A bit of jargon: in everyday language we tend to call the solid form of fat fat and the liquid form oil. These are both based on room temperature conditions. In the body we tend to use the collective term lipid to refer to both fat and oil.
Doctors most commonly use the term lipid when talking about blood lipid profile, e.g. triglycerides and cholesterol. But lipids are spread throughout the body and essential to life.
Lipids make up the membrane around our cells, so not a single cell is fat free. Also, most cells can use fat as fuel, so it’s normal to have some fat in our cells and therefore pretty much throughout our body.
That should already make it clear that lipids are part of our very structure. Lipids also form key parts of the communication systems of our bodies.
Our cells use specific lipids to communicate with each other, for example a group of molecules called eicosanoids are made from specific fats and help regulate inflammation.
Cholesterol, yet another lipid, is needed to make steroid hormones such as cortisol, oestrogen, and testosterone. Although that's not all it's needed for.
So lipids are part of our structure and a means of communication in the body. What we probably think of though when we think of fat is energy.
Whereas we can only store a few hundred grams of carbohydrate, and don't really store protein at all, we can store many kilograms of fat.
Where and how we store this fat plays a big part in determining our health prospects.
Where's the fat?
While carbohydrates are stored predominantly as muscle and liver glycogen, we actually have specialised cells to store fat. These cells are called adipocytes. Clusters of adipocytes are often referred to as adipose tissue.
We tend to divide adipose tissue into two pools. We call the most visible fat, the fat under the skin, subcutaneous fat. And we call the fat around the organs visceral fat.
It's normal to have some fat in our cells for the purposes described above, but we don't normally see the kind of distinct droplets you see in the adipocytes in other cells.
However, in conditions like fatty liver (1), fatty heart (2), and fatty pancreas (3), cell types other than adipocytes begin to store excess fat. Excess fat in our muscles can also be a problem (4).
In the case of the liver we call these cells hepatocytes. You can see white droplets of fat in the hepatocytes of someone with fatty liver below. Healthy livers don't have this salami-like look!
Fat isn't all created equal when it comes to health. Subcutaneous fat, although it gets the most discrimination in the media and real life isn't the big problem, at least not for our metabolism.
You’ve probably heard that ‘belly fat’ is more detrimental than general subcutaneous fat. The research confirms this by clearly showing close associations between visceral, and to some extent abdominal subcutaneous fat, with cardiovascular and metabolic (e.g. diabetes) health (5-12).
Beyond where the adipose tissue is, it can itself be more or less healthy (7). Good quality adipose tissue has the capacity to expand and store surplus fat.
When fat storage in subcutaneous fat is impaired for some reason, any excess fat gets shunted to the visceral adipose tissue or into the organs.
The problem is we can't see when this is happening because this 'ectopic' fat doesn't necessarily reflect the visible fat we can see or measure. Consider the two livers below.
Liver fat of greater than 3-5%, depending on the method of assessment, is considered excessive (14-16), so both these people qualified as having non-alcoholic fatty liver.
Visceral fat often gives a good indication of liver fat such that when visceral fat is high, so is liver fat and liver insulin resistance (9).
That said, the problems of visceral fat and liver fat don’t always go hand-in-hand (17). In other words, fatty liver on its own is a problem.
The problem of excess liver fat
The most common type of fatty liver these days is non-alcoholic fatty liver disease or ‘NAFLD’ (18), which is exactly what it sounds like – fatty liver in people with moderate-to-no alcohol consumptions. Another major type of fatty liver is alcoholic fatty liver (19).
Fatty liver can cause a number of liver conditions, but because the liver is so central to how our body deals with lipids, carbohydrates, and most everything else we ingest or otherwise absorb, the effects of impaired liver health can often be seen throughout the body.
The liver specific problems include progression to inflammation and fibrosis (scaring) called steatohepatitis, most commonly non-alcoholic steatohepatitis or ‘NASH’ (20). In some cases this can then lead onto liver cirrhosis and/or liver cancer, specifically hepatocellular carcinoma (21,22).
