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We know carbohydrates are used for immediate and stored energy (glycogen) before being converted to body fat.

We also know protein is used to repair tissue, mainly muscle for bodybuilders, before being converted to body fat. Conversion of protein into fat is 20 times less efficient than converting carbs into fat, so the body doesn't readily jump-start this conversion.

So what is dietary fat used for before turning into body fat? How readily does the body do this conversion compared to carbs and protein, and at what relative rate does it do it at? I ask this question because the word "fat" may be as misleading as "cholesterol". I learned from a previous question here that dietary cholesterol doesn't necessary increase cholesterol in the bloodstream. Maybe the same principle applies to fat...

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FWIW: for a massive investigation of this question, a full explanation of the ATP cycle and so on, and the two types of energy mechanisms in organisms, check out the book "Life Without Bread" by Dr. Lutz. –  Joe Blow Oct 21 '11 at 17:07
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It's really not as simple as carbs are either used for energy or they turn into fat. Lyle McDonald has a great article that talks about Nutrient Storage and Oxidation.

Nutrient Storage

Carbohydrates can be stored as liver or muscle glycogen, under rare circumstances they are converted to and stored as fat. Dietary fat is stored either in fat cells or can be stored within muscle as intra-muscular triglyceride (IMTG). Under certain pathological conditions, fat gets stored in places it’s not supposed to go, a situation called ectopic fat storage. In a very real sense there’s no true store of dietary protein although amino acids from protein digestion are used to make various proteins and hormones in the body. Skeletal muscle is, in essence, a ‘store’ of protein in the body. There is no store of alcohol in the body.

Fat Oxidation

But for the most part, ingested dietary fat has little impact on fat burning in the body; that is, when you eat dietary fat, your body doesn’t increase fat oxidation. One exception is if an absolutely massive amount of fat (like 80 g) is consumed all at once but even then the effect is fairly mild. Some specific fats, notably medium chain triglycerides, are somewhat of an exception to this; they are oxidized in the liver directly. Rather, the primary controller of dietary fat oxidation in the body is how many carbohydrates you’re eating, which I’ll explain momentarily.

Carb Oxidation

[Since the body only has around ~500 grams of carb stores it]is extremely good at modulating carbohydrate oxidation to carbohydrate intake. Eat more carbs and you burn more carbs (you also store more glycogen); eat less carbs and you burn less carbs (and glycogen levels drop). This occurs for a variety of reasons including changing insulin levels (fructose, for example, since it doesn’t raise insulin, doesn’t increase carbohydrate oxidation) and simple substrate availability. And, as it turns out, fat oxidation is basically inversely related to carbohydrate oxidation.

Protein Oxidation

As I mentioned above, an under-appreciated fact is that about half of all ingested dietary protein is metabolized in the liver (details on this can be found in The Protein Book). Some of it is oxidized for energy while others are converted into other things (including glucose and ketones) for use elsewhere. But, protein oxidation rates do change in response to intake. So, when protein intake goes up, oxidation will increase; when protein intake goes down, oxidation rates decrease. This change isn’t immediate (as it more or less is for carbohydrates) and takes 3-9 days to occur but mis-understanding of this process has led to some goofy ideas such as protein cycling.

Having gotten that out of the way, we can start to understand the role of fat from A Primer on Nutrition pt 2

Of course, a primary role of dietary fats in the body is to be used for energy and it was assumed for many years that this was the only real role of fat, to provide energy storage. This was especially true of stored body fat which was thought for decades to provide only a passive storage depot of energy; rather it turns out that fat cells do much more in the body, producing hormones and such that affect myriad processes elsewhere in the body (a topic I’ve discussed at length on the site and in my books).. Fats are also found in the cell membranes of various tissues (and the type of fat stored there can affect various cellular processes). As well, fats can be used to make eicosanoids, chemical messengers made from specific fatty acids that affect numerous biological processes. Specific dietary fats can also affect gene expression in certain cells, impacting on things like fat storage and oxidation and many others.

So what is dietary fat used for before turning into body fat?

Fat is mainly an energy source and the body's favorite one to store. It also has a host of other known and unknown functions in the body as per the quote above. Fat is also the only source of essential fatty acids, fat soluble vitamins, and other nutrients.

I learned from a previous question here that dietary cholesterol doesn't necessary increase cholesterol in the bloodstream. Maybe the same principle applies to fat...

As always, the primary determinant of whether stored fat stays stored or gets used for energy is overall caloric balance. If you eat caloric excess, you will store fat. From carbs and proteins, this will occur once lean storage depots are full or inaccessible. Although fats are stored very efficiently, those stores will be tapped for energy (for example during sleep) unless you remain in an overall caloric surplus for the day.

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What is "oxidation"? –  JoJo Oct 22 '11 at 5:12
    
Burning. Its just the opposite of storage. Like your body oxidizes sugar for energy. en.wikipedia.org/wiki/Redox –  mike Oct 22 '11 at 5:54
    
It's been 6 years since I took a chemistry class ;) –  JoJo Oct 22 '11 at 7:01
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