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In previous posts we’ve talked about the starvation response (aka starvation mode), metabolic adaptation (aka metabolic damage), the thermic effect of food, and thermogenesis. We’re currently running a series on macronutrients, and in our previous posts we’ve paid special attention to carbohydrates and proteins, focusing on their classification, how they are digested, and their food sources. In this week’s post we’ll do the same thing for lipids. Just like before, given the sort of content that we’ll be dealing with in this post, rather than rely on scientific papers, we’ll use the following references:
- Gropper, S. S., & Smith, J. L. (2012). Advanced nutrition and human metabolism. Cengage Learning.
- Wildman, R. E., & Medeiros, D. M. (2014). Advanced human nutrition. CRC press.
- Hankey, C., & Whelan, K. (Eds.). (2018). Advanced Nutrition and Dietetics in Obesity. John Wiley & Sons.
What are lipids?
Lipids are substances that are defined due to their solubility in organic solvents like ether, chloroform, and acetone. Because of that definition, there are more chemical compunds that qualify as “lipids” than just the dietary sources of energy that we usually call fats. In other words, if you’re used to only thinking of lipids as fats, then you’re in for some learning.
Just like happened in our post on proteins, we’ll follow the categorization provided Gropper and Smith, which is based on the physiological function of the different kinds of lipids.
Biological functions of lipids
As fatty acids. In terms of structure, fatty acids are the simplest form lipids come in, and this is because they consist of a hydrocarbon chain with a carboxylic acid group on one of its ends. They also serve as building blocks for more complex kinds of lipids. When it comes to physiological function, fatty acids are responsible for most of the calories that come from dietary fat. The hydrocarbon chain that makes up a fatty acid can be between 4 and 24 carbon atoms, and they can be saturated, monosaturated or polyunsaturated. What does that mean? Well, I hope you remember a little from your chemistry classes, especially when it comes to double bonds:
- Saturated fatty acids: having no carbon double bonds. They tend to come from animal sources.
- Monosaturated fatty acids: having at least one carbon double bond. They tend to come from plant sources.
- Polyunsaturated fatty acids: having several carbon double bonds. They also come from plant sources.
Why do we even bother with this sub-categorization? It turns out that the degree of saturation of a fatty acid is related to its geometry, and a less saturated fatty acid (one with more double carbon bonds) can bend and kink more than a saturated one. Wether the molecule bends or not is then related to it having a cis or trans isomerism (same chemical formula, but different geometric structure). You’ve probably heard of trans fats, and how they may be related to adverse nutritional effects. We’ll deal with them on a later post though.
As triacylglycerols (aka triglycerides). Once our body stores fat, it does so in the form of triglycerides. By the way, I hope you don’t mind me calling triglycerides TGs, it’s just that i keep messing up the spelling. TGs are formed by three fatty acids bonded to glycerol. Depending on the acid groups, TGs can be simple (the three fatty acids are the same) or mixed (the three fatty acids are different). TGs can be found in solid (called fats) or liquid (called oils) form at room temperature, and this depends on their constituent fatty acids: the more saturated fatty acids, and the longer their hydrocarbon chains, a TG possesses, the higher its melting point.
As sterols and steroids. Both sterols and steroids have a core structure composed of four rings which is often refered to as a steroid. Sterols build up on a steroid core by adding an alcohol group to it. You may already be familiar with one example of a sterol: cholesterol. While cholesterol can only be found in animal cells, sterols are also found in plant cells. Despite its bad reputation, cholesterol is a key component of cell membranes, and it also is a precursor for other steroids produced in our body, like bile acids, the sex hormones (estrogens, androgens and progesterone), and adrenocortical hormones. Compared to other lipids, sterols and steroids make up a tiny amount of our dietary intake at around 5%.
As phospholipids. As the name implies, phospholipids contain phosphate as well as fatty acids, and they come in two kinds: glycerophosphatides (if their core constituent is glycerol) and sphingophosphatides (if their core constituent is sphingosine, an amino alcohol). In general terms, phospholipids are important components of cellular membranes, and they’re also a source of other physologically important compounds.
As glycolipids. Just like phospholipids have phosphate in their structure, glycolipids have glucose in theirs. This doesn’t mean that their main function is as an energy source, but rather as a structural component of the nervous system. Glycolipids come in two flavours: cerebrosides and gangliosides.
It’s clear that lipids play a key role in sustaining life and its processes, and only focusing on lipids as energy sources limits our understanding of their importance. Let’s now take a look at how lipids are digested.
Most of the lipids in our diet come in the form of triglycerides, phospholipids, and sterols. The ordering of those lipids isn’t casual, as triglycerides make up the bulk of our lipid intake while sterols the least. Upon ingestion, digestive enzymes begin breaking down lipids in the gastrointestinal tract. Different enzymes are responsible for breaking down each kind of lipid. As there are different digestive processes that take place depending on the kind of lipid, we’ll first deal with triglycerides, and then with phospholipids and sterols.
- Triglyceride digestion: TG digestion begins in the stomach, as the digestive enzymes involved are secreted by the chief cells of the stomach and by the serous gland under the tongue. It is important to note that fats are hydrophobic, while digestive enzymes are hydrophilic, and so TGs must be emulsified in order for them to be digested. Part of the emulsification process is done through muscle contractions of the stomach, but the reality is that only a tiny percentage of the TGs present in the stomach will be fully digested, and digestion will truly take place in the small intestine. However, there is an upside to the presence of undigested TGs in the stomach: their presence delays the rate at which stomach contents empty, so it can be said that TGs have a high satiety value. TGs are completely digested in the small intestine due to the less acidic environment, the presence of more digestive enzymes and emulsifying agents (like bile), and the fact that absorptive cells are also present. The end result of TG digestion is a mix of diaglycerols, monoaglycerols, and free fatty acids.
- Phospholipid and sterol digestion: In this case the process is very similar to the previous one, with the production of free fatty acids and cholesterol. The difference lies in that, during digestion, polymolecular aggregates called micelles are produced. Micelles are composed of monoacylglycerols, lysolecithin, cholesterol, and fatty acids. During the last stage of digestion, those micelles are broken down into free fatty acids and cholesterol.
As was the case with carbohydrates and proteins, the products of lipid digestion are then trasported to the appropriate cells so that more complex compounds can be produced.
Sources of lipids
Although fatty acids can have 4 to 24 carbon atoms in their hydrocarbon chain, those with a number greater than 14 are the most important in nutritional terms. Among them we can find palmitic acid, stearic acid, oleic acid and linoleic acid, all of which make the bulk of our diet’s fat content.
Just as was the case with proteins, some fatty acids are essential. In this case however, it’s some of the unsaturated fatty acids that can’t be synthesized in animal cells and must come from plant sources.
Fatty acids can be found in milk, olive ol, coconut oil, fish like tuna and mullet. They can also be found in animal and plant food sources, as well as seeds like sunflower seed and linseed.
When it comes to cholesterol, it can be found in meats, dairy products and egg yolks. Just remember to keep an eye out for how much you eat.
So now we know a little more about one of the macronutrients: lipids. We have an idea of what they are, how they are digested, and where we can find them. Next week we’ll deal with alcohols. Hope you enjoyed this week’s post, and see you next week!
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