When fat is burnt, where is it taken from?

Question

I'm just curious if there is a general pattern of where fat is taken when it is spent during exercise. Is it taken more or less equally from all fat in the body, are some areas used more than others, is one area "depleted" before moving to the next?

Also, is it consistent with where fat is built up when stored? So if I gain x amount of fat and then burn x amount of fat, will my body "look the same"?

(I know it is probably a lot more complicated, but just looking for some general surface lines of what's going on out of curiosity)

Answer 1

You have to understand two things:

• A lot of people think fat "spot" reduction can be done, which is false
• The places where you Start "adding/removing" fat from first are genetically defined. Each person is different, some people start with their abdominal, other with their thighs...etc... so the place you "put" fat first is the last place from where fat will be removed. And fat is removed from the whole body, just as we said, where it starts/goes to, is genetically defined.
  
  

UPDATE (6/20/2012):

Based on requests on this site, to add scientific evidence, there is an article about this topic:

Source: Yale scientific magazine:Targeted Fat Loss: Myth or Reality?

The article mentions that Targeted fat loss is a myth and that fat is lost from overall body as i mentioned above. The magazine talks about two researches made by the University of Connecticut one in 1971 with tennis players and One in 2007 with:

104 participants completing a twelve-week supervised resistance-training program in which their non-dominant arm was selectively exercised.

Concerning the 1971 study, the scientific magazine states:

Targeted fat loss, also known as “spot reduction,” is a popular idea partly because it appeals to our intuition. After all, it seems perfectly reasonable to assume that the fat you burn while exercising comes from the area around the muscles you are using. Yet a 1971 study conducted by the University of California, Irvine on tennis players found that this is not actually the case. Tennis players constitute a population whose right and left arms have been consistently subjected to very different amounts of exercise over several years. Consequently, if spot reduction were a valid concept, one would expect the players’ dominant arms to have thinner layers of subcutaneous fat compared to their non-dominant arms. When the researchers measured the thickness of subcutaneous fat at specific points along the players’ arms, however, they found no statistically significant difference between right and left arms.

Concerning the 2007 study, the scientific magazine states:

More recently, in a 2007 study led by the University of Connecticut, 104 participants completed a twelve-week supervised resistance-training program in which their non-dominant arm was selectively exercised. MRI assessments of subcutaneous fat before and after the program revealed that fat loss tended to be generalized, rather than only occurring in the trained arm.

An important paragrah explaining fat loss:

The fat contained in fat cells exists in a form known as triglycerides. Muscle cells, however, cannot directly use triglycerides as fuel; it would be analogous to trying to run a car on crude oil. Instead, the fat must be broken down into glycerol and free fatty acids, which then enter the bloodstream. As a result, the fat broken down to be used as fuel during prolonged exercise can come from anywhere in your body, not just the part that is being worked the most.

The 2007 research's official publication (which yale's scientific magazine mentioned) mentions in their abstract's conclusion:

CONCLUSION:
Skinfold findings indicate that spot reduction occurred in men but not in women. In contrast, MRI found a generalized subcutaneous fat loss independent of gender, supporting the notion that spot reduction does not occur as a result of resistance training.

which means, MRI, a very accurate technique, found that fat loss is overall from the body and not targeted (spots).

PERSONAL experience:
Beside scientific researches, it is always good to share personal experience about a topic (sometimes it is more efficient than a research).
6 years ago i gained huge amounts of fat and then i went back to gym and diet to lose them. I had to lose around 60 pounds to get back in shape. Most of the fat i had was belly fat.

While my dieting and gym techniques were very efficient, i noticed that i was loosing fat from everywhere except my belly (very little from the belly).

• At some phases i had very skiny legs, fat upper body.
• Then skiny legs and arms but fat back and abs.
• Then only fat abs.

Until two summers ago where i got rid of 90% of my belly fat and now am back in shape.

From my personal experience, fat loss is genetically defined, but as i am not a doctor i cannot claim this is a general rule. But however the scientific researches i mentioned approve what i am saying.

Answer 2

For a more detailed and scientific explanation, OzmandiasReborn's post on reddit is valuable:

When you eat, your body detects the raised levels of glucose and fatty acids in your blood (having been absorbed through your intestine), and your pancreas releases insulin. Insulin essentially signals to your various tissues "WE ARE IN FED STATE, GUYS! EAT UP." This means that your liver stops making glucose on its own (gluconeogenesis drops), since this would be wasteful. Your liver starts breaking down the glucose (glycolysis: breaks down glucose to pyruvate, Krebs Cycle: Oxidizes acetyl-CoA (made from pyruvate), essentially stripping off electrons, oxidative phosphorylation: Takes those electrons and makes energy (ATP) out of it).

Simultaneously, your liver is replenishing stores of glycogen (a storage form of glucose). Your fat cells are taking up the extra fat in the blood, and turning those fatty acids (which are toxic) into a safe, neutral form: Triacylglycerides (TG or TAG). Your muscles are taking up glucose and fatty acids from the blood, and breaking them down the same way as in the liver: glycolysis--> krebs cycle --> oxidative phosphorylation, or for fat straight into beta oxidation.

NOW, on the other hand, if you haven't eaten in a while, your body needs to start using up its stored forms of energy, as well as make its own. This is signalled by glucagon (or in the case of stress, adrenaline/epinephrine). Your liver needs to break down the glycogen into its substituent glucose, and send this out to the body. Since your brain requires glucose (and at later stages can use ketone bodies), and can't deal with fatty acids, your liver needs to constantly be making glucose for your brain. The fat tissue (white adipose) is signaled to release its stores of trigylcerides into the blood, which are taken up my muscle (same process as before) and liver. The muscle can only break down the fatty acids into energy, but the liver can actually use energy from breaking down fatty acids and make new glucose, which it can then export into the blood.

So proximity doesn't really matter in the way you think it does. Imagine I live in an apartment building where a pipe loops around passing water through every apartment. Every apartment has a different type of filter that pulls different things out of the water. Every apartment also dumps their own special things into the water. This water is then chock full of all kinds of things related to the normal functioning/signalling of every apartment: the guy in 2B wants to give the woman in 10A something, while the woman in 6B wants to say to the landlord in the basement that he's hungry and needs a delivery of food.

So if I dump something in this pipe, it automatically becomes communal. If somebody is filtering stuff out right next door to me, then sure, they have a greater probability of picking up the actual fat molecule I delivered. But I don't know how useful it would be to think of it that way.

Also, fat is distributed either subcutaneously or viscerally. The former is fat deposits under the skin (e.g. thighs, buttocks, belly, etc.), while the latter is fat deposits on your organs (i.e. liver, heart, etc.). The latter is more typical of men than women, and is also a much stronger indicator of potential metabolic disorders like diabetes and insulin resistance. After menopause, fat redistribution to visceral is common in women.