in this video we're going to be focusing on lipids so the term lipid is sort of an all-encompassing term meaning molecules that are sort of hydrophobic there are two main uh lipids uh the two main dietary sources are going to be the triglycerides so triglycerides if you recall way back when in our discussion about biochemistry triglycerides are composed of a threecarbon molecule called glycerol plus three fatty acid chains so these are hydrocarbon chains that have a caroxile group at the end hence why they're called acids so these three fatty acids usually there's an even number of carbons here on each fatty acid chain so the dietary sources of the triglycerides are known as neutral fats uh the saturated versions which remember saturated just meant that all the carbons had single bonds so these are found in meats uh dairy foods basically animal products uh in some tropical oils as well the unsaturated fats these are found in seeds nuts and most vegetable oils unsaturated just means that some not all but some of the carbons actually have a double bond okay and the main difference is when you look at the saturated fats these tails are all sort of linear so you can stack I'm kind of drawing a phosphoipid on accident here um so you have your glycerol and then you have your three fatty acid chains and then the next one because they're all linear they kind of line up like bricks so saturated fats tend to be solid at room temperature think of something like lard well unsaturated fats what the double bonds do is they introduce let's say these are too saturated and this is unsaturated it creates these kinks that prevent the molecules from stacking correctly and so it's going to be liquid at room temperature hence usually these are fats that are called oils as opposed to just fats and then lastly you have trans fats these are very unhealthy fats that are found in forcibly hydrogenated oils uh and basically what happens here is that the orientation if I was to look at the double bond um in this carbon in a normal unsaturated fat if I drew a line through the double bond the remaining carbons before and after are on the same side in chemistry we call this the cy configuration so if I drew this as sticks it would kind of look like that okay so enzymes can recognize this and properly break it down what happens in trans fats because you're forcibly hydrogenating it the carbon chains are on opposite sides so it's what we call the trans configuration and hence why they're called trans fats and so they kind of look like this and enzymes can't really recognize this shape so they're not going to be absorbed in many cases and they'll be found in the stool so it leads to an oily stool the second dietary source of lipids are going to be cholesterol so this is found in egg yolks some meats milk products but the majority of the cholesterol that the body has is actually produced by the liver based on the uh needs of the body and it's not really taking into consideration the cholesterol that you are taking in through dietary means now the liver is capable of not only producing cholesterol it can actually convert one type of fatty acid into another and we'll talk about that process in a later video now with regards to fatty acids we can't synthesize all fatty acids so there are some fatty acids that are considered essential uh two of the ones that are found in vegetable oils are lenolic and linolenic acid these have to be consumed in the diet as we cannot produce it so what do we use lipids for well lipids because they're hydrophobic allow us to absorb fat soluble vitamins like vitamins A D E and K lipids are a major fuel source for skeletal muscle as well as hpatocytes those are the cells in the liver um and then we can convert some of these lipids into phospholipids where if you recall the phospholipid is glycerol with two fatty acids and the third fatty acid is actually replaced by a charged phosphate containing group uh so it gives it that dual characteristic of being hydrophobic on the fatty acid side and hydrophilic on the phosphate head side uh lipids are going to be stored in atapost tissue and there it's not only used as a fuel storage mechanism but it's there to help protect vital organs as well as act as insulation for us to retain body heat uh prostaglandins these are uh sort of lipid-based signal molecules they're going to have a role in uh smooth muscle contraction uh we'll also talk about their role for example in blood pressure control the inflammatory response fever um and then lastly cholesterol cholesterol is a very important component uh that we use as a building block to make various hormones which we call steroid hormones so think of the sex hormones uh cortisol the ces uh the uh stress hormone another hormone eldoststerone which is important for sodium balance in the kidneys and cholesterol is intercolated in between the phospholipids so it adds fluidity and rigidity to the membrane uh it alters the membrane's permeability as well lastly cholesterol can act as a very important precursor to the formation of bile salts so what are the dietary requirements of lipids well fats should account for less than 30% of your total caloric intake and if we're talking about saturated fats that shouldn't be more than 10% now of course in a traditional American or typical American diet lipids are more than 40% of the total calories so you can see sort of the coincidence between American diet versus the obesity and epidemic that we face in this country now with regards to cholesterol cholesterol should not exceed more than 300 milligrams per day now we can measure circulating cholesterol in the plasma or the extracellular fluid and that level should be below 200 mg uh per deciliter of blood so the last thing that I want to touch on are sort of these uses of fat substitutes um so we can use air beaten into food so if you see whipped for example uh modified starches uh milkway protein and there was a fat substitute that came out about 10 years ago I think it is and I don't really know if they still use it um Ostra uh this was found in a lot of chips uh and it's made from cotton seeds now the problem with these gum derivatives and is that we don't metabolize it which means it stays in the stool so it gives you sort of this fatty uh icky uh kind of stool uh very loose stool um the rest of these uh like the milk whey protein will be metabolized and be converted into energy so they have caloric value now some of the disadvantages of these fat substitutes is they don't withstand the high heat of cooking and so their chemistry can become altered when we use high temperature cooking uh the taste dramatically differs from real fats and because they're not fat uh they're fat substitutes they can interfere with certain absorption so fat soluble drugs vitamins beta carotene uh you know so that's the precursor to vitamin A uh it can disrupt its absorption and it can lead to things like diarrhea loose stool we mentioned as well as flatulence