The body's overall metabolic status is controlled by a master hormone called leptin. It also happens to be upstream of just about every process in the body that is related to metabolic function. In other words, having a normalized amount of leptin is something you want.
Leptin signaling is strongly linked to body fat levels, as well as to calorie intake. If you're carrying a decent amount of body fat and eat well, you do not have to particularly worry about this. On the other hand, if you're on a contest diet of 1000 cals per day and have crazy low levels of fat, then you're likely near bottomed out.
This has interesting implications. Centrally speaking, leptin is linked to a whole bunch of neurochemical mess in the brain. It's downstream of the dopamine pathway, which is responsible for things like pleasure-reward feedback. This might well implicate it in eating disorders. The hormones that control appetite also hang around this area. Again, no shock.
Thyroid, androgen, and adrenal output are all modulated by leptin as well. Which means that if you diet too hard, too long, you'll basically “crash”. The body appears to have an equilibrium point where it “wants” to be, in terms of body mass and composition, which is referred to as the setpoint. Going below the setpoint, along with chronic underfeeding, is what causes these problems.
We can take a few things away from this. Firstly, someone that's above his or her setpoint doesn't have to worry so much about the partitioning issue I mentioned before. There's plenty of leptin signaling going on, assuming a sane calorie deficit is in place, and proper strength training can help preserve muscle tissue.
Secondly, starvation diets are not a good idea. At least, over the long term. There's been quite a buzz about the phenomenon of so-called “metabolic damage”. Now, insofar as I know, which admittedly is not an infallible answer, the only way to truly damage the metabolism is to physically or chemically do something to the thyroid gland that causes it to stop functioning. The down-regulation of metabolism, the “crash”, has been shown in research to be limited to about 30% or so of baseline, and is easily undone by raising calories for a few weeks. There's nothing permanent about it, again barring actual damage to the thyroid gland (admittedly, not uncommon in women).
Now, applied properly, leptin-crushing low calories can be used. However this requires periodic increases in calories, and especially carbohydrate intake, called a refeed. Refeeds will vary in composition, and strictly defined they are simply periods of over-feeding. However, carb-based refeeds are the preference in many instances, due to the fact that glucose uptake by fat cells is the source of leptin signaling. Carb-based refeeds can span anywhere from 2g/kg to 16g/kg of carb intake, although the higher end of that is generally not a requirement. For most purposes, 2-7g/kg is likely plenty. In the research, intervals of less than 5 hours have been pretty useless for increasing leptin, so that's something to take into account also. This ties into the concept of cyclic dieting, which I'll cover a little more later.
Regarding fat intake, you'll hear anything from ZOMG FAT EVAL!! to people that think fat should be a major source in the diet (ketogenic eating).
The truth, as usual in this type of thing, is somewhere in the middle. You need fat in your diet, for a whole lot of things. Along with protein, fat comprises most of your tissues that aren't water. Fats are stored in adipose tissue as triglycerides, which also provide a source of fuel for aerobic metabolism when released into the bloodstream.
There are different types of fat, which fall into the categories of saturated, monounsaturated, and polyunsaturated (which includes the essential fats).
Essential fats are divided into the omega-6 and omega-3 categories, derived from two polyunsaturated fats called alpha-linolenic acid and linoleic acid. Essential fats are crucial to a number of processes, including inflammation and healing as well as neurological and cardiovascular health. The omega-3s in particular have also displayed a positive effect on partitioning away from fat cells and towards muscle, although this is a subtle, long-term benefit.
Virtually all of the essential fats are derived from polyunsaturated fat, and these include flax, canola, safflower, and borage oils, certain nuts, and fish. Plant sources do not contain EPA (eicosapentaenoic acid) or DHA (docosahexaenoic acid), which are the two omega-3 fats. These are found in fish and fish oils.
The essential fats can all be synthesized to some degree or another from other omega-6 or omega-3 fats, but having a dietary source of EPA and DHA is valuable as this conversion does not always provide adequate amounts.
In terms of daily recommendations, fat intake generally falls around 0.5g per pound, although this is a fairly arbitrary intake. In situations of lowered carb intake, dietary fat is used as “caloric ballast” to fill the remaining energy requirements once protein reaches the desired levels. Regarding essential fats, the only suggestion is to get 5-10g of omega-3s per day from dietary sources (fish oil is a good place to start).
As a rule of thumb, any remaining fats should come from other polyunsaturated sources, or from monounsaturated oils like olive oil. Olive oil in itself is an overall “healthy food”, containing large amounts of vitamin E and known as having strong antioxidant properties.
Saturated fats from animal sources are generally not desired as large parts of the diet, but like everything else it's not an all or nothing proposition. Getting a little in won't kill you, and is likely beneficial.
In addition to this, there are also the medium-chain triglycerides (MCTs) and diaglycerol (DAG) oils which can be a part of fat intake. These fats differ from others in that they are preferentially used for energy by the body, which can make them useful in periods of lowered carb intake. Both of these have been implicated in metabolic increases and positive changes in body composition, but so far as I can tell there's nothing actually supporting that claim. Coconut oil is a good source of MCTs (although it is high in saturated fat). DAG oil can be purchased under the label Enova Oil.
Contrary to popular belief, dietary fat does not make you fat. Eating too much, period, makes you fat. Now, strictly speaking, fat absorbed into the bloodstream from the gut does enter fat cells. The issue is, in a calorie deficit, it simply doesn't stay there. This is not a matter of insulin, or clean eating; it's a simple fact that the body is not going to store energy when it cannot fuel its basic functions. The body is designed for survival, and storing valuable, needed energy when it's already not able to cover its basic needs is not a survival strategy.
Thermodynamics will win out over insulin every time.
Certain diets that are moderate in protein, high in fat, and very low in carbs have shown themselves to have interesting effects in the body. In a chronically carb-deficient state, the body metabolizes fat into compounds called ketones, which can be used for fuel in place of glucose, albeit in a limited sense. Being in ketosis is often accompanied by fatigue, mental “fuzziness”, and occasionally with a feeling of having extra energy due to an increase in catecholamine levels.
Interestingly, these ketogenic diets have shown fat losses with calorie intakes closer to maintenance level than “glucose based” dieting, and studies have been done showing no negative impact on lipid profiles (cholesterol, triglyceride levels, etc).
This is not to say that ketogenic diets are somehow superior to other approaches. Rather, it's to point out that high fat intakes do not correlate with body fat. Calorie balance is the ultimate determinant, followed by essential protein and fat intakes.