Many of the dehydrated "food storage units" available these days specify that you need a certain amount of fats or oils to supplement their unit. You probably know these units, they generally sell as "1 person, 1 year" type of packages and they contain a variety of grains, legumes, fruits other essentials. You might wonder why they don't just include a container of oil to complete their units. Or even why we need them.
Fats are pretty chemically simple, carbon with oxygen and hydrogen attached. Not as complex as amino acids (proteins). This goes for saturated, unsaturated and polyunsaturated fats, the omega fats everybody loves to talk about these days, triglycerides and the cooking oil in your cupboard. Animal fats, vegetable fats are all this composition.
Fats (oils, lipids) are one of our body's essential nutrients. While our bodies can manufacture some of the fat we need by using other nutrients, we can't make enough of them. Fats are our body's method of storing energy, lubricating joints, and it turns out we need them to absorb a list of essential vitamins. They are not optional; we get sick and die without them. And in times of starvation, our body burns off stored fat by converting it into energy (mostly by turning it into glucose, which is the favored food of our cells).
Unfortunately, the presence of these hydrogen and oxygen molecules aren't all that stable and the hydrogen and oxygen tend to become attracted to, and run off with, the milk man so to speak. They can get together with each other and create water which will induce a milky or emulsified kind of appearing oil, and this would be hydrolysis. I usually see this with oils that have been "annealed", or subject to repeated heating and cooling.
The other thing is they can combine with oxygen and we have oxidation. The latter is the issue we call "rancidity". This is when you can smell it and you can taste it.
The major problem with rancidity is it first gives us unpleasant taste and odor. This progresses until the stuff is pretty much unpalatable. Oils and fats coat the inside of our mouths, making our taste buds more receptive to taste which is ordinarily great, but if that taste is foul, it's foul with an electric guitar and amp.
There aren't the kind of immediate health risks with rancid oils as there are from bacteria or other mean little bugs that grow on spoiled food. However, they won't be able to do their job for us chemically as effectively as we need them to. This is that vitamin uptake thing. It's also felt that rancid oils probably contribute to long term health issues like obesity. There might be some physiology where rancid oils burn less efficiently, and what the body can't use or use as readily as carbs, it might have a tendency to simply store them. I'm not 100% certain on that.
However, rancidity goes for all oils all the time; even oils in the foods we store up. Low fat beans store longer than high fat ones, white flour stores longer than whole wheat, white rice stores longer than whole grain or brown rice. Nuts go great in cans, but nothing can stop the oils in them from going rancid over time. This is why cheap nuts often taste bitter. They are older stock.
The cooking oil on the supermarket shelf might have been in storage for months and this chemical change thing called rancidity has been happening the whole time. We can slow it down, but we can't stop it. The ways to slow it down are the very same general rules we use for all foods in long term storage:
1. Keep it cool.
2. Keep it out of sunlight.
3. Try to keep oxygen away from it.
4. Don't let water adulterate it.
As well, it seems that the less refined-read SATURATED fats-seem to hold up best. Cold pressed extra virgin olive oil fits the bill, but so does lard. Some of these cold pressed oils will hold up for months in proper storage and lard does great in cans (no light or oxygen penetration plus it's free of acids and flavors found in other oils). And if it starts to taste a little harsh, well you just kind of deal with it until you get some more.
Perhaps most interesting to me is how fats operate in freeze-dried scenarios and upon rehydration. The fats are pretty much still there even with the removal of all that hydrogen and oxygen, and I have to remind myself that in fats these are chemically bound to carbon. It's not water; it just contains the components thereof. I suspect there's more going on there than I know at this time. Given as we can freeze dry meat, this is a real advantage for long term freeze-dried storage. With normal dehydration, these oils are basically unaltered and apparently more prone to spoilage.
If your diet is severely lacking in fats and you can't find bacon, eat more whole grains. Eggs, milk, cheese all contain it. Corn is such a wonderful source of oil that if you grow enough, you can press your own oil. The cautions come in if you are utterly dependent upon your stored food and have no hope of obtaining food (with fat in it) from outside sources. For me it's hard to imagine this scenario, but other preppers presume this level of isolation even for long periods. The RDA (gubment recommended daily allowance) of fat is about 60 grams, so that's about two avocados worth. Avocados are wonderful sources of dietary fat, but again most of the other foods you eat have fat as well.
L Joseph Mountain recently published Hidden Harvest, Long Term Food Storage Techniques For Rich And Poor. Available on Amazon. He keeps a website at LongTermStorageFood.com where articles are sometimes archived, info is irregularly updated, and questions are occasionally answered. 
