If you have played the game by now (nobody reads the whole manual before having a go at a new game, do they?), you will have noticed that Norns have three different forms of hunger: hunger for starch, hunger for protein, and hunger for fat. There are, indeed, three different food groups within the Norn's diet. A Norn should get a mixture of all of them to be healthy.
The nutrition any individual Norn gets from its food depends upon its genes, but as a general rule, generation 1 Norns get starch and fat from seeds, protein and starch from fruit, and fat and protein from food items. It is important to realise that foods with fat in aren't 'bad'. Fat is just an important a source of energy as protein and starch. Once the Norn eats starch, fat or protein, they are broken down into smaller molecules - glucose, fatty acid and pyruvate respectively. These are then used to power all the Norn's actions, and used up in the process. However, if the Norn simply turned all food straight into energy, they would either collapse, or have to eat continuously, just to stay alive. Therefore, they store glucose, fatty acids and pyruvate as glycogen, adipose tissue, and muscle tissue respectively. As they expend energy, they use up these stores. As glycogen gets low, a Norn gets hungry for starch, as adipose tissue gets low, the Norn gets hungry for fat, and as muscle tissue gets low, the Norn gets hungry for protein. If a Norn is low in glycogen, they'll often lie down and groan. This is a good sign that your Norn has not been eating enough, and it's best to isolate them and give them a good feed. Female Norns that have laid a number of eggs often need isolating in this manner, to give them a chance to rest and eat!
As a rule of thumb, a Norn with a high amount of glycogen, adipose tissue, and muscle tissue is a healthy, well-fed Norn.