White and Brown Fat
Probably the most important nutritional problem in today’s society is obesity.
Now, what is obesity? Essentially obesity is having too much fat in your body.
Now, what is body fat? Body fat is composed of adipocytes or fat cells stored in adipose tissue in the first place.
Let’s Take a Closer Look at Adipose Tissue
Adipose tissue has several functions, and a significant one is that it acts as heat or a cold insulator depending on how you look at it.
Individual animals have to endure freezing temperatures as they have a thick layer of fatty tissue to keep them warm. Typical examples would: a seal or a polar bear.
We also have a layer of fat, but we don’t need to keep ourselves warm because we carry clothing or we have central heating. So this function of adipose tissue may be necessary for individual animals, but for us, it has lost most of its appeal.
The second, and crucial function of adipose tissue is, it acts as an energy storage depot, and we all can recognise that when we overeat energy, the excess gets stored in the fat tissue.
The storage of fat was significant in the past. In prehistoric times when people suffer from long periods of famine, they need to effectively store the food as a readily available fuel in the form of fat tissue. And then during periods of starvation, they could recruit and mobilise the fat and use it to do their work and activities.
Nowadays, food is available everywhere. We don’t need to stuff ourselves during a certain period of the year to help us survive the winter. So this means that the energy storage function of fat tissue has become a lot less critical in the current human being.
The third function of adipose tissue emerged about 20 years ago when Friedman et al discovered leptin, and that is the so-called endocrine function of adipose tissue.
The ability of adipose tissue to produce hormones is vital as they help the adipose tissue communicate with the rest of the body, for instance, in regulating food intake. What’s interesting, most problems that arise in people with very little body fat are not because they don’t store fat, but because the production of certain hormones is disturbed.
Ok now, let’s take a closer look at a fat cell. Fat tissue is composed of fat cells or adipocytes that are very peculiar. And their main feature is a colossal lipid droplet. Also, it contains the essential organelles that other cells carry as well, that’s nuclei, mitochondria. They are very much pushed to the outside and are an efficient energy storage cell in the body.
Now how does fat get into these adipocytes in the first place?
So let’s start with the consumption of a fatty meal. We saw that the fat in our diet is processed in our GI tract and put into specialised particles called chylomicrons. Then, they deliver fat throughout the body. And it mainly goes towards two organs: the muscle and the fat tissue. And these two organs are characterised by the presence of a specific enzyme called lipoprotein lipase. Lipoprotein lipase breaks down the triglycerides in the chylomicrons, allowing the fat taken up into these tissues.
Now the activity of lipoprotein lipase dictates where the fat is going. Thus, if you decide to go for a run after that fatty meal, then the lipoprotein lipase in your muscle will be very active.
And most of the fat that you consume will be sucked into your muscles to be used as fuel. On the other hand, if you decide to sit on a couch and watch a movie, you’ll store most of the fat because the lipoprotein lipase in the fat tissue will be most active. So that’s how fat gets stored into the adipocytes.
Fat ~ Going In and Out of the Cell
What happens in the adipocytes, there is a constant balance between how much fat is going in and out. So what you see is that when people are gaining weight, more fat is going into the fat cell compared to the amount of fat that leaves it. In contrast, people that are losing more fat are leaving the fat cell compared to the amount of fat that goes in. At least, chronically, over an extended period.
What we see, if you look throughout the day is that a fat cell constantly switches from being a net consumer of fat to being a net releaser of fat. So if we start in the morning, when you wake up and your stomach is empty, you rely on your internal fat storage to mobilise energy to allow you to function that serves as an energy source.
Hence, fat breaks in your adipocytes, so more fat is leaving the fat cell than the amount of fat that comes in. After that heavy breakfast, it switches so more fat is now going into the fat section as opposed to the amount that leaves the fat cell. And that continually changes throughout the day, and you’d hope throughout 24h, the two balance out each other. It means that over 24h the amount of fat that goes in equals the amount of fat that goes out and you would be in fat balance and therefore in energy balance, making you weight stable.
Ok, now, total fat mass is determined by the number of fat cells and the size of the individual fat cells. Which means that the size of a fat depot can increase because of two reasons:
- the individual fat cells can grow; this is called hypertrophic obesity.
- The number of fat cells can improve; this is called hyperplastic obesity.
Now in most people, the number of fat cells remains constant throughout adulthood.
That means moderate changes in fat mass mostly occur via changes in the size of the individual fat cells. So this means that the fat cells expand when you gain weight and they shrink when you lose weight.
What we also know is that the average lifespan of a fat cell is about ten years. And after those ten years, a fat cell is renewed. So when we lose fat cells, we also gain them, and this is called fat cell turnover. Now, at the same time, the content of the fat section is renewed much more quickly at a rate that is about six times faster.
The Brown Fat
Now I’ve talked to you about white fat, white adipose tissue (WAT) but there is also another issue that I haven’t addressed yet, and that’s called brown fat.
Brown fat exists in certain mammals. Brown fat is involved in a process that we call cold-induced thermogenesis. It means that the tissue can produce heat and especially during the cold, so it allows the animal to keep itself warm by a process that is called chemical uncoupling.
Similarly, to do the process mentioned above, the brown fat needs a lot of mitochondria which are very abundant in brown fat cells. And another difference between brown and white fat cells is that it doesn’t contain one large lipid droplet but multiple smaller lipid droplets.
Now brown fat is abundant in a great variety of species including hibernating animals and cold-adapted small animals such as mice and rats. And it is activated when these animals are exposed to the cold resulting in the production of heat and allowing the animal to keep itself warm.
Now, the reason why I am telling you about this is that for several years we thought that brown fat is not particularly important for adult humans, but this all changed in 2009. As many papers that were published together in the New England Journal of Medicine showed that adult people do contain brown fat.
The technique called PET-CT scan discovered brown fat. Basically what you do with this technique is you monitor the uptake of radioactive glucose in different tissues of the body. What they say is that when these people were cold, they saw certain areas of the body light up and they realised after a while that this represented the uptake of glucose into these brown fat cells.
Since then, this whole field has exploded because people have realised the potential importance and impact of this finding, and it means that people could lose weight
by stimulating the heat production in the brown fat cell. It all depends on how crucial brown fat turns out to be in humans; at this point, we don’t have the answer yet. But the results we have so far are quite promising. What researchers also found that obese individuals seem to have less brown fat!
So, in summary, what I have told you is that fat tissue is vital. That has several functions including insulation, energy storage and an endocrine, hormone-producing organ. We have learned about how fat gets stored in the adipose tissue. And we have known how it is mobilised and undergoes a constant switch throughout the day. And in the end, I have told you about a new type of fat called “brown fat.” That has excellent promise as a future target potentially for obesity!
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