"The only issue with not adding healthy fat or oil to your diet is this:
When you are physically active (unlike hanging out in a cubicle all day at work) your body will at some point NEED the fat. Otherwise you will start consuming your muscles for needed calories."

Assuming we have a healthy body here, with normal metabolic channels functioning, if there's not enough fat (or any kind of substance) available in the diet, then the body will dip into its reserves. There are cellular reserves, tissue reserves in the form of fatty tissue, aka adipose, as well as other reserves. In a time of shortage, the body would tap all those first. The body will only consume muscle tissue when there is nothing coming in, and ALL its reserves are exhausted. This only occurs when a person is emaciated and starving. In that case it would consume muscles first, then proceed to the organs, starting with those that are less essential to staying alive. Fat occurs naturally in foods. A serving of bananas or pears have about 1/10th the amount of fat as a serving of Oreo cookies, but they still have fat. Plant derived foods have lots less fat content than animal derived foods, and they are associated with heart health. Vegans have less heart disease than anyone. Their fat consumption is low, particularly of the saturated variety.

"As well, for long term hiking don't discount a little oil added to dinner to keep one "regular"."

Whole grains/legumes are super high in fiber, which results in regularity. Oil is not required. Oil may help if a person eats a low fiber diet, for example a diet high in animal products. In that scenario, oil may help compacted materials to slide out. In fact, taking a shot of oil used to be a treatment for constipation. Today, adding plant-based foods to a diet will increase all the elements needed for improved nutrition, digestion, and elimination.

"While one doesn't want to go on a fat fest something to consider is the average 20 something thru hiker of the PCT. By the time I see them in mid Washington in late August they all look the same: ultra thin, just wiry thin muscles and they eat all freaking day long. They literally cannot get enough food in by then - and fat tends to a big choice."

The Boston Red Sox shortstop, Nick Green, is so thin he looks a little cadaverish. We here in the US are so used to being husky, hefty, hunky, etc, that when we see a world class athlete like Nick Green or other elite athletes (like some thru hikers), we think they look gaunt, sick, unhealthy. They have few reserves of adipose tissue, and they do feel the need to eat plenty. People tend to associate a well-fed person with health, but scientific inquiry has shown that slightly underfed laboratory animals live longer, are more active, and are less likely to fall ill. My personal goal is to become thin again. When I weighed 165 pounds (at 6'0" height), I could run ten miles in 70 minutes without breathing hard or my pulse rising above 100. I was thin, but I was all muscle. My resting heart rate was 38. That didn't mean I was about to die, haha. It meant I was a stud. Don't worry about those thru hikers. If they have energy to hike and pack, they're probably getting enough to eat.

Being hungry is a natural and healthy thing. Most people in the US probably don't feel hungry very often, 'cause mealtimes are so regimented here. It's better to just eat when you're hungry, and eat as much as you want. The secret is to eat the right things - not junk. And NOT EATING (God forbid!!) is a centuries old technique to improve health, awareness, and self-control. Thin is not IN, but it ought to be!