The definition of nutrition as provided in Wikipedia is:
Nutrition is the science that interprets the nutrients and other substances in food in relation to maintenance, growth, reproduction, health and disease of an organism. It includes ingestion, absorption, assimilation, biosynthesis, catabolism and excretion.[1]
The diet of an organism is what it eats, which is largely determined by the availability and palatability of foods. For humans, a healthy diet includes preparation of food and storage methods that preserve nutrients from oxidation, heat or leaching, and that reduces risk of foodborne illnesses
Another good definition of nutrition by medicalnewstoday.com is
Nutrition is the study of nutrients in food, how the body uses them, and the relationship between diet, health, and disease.
So as you can understand from the above definitions, nutrition is a vast field with a large number
of topics and concepts.
OK, now that we know the definition of nutrition, let’s understand what we mean by nutrients.
Nutrients are substances that allow your body to make energy, build and maintain tissues and regulate
bodily processes and nutrients are mainly categorized into six categories the three macronutrients,
carbohydrates, proteins and fats, and they’re required in large quantities.
Then there is vitamins and minerals, which are the micronutrients required in small quantities.
The sixth nutrient water is also categorized as a nutrient, and we’re going to discuss it later
and we are also going to discuss fiber, both soluble and insoluble types, which is classified into
carbohydrates and probiotics.
English 
Thai