The double bond in unsaturated fat can create 2 different configurations of the hydrocarbon chain. If at the double bond, the hydrogens are on the same side then it is known as cis, and this causes the hydrocarbon to bend, but if at the double bond, the hydrogens are on opposite sides then it is known as trans, and the hydrocarbon doesn’t bend, this is known as trans fat, and it is the most unhealthy form of fat. Trans fat has been shown to raise LDL and reduce HDL. Trans fats rarely occur naturally so the body hasn’t evolved to have the necessary enzymes to break them down. Trans fats are mostly found in partially hydrogenated oil products such as margarine as well as frozen pizza, cakes, cookies, microwave popcorn and pies. Certain foods have hydrogen added to them by food manufacturers in a process known as partial hydrogenation, the purpose of this is to prolong their shelf life, as unsaturated fat is less stable due to the curved hydrocarbon chain but partial hydrogenation straightens out this chain so it more resembles the shape of saturated fat. Partial hydrogenation is also used by commercial bakers to make foods more solid, this makes the production process cheaper and creates a certain desirable texture in foods. Thankfully the partial hydrogenation of oils has been banned by most advanced countries.

Monoglycerides and diglycerides

99% of fats consumed in the diet are triglycerides, meaning that they have 3 fatty acid tails, however, if a glycerol head has 2 fatty acid tails it is known as a diglyceride and if a glycerol head has 1 fatty acid tail it is known as a monoglyceride. Very small amounts of these 2 glycerides can occur naturally in some seed oils, such as olive oil, grapeseed oil and cottonseed oil, and they can not cause us any harm like this, but most occur when triglycerides containing animal fat or vegetable oil have heat and an alkaline catalyst added to it. Monoglycerides and diglycerides can then be separated through distillation where it may go through even more processing before it is added to food. Because these glycerides can be sourced from animal fat they may be problematic to consume for vegans or those with particular religious beliefs. Monoglycerides and diglycerides are added to food to help water and oil blend and provide the desired consistency and texture of foods as well as prolong shelf life. Monoglycerides and diglycerides are commonly found in baked goods, as well as frozen and packaged foods

Monoglycerides and diglycerides contain small amounts of trans fats, but because these are considered emulsifiers (as they help oil and water blend) and not lipids they are still legal in almost all countries. So food companies may switch to adding monoglycerides and diglycerides to foods instead of hydrogenating their products which still has a negative impact on the consumer's health.

Essential fatty acids

The body is capable of synthesising all essential fatty acids except for the polyunsaturated fatty acids omega-3 and omega-6, these are known as essential fatty acids and they must be obtained from the diet. Omega-3 makes certain hormones which regulate blood clotting, inflammation and the contraction and relaxation of artery walls. Omega-6 helps maintain healthy bones, stimulates skin and hair growth, regulates metabolism, and maintains a healthy reproductive system. You can obtain a good quantity of these essential fatty acids from oily fish and some seeds and nuts.
 

Fatty acids and cell membranes

Fatty acids form the membranes of all bodily cells. The unsaturated fat leaves gaps in the membrane providing membrane fluidity, so chemicals can pass in and out of the cell, but both unsaturated and saturated fat is needed to create the optimal composition of a cell membrane. If someone consumes too much saturated fat in the diet however then membrane fluidity decreases and molecules will struggle to move in or out of the cell optimally, the nervous system will be the system worse affected by this as membrane fluidity is needed for fast communication between neurons and the myelin sheath insulating the neurons which speed up the conduction of electrical impulses will struggle to form without a good quantity of unsaturated and saturated fat. 

