Your current nutrition is effectively the average nutrition of the last 20 meals you've eaten. Eating raw ingredients (which have very little nutrition / item), means you will on average, have very low nutrition. Eating the same item over and over, you will see no change in your overall nutrition. Your nutrition and maximum health do not decay passively. They will only ever change when eating foods.

The best way to maximize your nutrition is to eat foods with a high total nutrition / food item. In order to gain maximum nutrition, you need to consume 1 of each nutrient per food item (drinking milk, and using a variety of soups and sandwiches is often the best way to do this). Eating raw ingredients is a very ineffective way of getting high nutrition since they tend to have very small amounts of nutrition per item.