When you do not know something, what is the first thing you do?
Ask Google.
So According to Google…

This is quite a self explanatory definition. Basically, it is the study of food and how it is used in our body for growth and survival. Animals, like us, need nutrients in order to maintain their health, grow and survive. If the body does not receive adequate amounts of nutrients, the body will be unable to maintain optimum health and be susceptible to many health issues. The interesting part about nutrition is that it works the opposite way as well. If we absorb too many of one vitamin or consume too much of a specific nutrient, it would result in health issues as well. So, the key to good nutrition is having just the right amount. Before we understand what the “right amount” of nutrients is for dogs, we must first learn a bit more about nutrition. 

First, there are two types of nutrients, essential and non-essential. Not all subcategories have nutrients that are essential and non-essential. But when these two terms do show up, here is the definition.
Essential means that the body needs this nutrient since the body does not produce this at all or enough.
Non-essential means that the body produces this nutrient in sufficient amount. Nutrients that are categorized as non-essential does not mean we should not intake these nutrients, it just means that the body produces just enough of it.

And so, nutrients are divided into separate categories:
1. Macronutrients, which is further divided into 3 main categories:
– Carbohydrates: usually the main source of energy for the body, it is also necessary in order to digest and process other nutrients
This is again divided into soluble and insoluble (fibre) carbohydrates. Soluble carbohydrates are usually broken down and absorbed as glucose to be used for energy. Where as fibres are rarely broken down by canines since their digestive systems are fairly short. Usually if a dog is fed adequate amounts of protein, carbohydrates are not as important in their diet, so the amount of crude carbohydrate in a package of dog food is rarely listed (Eskew, 1999).
Examples of food that mainly contribute carbohydrates: grains, flour, corn, rice

– Lipids: also known as fats, are separated into essential and non-essential. Focusing on the essential, this comes from animal and plant matter. Usually healthy dogs are able to absorb and utilize 90-95% of the fats they consume, so dogs can become obese very easily. Fats are needed to absorb nutrients that are “fat soluble”. These include vitamin A, D, E and K, so fat is important in a dog’s diet in order to absorb these necessary vitamins (Eskew, 1999).
Example of food that mainly contribute lipids: fish oil, chicken fat, sunflower oil

– Proteins: proteins are the hardest nutrients to manage. Dogs need 22 amino acids compared to the 20 amino acids humans need. Amino acids are important for the body since they act as enzymes and hormones, make up a component of body secretions and is needed to build tissues. Not all foods provide all the necessary amino acids needed, Some have higher amounts of certain amino acids and lower levels of others. Therefore, dog food manufactures must incorporate specific combinations of foods in order to meet the amino acid requirements in a canine’s diet. At the same time, as dogs enter different stages in life, the amount of each amino acid needed varies. For example, puppies need four times the amount of arginine compared to adult canines. Dog food producers also need to ensure that the amount of amino acids given in the food are digestible. So there is a large amount of calculations placed into every bag of dog food (Eskew, 1999).
Examples of foods that contribute proteins: chicken, beef, salmon, lamb

2. Micronutrients:
Micronutrients are separated into vitamins and minerals, we will be focusing specifically on the essential micronutrients.
These include:
Vitamin A & E: acts as antioxidants for fighting diseases, helps with burning fat
Vitamin B12: important for cell growth
Calcium, Phosphorus and Vitamin D: helps with bone formation and strengthening
Iron: transports oxygen throughout the body
Potassium: maintains health of the heart
Vitamin C: helps with healing and fighting off illnesses by boosting the immune system
(Boatman, n.a)

Aside from knowing the amount of nutrients in a package of pet food, we also need to know the amount of usable energy left for the dog. When you subtract to leave the actual digestible and metabolizable energy, the net energy provided is much less.

Utilize energy:
Carbohydrates – 3.2kcal/g
Proteins – 2.2kcal/g
Fats – 8.2kcal/g

So ultimately, fats produce the most utilizable energy for canine

(Pibot, Biourge & Elliot, 2006)