Cholesterol

Fatty acids and mono-glycerol from fat and dietary cholesterol and apoproteins all combine in the intestinal cells to form chylomicrons, which travel to the liver in the bloodstream to be used in the process of forming VLDL and empty HDL. In the liver cholesterol actually forms through the metabolism of glucose in the mitochondria of hepatic cells. To make it clear dietary cholesterol has little to no effect on blood cholesterol for most people. Triglycerides, apoproteins and phospholipids combine in the Golgi apparatus of cells to form either very low-density lipoprotein (VLDL) or empty HDL. VLDL has a high ratio of lipids compared to protein, whilst, HDL has a low ratio of lipids compared to protein. VLDL transports triglycerides in the blood around the body in the bloodstream to be stored as fat or to be used for energy. VLDL comes across lipases which will release the VLDL’s fatty acids to be stored as fat or to be used for energy, the VLDL then turns to intermediate density lipoprotein (IDL). IDL can then form LDL (also known as bad cholesterol) when lipase reacts with it further releasing more fatty acids. LDL now contains a lot of cholesterol when compared to fat, and it can give cholesterol to cells to make cell membranes, vitamin D and to make hormones. LDL can then return to the liver to be recycled by the Golgi apparatus to form more lipoproteins or it can be excreted through bile. There will be a greater excretion of LDL through bile if LDL is in excess in the bloodstream.

Now to go back a second to that empty HDL. Empty HDL picks up excess cholesterol in cells, tissue and plaque, forming high-density lipoprotein (HDL). This is why HDL is also known as good cholesterol. HDL can then also travel to the liver where it can be recycled by the Golgi apparatus forming lipoprotein or be excreted from the body as bile.

LDL is also known as bad cholesterol as it can form plaque on the inside of arteries and make them narrow. A chemical reaction caused by the build-up of plaque causes cholesterol to oxidise, which damages endothelial cells and therefore initiates an inflammatory response. Monocytes enter the area and convert to macrophages, after stimulation from the oxidised cholesterol. Macrophages ingest and eliminate cholesterol, consequently, macrophages turn into foam cells, which accumulate together forming more plaque. The arterial wall hardens and thickens and smooth muscle cells in the arterial walls multiply and form a fibrous cap over the plaque. The fibrous cap can eventually erode and release plaque into the bloodstream, the plaque can travel through the bloodstream until it gets caught in a blood vessel, causing a blood clot, which limits blood supply to the tissue which relies on that blood vessel, this will lead to the death of certain tissues. If a blood clot forms in the coronary arteries of the heart it will cause the death of the tissue in the heart resulting in a heart attack. If a blood clot forms in one of the arteries supplying blood to the brain it can cause the death of neurons resulting in a stroke.

LDL-P is the amount of LDL in the bloodstream, this is the most important measurement of cholesterol as the more LDL the greater the risk of plaque formation in arteries. LDL-C is a measure of the amount of cholesterol carried by LDL, this isn’t as good of an indicator for health as LDL-P, as 2 people may have the same LDL-C level but different LDL-P levels due to different sizes of LDL. A healthy LDL-C level is under 200 mg/dL, and a healthy LDL-P level is under 100 mg/dL.

HDL-P is the amount of HDL in the blood. A healthy HDL-P level is over 60 mg/dL.

HDL-C is a measure of the amount of cholesterol carried by HDL. The ratio of LDL-C to HDL-C gives a better indication of health than just LDL-C alone. A healthy HDL-C level is over 60 mg/dL.

The triglyceride to HDL-C ratio can come in useful when someone has an LDL-C level of below 160 mg/dL. A ratio of above 3.8 is a sign that someone has small, dense LDL which is susceptible to oxidisation. Studies have been performed where the participants reduce their saturated fat intake but it only has a minimal beneficial impact on the triglyceride to HDL-C ratio

