Your question answered:

What is happening to people who seem to be able to go without food all day?  How do they do that and not feel hungry?

Your body can adjust to whatever routine you set for it. People will have different styles of eating and their bodies will be forced to adapt.  In fact your body is fabulous at adapting short term or adopting coping mechanisms.  The problem is that long term some of the habits you may have formed can start to have undesirable effects.

When you go for prolonged periods of time without eating you start to disrupt your metabolism and can enter your body into "fasting" and then "starvation" state.  In these states your body in a desperate attempt to get fuel for your brain and your muscles to function, it will break down its own tissue to make glucose.  Once your blood sugar drops and your glycogen stores are exhausted, your body turns to protein.  It will start to break down its own muscle before it uses fat.  The problem with this is that your metabolism is primarily set on the percentage of lean muscle mass you have.  So skipping meals or not eating regularly can result in a lower metabolism, and your body can become really efficient at storing fat, because it doesn't know when its next meal is coming.

Why don't they feel hungry?  Sometimes it can be because their metabolism has slowed down, sometimes their hormones and chemical messengers have been disrupted by their irregular eating habits.  Sometimes when people wake up in the morning and they are not hungry this can be because they don't have the right enzymes to digest their food and they are not digesting their food quick enough so they are still registering as having food in their system.

It is a good idea to eat regularly, eat enough fibre and if you have digestion problems or don't feel hungry or skip meals, it may be a good idea to speak to a nutritionist or dietitian to check your body is working properly and you are getting what you need.

Feel free to email me with futher questions.