An extremely common modern misconception about food is that eating fat makes us fat. But the truth is: fat makes us thin, and sugar makes us fat. (There is a TL;DR at the end)
Dietary Recommendation History
The idea that fat is bad for us started after U.S. President Eisenhower suffered a heart attack and our nation was looking for an explanation for why he had the heart attack and how to avoid heart attacks.
Scientists quickly pointed out that such heart attacks were/are caused by a buildup of cholesterol within veins. However, no one knew for sure how the cholesterol built up within veins. Two theories were put forward. The first is that consuming a lot of sugar results in cholesterol buildup. The second theory was that consuming fat results in cholesterol buildup.
Scientists settled on the second theory and our society has been transforming itself accordingly ever since. It's an easy-to-understand theory and matches how we intuitively think our bodies work; eat fat and it instantly becomes a part of our bodies, permeating us through and through.
Because of our cultural expectation to reduce our fat intake and companies have been stripping fat out of foods to make them low-fat or non-fat. Removing fat often sacrifices flavor and because
flavor predicts nutrients, stripping out fat means stripping out nutrients.
To compensate for the reduced flavor and to keep food appealing, companies often add sweeteners to their products, primarily sugar. That sugar might take the form of apple juice, grape juice, orange juice, or some other healthy-sounding ingredient, but it is sugar nonetheless.
Sugar is the true cause of cholesterol buildup, so substituting sugar for fat unwittingly compounds our national obesity problems.
Digestive Defenses
Our intestines are like our skin. The digestive tract is designed to keep unwanted or unneeded things out of our body. And by "body", in this case, I mean bloodstream. Once something enters the bloodstream it can travel anywhere in the body quickly and can start causing problems almost immediately. Having a strong protective barrier reduces the chances of something getting through.
Mostly this protection results in keeping bacteria out. The acid in our stomachs kills most bacteria which come along with food. To catch the bacteria which might survive the stomach, the intestines have a tight link with the immune system.
This protection also applies to keeping molecules which we cannot use outside of our bodies. Sometimes the intestines will neutralize molecules, sometimes it will transform molecules, and sometimes it will purge molecules (including violently).
On the flip side, the body needs nutrients in order to function. While the digestive tract is keeping bad molecules and bacteria out, it will allow good molecules into the body and will allow good bacteria to continue to live in the gut. Good bacteria will either transform larger or bad molecules into useful molecules, or will neutralize or absorb such molecules.
The intestines are lined with a tight-nit collection of cells which are the primary barrier protecting our bodies. Those cells will absorb molecules from within the intestines, transform the molecules if needed, and pass those transformed molecules through to the other side of the cell and into the bloodstream. When needed, those intestinal lining cells accelerate molecule absorption by loosening their barrier.
The body generates certain molecules which signal the intestinal lining to loosen. Normally those trigger molecules come from via the bloodstream. However, those trigger molecule can also come from external sources. External triggers can be good such as when we consume an NSAID-based pain killer. But external triggers can be bad when the trigger molecules enter our body unintentionally. For example, gluten is such a trigger for people who are either sensitive to gluten or have celiac disease; regular consumption of gluten causes a continual flood of unwanted molecules and bacteria which the body must continually fight. For people with an auto-immune disease, it appears that they are frequently (if not always) sensitive to gluten (although more formal scientific studies need to be performed).
But I digress. The real point is that the intestines are a barrier against unwanted bacteria and molecules and under normal conditions it has a significant amount of control over what molecules are allowed into the bloodstream. Some molecules which it doesn't have control over are those which trigger the intestines to loosen up.
Why Sugar Makes Us Fat
Most foods gathered in nature contain some sugar, and the levels of sugar are manageable by our bodies. We often consume food because we need energy, but it takes different amounts of time to extract the energy we need depending on the source of that energy: sugars, complex carbohydrates, fats, or proteins. Sugars are immediately usable but supply energy for about 20 minutes. Complex carbohydrates produce energy for about 2 hours after consuming them. Fats take about 2 hours to start getting energy from them and they can produce energy for roughly the next 2 hours. Finally, proteins take about 4 hours of processing before the body gets energy from them and they also supply energy for about the next two hours.
Our body absorbs sugar immediately because the body often needs some short-term energy. The energy supplied by an unprocessed food will be enough for the body before it starts getting energy from the consumed carbohydrates.
In modern times, however, humans can extract sugar, concentrate it, and add it to foods. That processing allows food to taste better but also supplies far more sugar than the body needs before the carbohydrates kick in. One of the signs of this oversupply is the so-called "sugar high" we might get, and the following "sugar crash" 30 minutes later.