Fat's journey of digestion

Unlike other nutrients, by the time fat enters the small intestine it still has a lot more digestion to undergo, thankfully bile will come along and finish the job. Food enters the small intestine from the stomach as a creamy paste called chyme. Bile is formed when cholesterol in the liver is altered by digestive enzymes forming cholic acid and chenodeoxycholic acid (which are bile acids), these move to the gallbladder through ducts to be stored if they're not needed immediately, along with many other components needed to make bile (these include water, phospholipids, cholesterol, ions, bile salts and proteins). The components needed to make bile are much more concentrated in the gallbladder than in the liver. The gallbladder squeezes its contents out and into the small intestine, when the hormone cholecystokinin, is secreted when a fatty meal (containing triglycerides, monoglycerides, phospholipids and cholesterol) passes through the duodenum and stimulates cells to release bile. The bile acids that don’t get stored in the gallbladder (which is about half), move straight down to the ileum (in the small intestine), where bacteria slightly metabolise the bile acids from primary bile acids to secondary bile acids, this forms deoxycholic acid from cholic acid and lithocholic acid from chenodeoxycholic acid. A very small amount of secondary bile acids move all the way through into our faecal material, but most of it gets reabsorbed into a part of the bloodstream, known as the portal system, where it goes back to the liver and combines with amino acids, to form bile salts. The amino acid portion of bile salts is hydrophilic, meaning it wants to combine with water, whilst the rest is hydrophobic, meaning it repels water, these factors make an amphipathic molecule. Some of the bile salts then move to the gallbladder to be stored and some move down to the small intestine, the bacteria in the intestine remove the amino acids and it is later reabsorbed into the portal system, so the cycle can continue, 95% of these bile acids and bile salts go through this pathway (known as the terra hepatic circulation) and the rest is produced by the liver, a very minor amount are excreted through faecal matter. The bile salts mix with the fat in the small intestine, the hydrophobic part connects with the fat, whilst the hydrophilic part faces outwards, this begins to break the fat apart, as parts of the fat are becoming surrounded, with the hydrophilic ends facing outwards. The fat can then be broken down by lipase released from the pancreas, and triglycerides will form monoglycerides and diglycerides, this makes fats possible to be absorbed into the bloodstream where it can form triglycerides once more. Fat-soluble vitamins are absorbed along with fats in the diet and are stored in the body's fatty tissue and the liver, these vitamins are A, D, E and K. Vitamin A is important for growth, immunity, cell division, reproduction and vision. Vitamin D helps your body absorb calcium in the intestines. Vitamin E acts as an antioxidant stopping damage occurring to cells. Vitamin K helps make proteins that are needed for the building of bones and blood clotting. Fat which has 12 or more carbons doesn’t enter the bloodstream straight from the intestines it instead first enters the lymphatic system and then the bloodstream, this is because most fat molecules are too large to be absorbed straight into the bloodstream, so instead, they travel in the lymphatic network to the largest lymphatic vessel, which is the thoracic duct, the fat then can therefore enter the bloodstream in the heart, this is why it is important that we keep our gut and lymphatic system healthy and we don’t consume too many fats, particularly the more unhealthy kind, in 1 meal. This process of digestion for fat is a lot longer than the other macronutrients so it will keep you full for longer when looking at macros gram for gram, however, fat has more calories in them than the other macros so you can’t consume as much of them if you don’t want to surpass a certain number of calories in a meal, making them suboptimal in some cases when looking at macros calorie for calorie, and calorie consumption appears to be the most important thing for fat loss, and it’s a myth that fat makes you fat even though it has to undergo fewer processes to be stored as fat in the body.
 

Fat causes the smallest increase in insulin over carbohydrates and protein, this is useful as when plasma insulin levels are chronically elevated it can desensitise the body's response to insulin and cause type 2 diabetes. These benefits are mainly useful to those who already have diabetes or prediabetes, but those with a healthy insulin response still perform high-fat diets due to its effects (or rather lack of effects) on insulin, but increasing your fat intake whilst maintaining the same caloric intake has a minimal impact on weight loss for healthy individuals.
 

How much fat to eat

I would recommend that about 40% of your total daily calories come from fats, which would be about 1000 calories for men and 800 calories for women, which sounds like a lot but this is only about 110 grams for men and 90 grams for women, this is because just 1 gram of fat has 9 calories. No more than 10% of total daily calories should come from saturated fat and the rest should preferably come from an even mixture of unsaturated fats, this is not including trans fats as they should be ideally excluded from any healthy diet.