Fat

Fat is a nutrient that ...

50% of body fat is stored directly underneath the skin.

Contents

1   Function

Fat serves as padding and insulation for the body.

2   Storage

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Adipose tissue is specialized connective tissue that functions as the major storage site for fat in the form of triglycerides. [1] Adipose tissue is found in mammals in two different forms: white adipose tissue and brown adipose tissue. Most adipose tissue is white. [1]

White adipose tissue serves three functions: heat insulation, mechanical cushion, and most importantly, a source of energy. Subcutaneous adipose tissue, found directly below the skin, is an especially important heat insulator in the body, because it conducts heat only one third as readily as other tissues. The degree of insulation is dependent upon the thickness of this fat layer. For example, a person with a 2-mm layer of subcutaneous fat will feel as comfortable at 15°C as a person with a 1-mm layer at 16°C. Adipose tissue also surrounds internal organs and provides some protection for these organs from jarring. [1]

As the major form of energy storage, fat provides a buffer for energy imbalances when energy intake is not equal to energy output. It is an efficient way to store excess energy, because it is stored with very little water. Consequently, more energy can be derived per gram of fat (9 kcal.gm-1) than per gram of carbohydrate (4 kcal.gm-1) or protein (4 kcal.gm-1). [1] The average woman with 20% body fat has about one month of energy stored as fat. [1]

There are some constraints on the use of fat as fuel. Most animals cannot convert lipid into carbohydrate. Tissues that function predominantly anaerobically (e.g.. erythrocytes) must rely on carbohydrate for energy and need to have an ample supply available. Additionally, under normal conditions the brain is dependent upon glucose for energy and does not use fatty acids. In unusual metabolic circumstances, the brain can use ketone bodies (a by-product of incomplete fat metabolism) when they are present in sufficiently high quantities. [1]

Brown adipose tissue, which derives its color from rich vascularization and densely packed mitochondria, is found in various locations, depending upon the species and/or age of the animal. During maturation, in non-hibernating animals, brown adipose tissue is metabolically less active, although cold exposure can activate it. In hibernating animals and neonates, brown adipose tissue is important for regulating body temperature via non-shivering thermogenesis. [1]

In adult mammals, the major bulk of adipose tissue is a loose association of lipid-filled cells called adipocytes, which are held in a framework of collagen fibers. In addition to adipocytes, adipose tissue contains stromal-vascular cells including fibroblastic connective tissue cells, leukocytes, macrophages, and pre-adipocytes (not yet filled with lipid), which contribute to structural integrity. [1]

The lipid droplets in adipose tissue can be unilocular and/or multilocular. Unilocular cells contain a single large lipid droplet which pushes the cell nucleus against the plasma membrane, giving the cell a signet-ring shape (Figure 1). Unilocular cells, characteristic of white adipose tissue, range in size from 25 to 200 microns. Mitochondria are found predominately in the thicker portion of the cytoplasmic rim near the nucleus. The large lipid droplet does not appear to contain any intracellular organelles. Multilocular cells, typically seen in brown adipose tissue, contain many smaller lipid droplets. A cell in brown adipose tissue may reach a diameter of 60 microns and the lipid droplet within the cell may reach 25 microns in diameter. [1]


Adipose tissue is made of adipocytes, which are cells that store triglycerides in the form of small fat droplets. Adipose tissue contains about 80% triglycerides and some 1-2% protein, and the remaining part is water plus electrolytes. During weight loss adipose tissue decreases: the actual fat loss will be about 80% of the actual weight loss. [6]

2.1   Distribution

People carry their adipose tissue in different anatomical location. The body stores adipose tissue predominantly on the upper body in males, and predominantly on the lower body for females

Fat distribution is determined by genetic background, gender, and age.