So why do we get that high and the following crash? Let's look at how the body processes sugar.
Sugar is a molecule which the body readily absorbs into the bloodstream. Common table sugar (
sucrose) is made up of 2 separate sugars in equal proportions:
glucose and
fructose. There are a lot of other sugars, but these are the two I will focus on here because they are the most common both naturally and within the standard American diet.
When we ingest sucrose our body immediately splits it into glucose and fructose. Then
several separate proteins in the intestinal lining transport those molecules into the bloodstream.
The absorbed glucose is either consumed by other cells in our body or is transported via insulin into long-term fat storage. If our body detects too much glucose in the bloodstream then it will pump out a lot more insulin to push that glucose into fat cells.
Fructose, is typically absorbed by the liver and it must go through many long chemical processes to be converted into a more-usable structures.
But in short, it looks like glucose must be consumed immediately or driven into cells for later use while fructose sticks around attaching itself to other molecules.
The exact details of how sugar causes obesity are contested in the scientific community. Some studies of fructose have shown a correlation while others have shown no correlation, although the sources of the negative studies are somewhat suspect because they are funded by companies which are heavily invested in sugar as an ingedient.
By the way, carbohydrates (simple and complex, and especially starches) are really chains of sugars (this is a gross simplification, so pay attention to the general ideas here). Our bodies cannot immediately absorb these molecules so our bodies rely on bacteria in its gut to breakdown the carbohydrates into their component sugars. Then those sugars are absorbed into the body. Consequently, lots of carbohydrates will yield lots of sugars...just not as quickly as raw sugar.
Why Fat Makes Us Thin
Fat helps us get thin for a couple of reasons. Most importantly, fat is needed for us to feel satiated. Secondly, fat doesn't immediately cross the intestinal barrier into the bloodstream; the body can choose when to absorb it.
Satiety comes because our bodies feel like they have enough nutrients. Our body needs fats/oils so that cells can manufacture essential molecules (e.g. neurotransmitters). In addition, as mentioned earlier, fat allows our body to have longer-term energy. Because fat is a precursor to essential molecules and it can be used for energy, having enough fat in our food helps our body feel nutritionally satisfied.
When our body is
nutritionally satisfied then it feels like its nutritional environment is safe, at which point it can start eliminating its fat reserves.
Bad Fats
Although fat can help us get thin, the kind of fat we consume is an important factor. Fats can also be bad when we consume them in the wrong proportions. There are several different fat categories: saturated, monounsaturated, and polyunsaturated.
Saturated fats have a longer shelf-life than the other categories of oil. They also tent to be solid at room temperature. Saturated fats are
not necessarily bad, but should be consumed in moderate quantities.
Monounsaturated and
polyunsaturated fats are considered unsaturated because there is room on the molecular chain to accept more hydrogen atoms. A single "kink" in the chain makes it monounsaturated. More than one "kink" makes it polyunsaturated.
Unsaturated fats have a shorter shelf life, are found more abundantly in nature, and are more usable by our bodies. Within the unsaturated fat category there are various sub-categories depending on where the "kink" is in the chain. In particular, the omega-3 and omega-6 variants are the most useful to our bodies because our natural cellular processes easily convert those two variants into other molecules.
We need omega-3 and omega-6 fats,
but in a proportion of 1:1 or 1:2 whereas most Americans get a proportion of 1:10; an extreme disproportion. In particular, although omega-6 fats are necessary, they activate inflammatory pathways within our bodies, which is why their proportion should be kept low.
To get the right fat ratios, the recommendation is to get those fats from more natural sources (eat that fat from your steak!). In particular you should eat lots more fish;
although you should avoid tilapia. Personally, since I learned how to cook salmon sous vide I get plenty of fish in my diet.
References
These points about fat and sugar have been reiterated to me from several places. The following books are great references for learning more:
I have not read the "Eat Fat, Get Thin" book, but based on the book's summary it appears to align with my understanding here.
A good movie which explains how sugar is processed by our body and causes weight gain is
That Sugar Film.
TL;DR
The reason that fat makes us thin is that our body has control over how much it absorbs; It can control when to convert the fat into more useful molecules as well as when to break the fat down into fuel. On the other hand, sugar makes us fat because the body has trouble controlling how much sugar it absorbs; it will absorb sugars into the bloodstream pretty easily and must either burn it immediately or push the sugars into our fat cells for storage.
