Nutrition and Digestion: Key Concepts Explained
Regulatory Nutrients
These are compounds our bodies cannot produce themselves. They control the numerous chemical reactions in a cell.
Vitamins
We need vitamins because our body cannot synthesise them, so they need to be ingested with food.
Characteristics of a Balanced Diet
A balanced diet must provide enough calories and the right amount of essential nutrients. Ideally, 55% of calories should come from carbohydrates, 30% from fats, and 15% from protein. Animal protein should not exceed 40% of the total amount of protein ingested. We should consume more vegetable fats, nuts, and fish, and less animal fat and cholesterol. The amount of vegetable fibre we consume must be sufficient to promote bowel movement, and the amount of water and minerals must compensate for the daily loss of these substances.
Basal Metabolism
Basal metabolism, or the energy consumed by the body’s chemical reactions even when at rest, depends on sex, age, and muscle-fat ratio.
Energy Content of Nutrients
Sugar contains 4 kcal/g, lipids 9 kcal/g, proteins 4 kcal/g, and vitamins 0 kcal/g.
Mediterranean Diet
The Mediterranean diet emphasizes the use of olive oil, an abundance of fresh, seasonal produce of plant origin (such as fruit and vegetables), consuming fish as a protein source, combined with lower meat consumption, and the consumption of dairy products.
Vegetarian Diet
Vegetarians avoid eating meat and fish, and sometimes other products of animal origin too, such as eggs, milk, gelatine, or honey.
Food-Related Health Problems
Overweight
People with a moderate accumulation of body fat.
Anorexia Nervosa
Rejection of food and the obsessive fear of gaining weight.
Bulimia
Eating large amounts of food, then using drastic measures (e.g., purging).
Stages of the Digestive Process
The stages are: ingestion, digestion, absorption, assimilation, and defecation.
Parts of the Digestive System
The digestive system includes: salivary glands, liver, gallbladder, pancreas, mouth, pharynx, oesophagus, stomach, small intestine, large intestine, and anus.
Digestion in the Mouth
Digestion begins with chewing and salivation. Saliva mixes with the ground-up food. Saliva contains an enzyme called salivary amylase, which transforms complex carbohydrates. It also contains water and bicarbonate.
Digestion in the Stomach
As the stomach fills, the stretching and chemicals it contains stimulate contractions. These allow the food to mix with the gastric juices. This breaks the food down more and converts it to chyme. The stomach walls contain digestive glands that secrete mucus, pepsin, and hydrochloric acid.
Nutrient Absorption in the Small Intestine
The inner mucosa layer of the small intestine is folded and has projections called villi. These are lined with epithelial cells called enterocytes, which have folds in their membrane called microvilli. Enzymes convert food into a solution of simple molecules that are absorbed in the mucosa of the small intestine.
Nutrient Absorption in the Large Intestine
In the large intestine, water and some vitamins and minerals are absorbed, making the digestive waste more solid. Main diseases of the digestive system include dental caries, constipation, and diarrhoea.