Following the Energy Balance Principle, in theory all you have to do to lose weight is make sure calories consumed are below your energy expenditure. Yet if it were that easy, we would not have an obesity epidemic. We would all be as skinny as desired simply by cutting calories, not eating, or exercising a lot.

The biggest challenge that stands it our way is what we will broadly call HUNGER -- it is any desire, drive, motivation or cue that causes us to eat food. What happens is that it overcomes our efforts to create a calorie deficit.

Sometimes, extremely overweight or obese individuals have little hunger when they first start dieting. But eventually hunger comes into play in one way or another with everybody, especially as you get more lean. The question is how to deal with it and minimize or eliminate it as much as possible.

What causes and regulates hunger, appetite and the drive to eat is extremely complicated. There are both biological and psychological/emotional factors, and they affect each other. Hunger signals are affected by stomach stretching, levels of nutrients in the blood, brain chemistry and changes in fat cell size, among many others.

Your Body's Calorie Control Systems

The body has two systems to monitor and affect calorie intake. The homeostatic system and the non-homeostatic system.

Homeostatic System

If you think of your body as a machine, the homeostatic system is an internal regulator of the machine. It detects energy levels and fat levels and your brain continually adjusts the machine to keep everything in a certain range (homeostasis).

Research continues to discover hormones that affect hunger in the homeostatic system. A few of them are: leptin (monitors fat levels), ghrelin, peptide YY, and GLP-1. They monitor how much you are eating, what you are eating, and your current weight. They then send signals to your brain (hypothalamus) which in turn alerts your conscious mind, "I'm hungry" or "I'm full". We may refer to this as biological hunger.

The homeostatic system acts independently of your environment, unconcerned about what's going on around you. It does a great job of holding on to fat as insurance against famine (especially when if you start getting really lean). But isn't so great when it comes to preventing fat gain from overeating. (This relates to the discussion of Metabolic Adjustments, and some people believe this system creates weight "set points").

Non-Homeostatic System

This is the biggest contributor to weight problems. This system mainly interacts with the external world -- the food you see in front of you, social eating (parties, work, etc), food advertisements, restaurant signs, etc. The non-homeostatic system also includes various emotional and psychological reasons for eating. We may eat about of habit, or in response to emotions (boredom, anxiety, sadness, etc) which are themselves triggered by some outside circumstances or events. We may summarize this as "psychological hunger".

All of those cues or causes of eating are outside of any true need for nutrients -- in other words, it overrides our homeostatic system and causes us to take in more energy than we can burn off -- more than our homeostatic system would normally want us to.

Hedonic System: This is connected to the non-homeostatic system. With "hedonic" eating, we decide to eat simply because it's pleasurable and rewarding; it just tastes good, looks good, smells good, or we like the texture. It activates the dopamine-reward pathway which is related to psychological cravings, emotional eating, and addiction.

The Problem of Non-Homeostatic Eating

A researcher named Juan De Castro looked at real-world eating habits and made some eye-opening discoveries. Invariably, the same patterns emerged. For example:

The more people at an event, the more food you eat (e.g., holiday gatherings)
People tend to eat more on the weekends than during the weekdays
The more food that is on your plate or the more that is presented to you, the more you'll tend to eat
Likewise, the more variety at a given meal, the more you'll tend to eat.

De Castro found that the amount of food we eat is affected by a surprising number of things: Number of people present, food accessibility, eating locations, food color, food temperature, smell of food, time of consumption, ambient sounds, temperature and lighting. It's easy to see why controlling food intake goes far beyond our basic idea that we eat because we're hungry!

Types of Hunger

There are many types of hunger. We're going to outline a few of them below. It's important to understand the differences so you can find the right solutions for managing food intake in the more effective way.

Remember when we say "biological" we are referring to the homeostatic system. It's just a shorthand way of referring to the same system. When we say "psychological" we are referring to the non-homeostatic system. The two can overlap, and they do influence each other.

True Hunger (Biological)
This hunger is manifest by the grumbling of your stomach or actual physical hunger sensations that draw attention to an empty stomach or when you haven't eaten for awhile. The hormone ghrelin is the main driver (think ghrelin=grumbling). This is the main hunger signal from the homeostatic system. It doesn't mean we have consume food immediately. Our body is simply reminding us the tank (stomach) is empty or getting close to empty.

Low blood sugar (Biological)
When blood sugar gets too low (hypoglycemia), it can causes feelings of low energy, light-headedness, fuzzy thinking, shakiness, sometimes headaches -- and of course, a desire to eat to combat the symptoms. The symptoms depend on the individual and severity of response. Sometimes low blood sugar manifests itself less forcefully but still gives you a sudden desire for some quick energy foods. All this is caused by a lack of glucose to the brain, which cannot function properly without a steady, reliable flow of fuel.

Cravings (Psychological)
The main cause of psychological cravings is deprivation of foods we really enjoy eating -- or have gotten into the habit of eating -- but are now "forbidden". It's possible for this type of craving to increase in intensity until it causes a binge-eating session. Other times it is manageable and simply goes away. Regardless a craving is recognized as something you simply want because you LIKE it (pleasurable eating) not because your stomach is empty or you feel weak or anything physical.

Emotional Eating (Psychological)
This type of "hunger" is when you eat food to make yourself feel better, or to deal with stress, boredom, loneliness, anxiety, or any other emotion. It's a substitute for proper handling of emotions and hard situations. Like psychological cravings, it's not related to real hunger -- although combined with real hunger it can quickly turn into an overeating session.

Addiction (Psychological)
This could be defined as any eating pattern that interferes with normal life, gets in the way of work, productivity, relationships. We often joke about being "addicted" to certain foods, when in reality we just really, really enjoy them and it's psychologically painful to stop eating them. Outright food addiction is a somewhat debatable topic. We might simply conclude that you are addicted if it is a food or bad eating habit that you want to stop for health reasons or weight loss goals, but you just can't seem to control it no matter what you've tried.

Feeling Full

Satiety (between-meal fullness)
This is a biological state where your body is not sending any hunger signals (which can last for hours after a good meal). Mostly related to the homeostatic system. Satiety cannot be fooled, because it's purely biological and depends on our homeostatic system of energy regulation. Our body only sends hunger signals until it senses an internal need for nutrients. If your body detects that you currently have enough nutrients, you won't get hungry.

Sensory-Specific Satiety
This refers to the moment you feel like you're done eating a certain food or part of your meal. The most common reason is because you're tired of eating it -- also called "sensory-specific satiety". We are designed to crave variety (we are omnivores, after all!), which ensures we get a variety of nutrients. During a meal, the foods we haven't eaten yet still seem appealing especially if they are different in taste, smell, etc. This is one reason why we still have room for dessert after a large meal. It also means that we eat more of food that looks, tastes, feels more appetizing than other food.