The liver is heavily involved in dealing with drugs, medication and otherwise, and its ability to do so is changes when it becomes fatty (23-26).
The most likely follow-on to NAFLD is cardiovascular (i.e. heart and blood vessel) disease (20). This is followed by various non-liver cancers (27-29), with the clearest association being to colon cancer (30-33).
Given that this blog has talked quite a bit about the importance of the liver in glucose regulation, it should come as no surprise that fatty liver is often associated with type 2 diabetes and the ‘prediabetic’ conditions of metabolic syndrome, impaired fasting glucose, and impaired glucose tolerance (20).
Lastly, liver fat seems to predict kidney disease even when we try to take the influence of diabetes out of the equation (literally as we use statistical methods to do this) (34-38).
These findings, and experimental work suggest that excess liver fat does have a cause and effect relationship, at least with the mechanisms underlying type 2 diabetes.
Fat and the pancreas
If you’ve been following this blog, you’ll know that the pancreas also gets fatty as well as shrinking and changing shape in type 2 diabetes (3,39-41).
Reduction of pancreatic fat occurs with weight reduction and closely mirrors improvements in beta-cell (the cells that produce insulin) function (3,39,42).
So in the pancreas too we see excess fat leading to impaired function, and reductions in intra-organ (within-organ) fat leading to improved function.
Fat and muscle might seem mutually exclusive until you think of a well marbled steak, or you look at the image below depicting two thighs. The one on the left is lean, the one on the right fatty, much like that well marbled steak.
That’s bad news when you consider that normally our blood stream is intended to only carry about a teaspoon of glucose, our liver is good for 100 g or so, but our muscles can take up a few hundred grams (43).
The exact amounts depend on how much blood, how big a liver, and how much muscle, but you can see there’s going to be a problem if the organ with the biggest capacity gets taken out of the picture.
What should happen after a carbohydrate containing meal is that a good proportion of that carbohydrate should be shuttled into our muscles. This doesn’t happen in type 2 diabetes, at least not while we’re sedentary, so the liver has to deal with the resulting glucose, and what it doesn’t deal with spills over into the blood and other tissue (44).
Here it’s not just a case of where the fat is, but why it’s there. Athletes also tend to have higher intramuscular fat, but exercise itself sensitises muscle to insulin (45). Will look at this in more detail in future posts.
Is there a fat threshold?
The question is why, when we clearly have the capacity to store fat in relatively safe subcutaneous adipose tissue, does it end up around and inside our organs?
We have several specific and highly detailed explanations involving endless acronyms that are hard to pronounce and stand for names that are even harder to pronounce. However, one elegant high level explanation is that we have a fat threshold.
In other words, we have a certain set capacity, that if exceeded leads us to store fat in the wrong places and in the wrong way.
A fat threshold elegantly explains why some people can have a very high body fat and yet have a relatively normal metabolism, and another person can be relatively lean and have the problems we commonly associate with obesity, including type 2 diabetes (46).
Have another look at the earlier image of the relatively lean man (20% body fat) with a similarly fatty liver to a woman with a body fat percentage of 45%, to see a real-world example.
So to recap. Fat under the skin isn’t the main problem, fat surrounding the organs, and excess fat within the cells of organs is the big problem. These types of fat aren’t always very closely linked, so what you can see won’t necessarily tell you about what you can’t see.
In part 2, we’ll explore how ectopic fat is measured, and how you can get an idea if it’s a problem without expensive scans. Most importantly, we’ll explore the concept of the fat thresh-hold and how understanding some of the reasoning behind it can help you focus on actions that will reverse the problem.
Click here to read part 2.
References: click here for a full list of references
To learn more about type 2 diabetes and what you can do to better manage, and often reverse it, watch my free video education series. If you want one-on-one help dealing with your type 2 diabetes, contact me here to book a consultation.