Beyond the generalized sex differences, there are a number of hormones that affect where and how we store fat - for ex. estrogen encourages the more 'classically feminine' fat storage patterns (hips and thighs) whereas stuff like cortisol and insulin production lead more to belly fat. As the latter two, cortisol in particular, are highly stress-dependent, people may even notice different fat storage patterns at different stages of life, depending on things like diet and levels of stress or of sex hormones.

Different ethnic groups can also have genetic differences in fat storage which change their risk towards cardiac health, etc. South Asians in particular are at a much higher risk of cardiac events compared to Europeans and European descendants. They're genetically predisposed to storing much higher levels of visceral fat, which is why their body type tends to be skinny with a big gut.

It's worth noting that with varying levels of estrogen/testosterone, the body's fat storage patterns will change. For example, if a trans man begins hormone replacement, the testosterone will cause fat in the body to be deposited in a more masculine pattern, and vice-versa.

3   Classification

There are four types of fat: monounsaturated fatty acids (MUFAs), polyunsaturated fats (PUFAs), saturated fat, and trans-fat.

3.1   Saturated fat

Saturated fats are found almost exclusively in animal products (two exceptions are coconut_ and `palm kernel oil`_). They are solid at room temperature.

3.2   Trans-fat

Trans-fatty acids (= partially hydrogenated vegetable oils) are a man-made fat made by bubbling hydrogen through `vegetable oil`_ to make it semisolid with a longer shelf-life. For example, margarine.

4   References

[1](1, 2, 3, 4, 5, 6, 7, 8, 9) Ann L. Albright and Judith S. Stern. 1998. Adipose Tissue. http://www.sportsci.org/encyc/adipose/adipose.html
[6]Paul Deurenberg. 2009. Body Composition.

If you mean adipose tissue, the adipocytes, which contain uni and multilocular cells, lose some of their size (they shrink). The main difference between the two is that the latter contain considerably more cytoplasm (the substance that makes up the cell's internal structure). When you lose weight, the triglycerides (stored fat) contained in those cells are turned into fatty acids by various hormones, such as adrenaline and noradrealine, and then bound to various proteins to form lipoproteins so they can enter the bloodstream. From there, they are used in lipolysis and result in the creation of Adenonine triphosphate (energy) through the electron transport chain of the body's tricarbolyxic acid cycle (one of the main metabolic pathways). However, not only fat is lost when weight is lost. Some muscle tissue is also lost. All dietary proteins are broken down into amino acids, which are sent to the body's nitrogen pool. From there, amino acids are used for various purposes, including the creation of new muscle tissue. But if the nitrogen pool has insufficient supplies of certain amino acids, muscle tissue will be broken down so its amino acids can be sent back there and used for more vital functions. Sadly, my knowledge of biochemical principles is not advanced enough to tell you to what extent, but it definitely happens.

That's why in fitness circles the adage of 1g protein per 1lbs of lean body mass is touted. Although even in complete novice beginners, there was no net benefit seen eating more than about 0.8g per pound of lean body mass, 1 is an easier number to work with and requires less maths, and if you're not eating complete proteins, you have a little bit of fudging room.


hen you consume any amount of fat in your diet, the cells in your small intestine will package those fatty acids into things called chylomicrons. When you think of a chylomicron, think of the fat-equivalent of a water balloon, but at the molecular level. Chylomicrons have a number of different molecules, but you might say that their primary function in the body is to transfer any fat you ingest to wherever the body needs it to go, be it for energy use or energy storage. All the fat that's being transported via these chylomicrons is done so in the form of triglycerides, which are three fatty acids attached to a 3-carbon molecule called glycerol.

After the small intestine cells package the fat into these chylomicrons, the chylomicrons are sent throughout the body by entering the lymphatic system, which then eventually pours into the blood stream via the subclavian vein. The blood takes these small packages of fatty acids throughout all tissues in the body. (Side note--this is oversimplified: fatty acids of different lengths have different pathways, but this will give you the basic idea without overloading you with too much detail that you don't care about.)

