How "Hungry" Works
So I've been doing a lot of research lately on nutrition and I learned something really interesting about how our body decided whether it's full or hungry. I figured I'd share it here because it really helped me understand how a baby's (or child's) hunger cues work. Which I think makes feeding them what they need much easier.
The first thing you have to know is that there are two different "sensors" in the stomach that the body uses to determine when it's full. One measures quantity (the physical volume of food in the stomach) and one measures quality (the caloric density, or richness, of the food you ate). When you reach the threshold for either sensor you body signals your brain that you're full and you stop eating. This is the same whether you're a newborn or an adult.
Generally these two sensors are coordinated so if you're eating the right kinds of foods you should trip both sensors at the same time. This means that your stomach will be filled up and you'll have eaten the correct amount of calories/nutrients. However, if you don't eat the right stuff your sensors won't match and that's when you run into problems. For example, if all you're eating is lettuce you'll fill up your stomach long before you reach you calorie needs. The quantity sensor will trip and tell you you're full. BUT because you didn't get enough calories/nutrients it will tell you you're hungry again as soon as any space frees up. Of course you can also have the opposite problem. If you're eating a really rich piece of food, like a cheeseburger, you'll hit your calorie needs long before you hit your volume needs. At first your body will tell you to stop eating because you tripped the quality sensor, but soon it will realize you're not actually full and tell you to eat again - even though you don't need the extra calories. That's how you become overweight. It's all a big balancing act.
For babies and young children the same thing is true, except that they have the added issue of growth spurts. There are times in their lives where their caloric needs suddenly go up and their quality sensors don't trip even if you filled them up with the right stuff. It can take a little while for their quantity sensor to catch up because the stomach needs to physically stretch out or grow. That when we notice them suddenly getting hungry more frequently. You can either feed them richer foods (as long as they're not too rich) at each meal or add another meal into the mix. Whatever works for you.
For those of you who still have babies who are nursing/on bottles there is another interesting twist to the story. Breast milk and formula deal with the issue of growth spurts in different ways. It starts out the same. The baby will get hungry more often and you'll probably be frustrated with extra feedings for a bit.
For formula fed babies you'll probably start offering a bigger bottle, which essentially stretches out the stomach so that they can take in enough food to hit their quality and quantity sensors at the same time. For breastfed babies the opposite happens. The extra feedings stimulate your breasts to produce higher-calorie milk so the baby can reach their quality sensor without having to stretch out the stomach. Of course there is always some stretching, but not as drastic as it would be for a formula-fed baby. That's why you'll generally see older formula fed babies with large bottles while the same age breast-fed baby may only need half as much expressed breast milk.
So now you know :)