small question on fat

[the last donut of the night]

basically, where does the fat stored in your body come from? it may sound like a dumb question, but i've read many different answers from different sources. some people say most of it comes directly from the fats you ingest, others say it comes from carbs being processed into fats by your metabolism. can someone answer this for me?

(sorry if it's a shockingly obvious answer)

[PhoenixAsh]

The simple answer is: from excess calories.




The more complex answer is:

Depending on your diet it may come from excess calories of different macro nutrients (fat, carbohydrates, protein or a combination there of) or a subdivision of these (especially trans-fat as a specific kind of fat and succrose as a specific kind of carb) and is influenced by your genetic predeposition to be able to metabolise them....or health factors which may impair metabolism.


Excess calories are caused by consuming more nurtrients (food) than you are able to burn and convert to energy.

Fat is not the primary cause for storing fat. And (without genetic or health defects to metabolism) getting fat does not necessarilly mean you eat or drink too much fat. It can also mean you eat or drink too much sugar....or too much protein. About anything...even eating too much fruit and vegetables can cause fat storage.

However...some macro nutrients have a higher propensity for fat storage or may disrupt the way your body store fat. Succrose and Transfat are the two that spring in mind. There are others.


1 pound of body fat equals approximately 3500 calories.

calories are measures of energy provided by the nutrients.


Conversely to the above...eating less than you burn per day will mean your body will use its reserves.

[TC]

basically, where does the fat stored in your body come from? it may sound like a dumb question, but i've read many different answers from different sources. some people say most of it comes directly from the fats you ingest, others say it comes from carbs being processed into fats by your metabolism. can someone answer this for me?

(sorry if it's a shockingly obvious answer)


When you eat, the presence of glucose, amino acids and fatty acids in your digestive tract triggers the pancreas to secrete insulin, and insulin causes your fat cells to absorb fatty acids, glucose, and amino acids - the cells literally get bigger.

While glucose and amino acids can be stored in fat, fatty acids are stored in fat much more efficiently - so while some of the energy in glucose and amino acids is actually expended in converting them into fat cells, almost all the energy in fatty acids is stored. So the diet mantra of "a calorie is a calorie and all the same whether its protein, dietary fat or carbohydrate" is actually quiet false on the cellular level.

...However...simple carbohydrates (like simple sugars) rapidly increase blood glucose levels and stimulate insulin release which is the mechanism that encourages fat storage - whereas when blood glucose levels are low the body breaks down stored fat - so simple carbohydrates also contribute to fat storage but in a very different way. You can see how there is a certain intuitive logic to both high (complex) carb/low fat and high fat/low carb diets.


This is of course overly simplified...but more than that, human metabolism is extremely complicated and involves many different hormonal systems and homeostatic relationships. It is an area with significant legitimate continuing scientific debate. There are unfortunately lots of really reductionistic almost metaphorical explanations (such as "its storing more calories than you burn" which while obviously true as a matter of thermodynamics, is also obviously useless because what you eat effects how much you burn and how much you store.)

[the last donut of the night]

thanks. my diet is largely based on fruit and vegetables and whole grains. although i eat meat, it doesn't make up a huge portion of what i eat daily. how does fat absorption work with a largely vegetarian diet, for example?

[Red Commissar]

The fat in your body is mostly formed from the glucose that is extracted from your diet or fats themselves you eat. Glucose enters into your body's metabolism in a process called Glycolysis (http://en.wikipedia.org/wiki/Glycolysis). The end product of this is Pyruvate, which can go a number of ways. For simplicity I'll say it goes into the Citric Acid Cycle (TCA cycle), which involves it being converted into a compound called Acetyl-CoA before it can go in this.

As this cycle goes on it causes a large amount of ATP to be formed. However the body doesn't need to use all the glucose you are taking in, so it will take the Acetyl-CoA left over and re purpose it into fat. By a number of steps starting with a conversion to Malonyl-CoA, eventually the Acetyl-CoA becomes a fatty acid. Fatty acid comprises the main component of lipids- three fatty acids are usually compounded into a glycerol head and makes a triglyceride.