The main way the body regulates where this fat is stored is by regulating the specific protein that these chylomicrons dock to that will cause the fatty acids to be transferred to whatever cells are in a given area. Keep in mind that these could be any cells in the body, including skeletal muscle cells (which would primarily metabolize the fatty acids for energy production), as well as adipocytes (the name for cells whose primary function is to store fat in the body for use during periods of fasting, starvation, or simply between meals in a given day). The name of the protein that is responsible for transferring these fatty acids from chylomicrons to neighboring cells is called lipoprotein lipase, and you'll see it commonly abbreviated as LPL.

This protein is located in capillaries throughout the body. When a passing chylomicron encounters LPL, a receptor on the exterior of the chylomicron known as Apo-C allows for the chylomicron to dock to the LPL protein, and LPL begins to drain the chylomicron of the triglycerides it contains and hydrolyze them to make free fatty acids--they're not attached to a glycerol molecule anymore. These free fatty acids then float near cells that are found near that capillary where this is all taking place and are free to pass through the membrane of whatever cell is in the vicinity due to how similar they are to the general structure of cell membranes. Once inside the cells, these fatty acids are restructured into a triglyceride by using a glycerol molecule that was created within the cell, and the triglyceride is now able to be stored or used for energy.

So now to your question: how does the body regulate where fat is stored? Why do we accumulate fat in our stomachs, butts, and thighs instead of our foreheads or the backs of our hands? This is primarily determined by where and to what degree LPL is expressed. I believe there are several ways LPL is genetically regulated, but they only ways I've learned about are hormonal regulations. For example, when you consume a diet containing both carbohydrate and fat molecules (which will be most any meal you eat, really), the insulin that's expressed will spread throughout the body and interact with all the cells it reaches in several different ways. One of those ways is going to be by inducing an upregulation of LPL in those parts of the body that the body desires fat to be stored or taken up. As others have mentioned, other hormones affect the body's fat distribution in several ways, and I admit I don't understand the specific mechanisms for how these hormones all work on LPL, but I know that insulin is one of the major hormones involved.


This is part of the emerging understanding that people don't get fat because they eat too much, they eat too much because they're getting fat.


Because there different kinds of fat for particular functions (particularly in females). Android fat, is the kind that accumulates across the whole of the body when there's an excess of calories. Gynoid fat, plays a role in fertility and accumulates to the breast and hips and does not fluctuate as drastically as android fat when your losing weight. This is why there's a common trope "Lost the weight, kept the boobs".


Fat is stored at an intracellular level in the cell plasma near and in mytochondria, especially in cells who need a lot of aerobic energy such as type 1 (endurance) muscle cells. It is well documented that endurance athletes have more and better positioned fat/lipid droplets with reguard to their mytochondria inside the cells for optimal aerobic capacity. The opposite is also well documented, as in a sedentary person has worse positioned lipid droplets inside the cells which makes their cellular aerobic capacity very limited. As for the storage, it is also documented that cells adapt to energy needs, as in the endurance athlete will store more fuel (fats as lipid droplets and carbs through glut4 translocation through the membrane) and more importantly be able to produce the energy at a much faster/more efficient way.


Mostly the adipocytes, the fat cells, when they're triggered just release fatty acids and the blood moves it around bound to some proteins. They're not locally targeted really. The liver sends the body signals when it starts to get a low on blood sugar supply and triggers the fat cells to send out free fatty acids. The liver doesn't direct it to a special place, and the blood just carriers it wherever cells need energy. Fat -> blood -> muscle. Which muscles you're exercising don't control where the fat comes from.

The body doesn't target which area of the body to burn off its fat storage first, but they do have priorities on where to store the fat first: the center of the body. The body decides to burn fat or muscle depending on your activity rate. Muscle gets stronger by being torn apart and then getting rebuilt, and uses fat (stored energy) only when there's nothing else to burn (your daily calorie intakes). If you're not tearing your muscle apart working out, though, then the muscle deteriorates because it's not being used, and the fat storage builds up because there's no need to use excess energy anymore.