First, what types of fats are there? There are many ways to classify fats (AKA lipids) but here are four simple categories:
Triglycerides - 95% of what human fat is, the rest are phospholipids and cholesterol. Triglycerides account for the largest energy stores in the human body.
Phospholipids - makes up cell membranes, protects cells from being dissolved in water (which our bodies have a lot of), and allows things in and out of our cells for metabolic processes.
Sterols/steroids/cholesterol- our body makes the majority of our total cholesterol so its not essential for us to get it in our diets, it's also not a source of energy meaning, we don't burn cholesterol when we burn fat. Cholesterol makes up our hormones (some of which are naturally steroids), some vitamins and bile salts which are essential for fat digestion. So cholesterol is important for our body!
Waxes- substances that are generally found in nature as a protective coating on plants. These can be harder for the body to digest, as all plant components are so having good gut health is important for digestion of these substances.
OKAY, but what about the fats we eat?
So, all fats are made up of carbon chains with hydrogens attached (see pic below, the black dots represent carbon molecules and the white dots represent hydrogens):
The longer the chain, the more it tends to be solid (like butter) and the more energy it has. The more energy a molecule has equates to the amount of energy we can get from the molecule, AKA, the more calories we can burn when we consume these long chains.
Saturated fats are fully saturated with hydrogens, so they are more packed together and solid at room temperature.
Trans fats are a type of unsaturated fat and rarely found in nature, which means they are primarily commercially made by adding hydrogens to vegetable oil. This makes them less likely to spoil, so foods can have a longer shelf life. Trans fats have been directly correlated with adverse cardiometabolic outcomes so they are being replaced with palm oil now. I'm not sure how much better this is though.
Unsaturated fats make up poly (many double bonds)- and mono (one double bond)- unsaturated fats. They are not fully saturated with hydrogen so they have double bonds in their chains, not well packed together which means they are liquid at room temperature.
The RDA guidelines for total fat intake suggest 95g of fat for men and 65g of fat for women but a Registered Dietitian (like myself!) can help you determine exactly how much fat you should be consuming!
Let's Chat Digestion and Absorption of Fat
It all starts in the mouth! Mechanical breakdown of food through chewing increases the surface area of food particles for enzymes to attach to. So, it's extremely important to chew your food enough!
Once our food makes its way through our stomachs and reaches our intestines, bile salts are released in response to fat. Bile salts are made in the liver from cholesterol and are stored in the gall bladder where they wait to be released in response to fat entering the small intestine.
FUN FACTS: 90% of fat digestion occurs in the small intestine. We produce about 30g of bile per day to help us digest fat!
The end products of fat digestion include fatty acids surrounded by bile salts (called a micelle), which are now ready for absorption into the intestinal cells.
Shorter fat molecules diffuse right into the blood and are taken to the liver where they can be used to create new glucose for energy (called gluconeogenesis) or they can be stored as fat in the liver. The body will typically only store fat in the liver if it has no where else to store it or if it can't use the fat at that time. This is not ideal, we don't want fat stored in the liver, only in fat tissue! This can lead to non-alcoholic fatty liver disease.
Longer fat molecules are packaged up into particles called chylomicrons and dumped into the lymphatic system. This skips the liver and goes directly to muscle and fat tissues to be used as energy or for storage.
When these particles get to muscle and fat tissues they release their fat and the remaining parts are transported back to the liver where they can be made into HDL and LDL cholesterols.
So now that you've been schooled on all things FAT,
let me tell you how HARD it is to actually gain fat!
The modified photo below is from an old overfeeding study done on young men who were overfed up to an extra 5000 calories a day mostly from carbohydrates (roughly 1% protein, 3% fat, 86% carbs) after first going through a depletion phase.
This study found that it takes roughly 500g of extra carbs consistently in your diet before even one gram is converted to brand new fat! In a case of overeating, the body is first going to use the carbs for energy (carbohydrate oxidation) then once we no longer need energy, the body will store the excess carbs as glycogen in the muscle and the liver to use for energy later (like when you're sleeping and not eating for long periods of time). Once the body's glycogen stores are completely full, the body will then make brand new fat termed de novo lipogenesis.
But it takes awhile for our bodies to be in a state where it no longer needs energy AND has completely full glycogen stores because if you think about it, your body CONSTANTLY needs energy, even just to breathe and survive.
But if and when this state does actually occur, that's when the body will make new fat from excess caloric consumption. It's easy for the body to make fat from glucose but not so easy for it to make fat from protein.
So to break this down, that would mean you would have to be CONSTANTLY eating in an EXCESS day after day after day for your body to reach a point where the influx of food is higher than energy demands and glycogen stores are full. THEN the body will make completely new fat cells.
For the average human, you would need to consume roughly 3-4 times your total daily energy expenditure over and over again to stimulate the product of new fat cells.
THIS IS WHY YOU CANNOT PHYSICALLY GAIN A POUND OF FAT OVER NIGHT!
And remember, bloating is not the same as fat gain.
I hope you found this information valuable and insightful! And my biggest hope is that you no longer fear fats if you did! Fats are an essential macronutrient that our bodies need and fat storage is a crucial component for survival!