. By the age of two, most children have moved onto a diverse adult-like diet, and the body significantly scales back the production of these infant-specific enzymes. Therefore, Option (A) two is the most accurate reflection of the end of the primary milk-dependent stage of human growth.

The options (B), (C), and (D) are common distractors used to test your precision regarding developmental timelines. Ages like three, five, or eight represent later stages of childhood where the dietary shift has already been long established. A common trap here is to confuse "gradual decline" with "functional presence"; while some enzyme activity may persist, the disappearance or significant reduction targeted by the question occurs much earlier. Always lean toward the earliest milestone that aligns with the conclusion of the breastfeeding/infancy phase when faced with these specific biological markers.