Fatty Acids are stored and can be broken apart when needed. One one path this yields Acetyl-CoA through beta-oxidation which can be fed into the TCA cycle for ATP production for energy.

Here is a diagram drawing it out

http://oi54.tinypic.com/2whgetk.jpg

Follow up to Acetyl-CoA and see that it leads into Fatty Acid. You'll see in the upper right then that fatty acid can be broken down and can lead back into Acetyl-CoA for energy production.

I'm really oversimplifying it but that's the gist of it. Most of your body's energy is in the form of fat and therefore by extension what you'll end up "burning" in the long run.

[Niccolò Rossi]

TC's actually done a pretty good job in her post. There are a couple of inaccuracies that should be rectified though.


When you eat, the presence of glucose, amino acids and fatty acids in your digestive tract triggers the pancreas to secrete insulin

Fatty acids have little/no effect on the secretion of insulin. Keep in mind, insulin's primary function is to regulation blood sugar. If your body does not produce (sufficient) insulin (ie. diabetes) your blood sugar levels would be elevated uncontrollably (hyperglycemia), followed by the onset of ketoacidosis and potential death. Insulin and blood glucose in people with healthy pancreatic function exist in a kind of see-saw relationship. As blood glucose rises (from for example the consumption of carbohydrates), insulin is secreted to shuttle the glucose off into fat and muscle cells. Amino acids (from protein) are also insulinogenic. Fats on the other hand are not (although large quantities of fat with a meal containing carbohydrates and fat may increase insulin secretion by causing a temporary insulin resistance).


While glucose and amino acids can be stored in fat, fatty acids are stored in fat much more efficiently - so while some of the energy in glucose and amino acids is actually expended in converting them into fat cells, almost all the energy in fatty acids is stored.The first half of this statement is entirely correct.

The second half on the other hand gives off the wrong idea. When glucose and fatty acids are present, glucose will be used preferentially by the body. So yes, upon the consumption of a meal containing carbohydrates and fats, glucose will be utilised by the body immediately while the bulk of the fatty acids will be stored as triglyceride in the body's adipose tissue (fat cells). However, taken over the course of hours, or an entire day, these stored triglycerides need not be a problem. Provided the total energy balance between energy consumption from food and energy expenditure from basic metabolic functions, physical activity, etc. is equal, the net result will be no weight gain or fat gain. In the intra-meal hours, when the avaliable glucose has been utilised, the triglycerides deposited in the adipose tissue from our meal will be liberated as free-fatty acids into the blood stream to be utilised for energy.

In a caloric deficit this is even more pronounced, where accumulated triglyceride formerly stored as body fat will be metabolised for energy. This results in weight loss and body fat loss.

Only in a net caloric surplus will we see a net gain in adipose tissue since the body will have no need to liberate triglycerides in the presence of sufficient calories from glucose. A caloric surplus will result in a net accumulation of adipose tissue.


whereas when blood glucose levels are low the body breaks down stored fatAgain, this is only the case in the presence of a caloric deficit, without which the body will have no recourse to liberating triglycerides for energy.

Probably related to this, a common misconception is that glucagon (a hormone that has the exact opposite effect of insulin in relation to blood sugar, being released when blood sugar drops too low, liberating glycogen from the liver to raise blood glucose), has a fat mobilising effect. Advocates of ketogenic/atkins type diets often claim this incorrectly. Whilst glucagon does exhibit fat mobilising effects in mice, it does not appear to have the same effect in humans.

Nic.

[Niccolò Rossi]

thanks. my diet is largely based on fruit and vegetables and whole grains. although i eat meat, it doesn't make up a huge portion of what i eat daily. how does fat absorption work with a largely vegetarian diet, for example?

No different than if you were eating meat. Fats are still fats. Carbs are still carbs. Proteins are still proteins.

Nic.