B.6.0 | Nutrition: Lipids, phospholipids, and more

HUFAS and Phospholipids

Given that our focus needs to be providing limiting nutrients in enriched rotifers, it must be said that in the past Highly Unsaturated Fatty Acids (HUFA) lipids and especially in Lipid Membrane form have been very difficult and critical to manage nutrients.

Most marine fish larvae need very high levels of the essential fatty acids DHA and EPA and just the right amount of ARA. It is important to provide these critical levels without developing an excess of DPA or other less critical HUFAs. Further, it is very important that the majority of these fatty acids be Membrane Lipids (polar phospholipids and glycolipids and non-polar sterols), as opposed to triglyceride oils.

Why? Larval fish build tissue, and especially neural tissue, very rapidly. Fish cell membranes are mostly made up of polar phospholipids and glycolipids constructed from DHA and EPA, and non-polar sterols. These lipids are especially important for neural and eye development. Fish larvae must have an abundance of these lipids to grow well and without deformities. While larval fish can easily form their HUFA membrane lipids from other HUFA membrane lipids, they have a very limited ability to produce phospholipid and glycolipid HUFAs from triglyceride (oil) HUFAs. Further, marine larvae lack the ability to generate HUFAs (DHA, EPA and ARA) from more plentiful shorter-chain fatty acids.

Why? The details involve enzymes (elongase, desaturase, reductase and more). These enzymes are deficient in larvae that have evolved to consume phospholipid-rich live prey (the details make my head spin). Suffice it to say that HUFA phospholipids and other membrane HUFAs are the form of nutrition found in most species of marine larval fish eggs. That is what nature offers and that is what nature demands. Unfortunately, when rotifers are enriched with the wrong feeds or protocols, they can accumulate triglyceride (oil) HUFAs and still be quite deficient in membrane lipid HUFAs.

If you want to know more I would suggest starting with Sarget, Toucher and Bell, in Fish Nutrition edited by Halvar and Hardy. From there go on to Sargent et al. (1999), Aquaculture 179 217-229. Good Luck!

Short Lipid Primer

Fatty acids are the building blocks that make up lipids. Fatty acids are defined first by their length. Thus EPA is C20:, (20 carbon chain) and DHA is C22: (22 carbon chain). Further, fatty acids can be saturated with hydrogen or unsaturated at different points along the carbon chain. The level of saturation determines their geometry and functionality. Saturated fatty acids are straight and tend to harden at moderate temperatures. Unsaturated fatty acids are kinked and tend to stay liquid at even low temperatures. The level of saturation and geometry of fatty acids affects their functionality in nearly all respects.

Lipid class refers to the different lipid structures created largely from fatty acids. These include:

  • Neutral triglycerides (oil) used for energy storage.
  • Polar phospholipids (phosphate head) and glycolipids (glycol head) whose polar nature allows for the construction of cell membranes, which are overwhelmingly composed of these lipids (plus embedded proteins, sterols and some other stuff).
  • Non-polar sterols which are also critical to neural cell function and cell membrane function in general.
  • Free fatty acids are unattached and “free” to be turned into more complex fatty acids through enzymatic action.
  • Other, smaller, classes include lipid vitamins, carotenoids, messenger lipids and more.

Enzymatic Lipid Transformation
Under the right conditions fatty acids can be transformed with enzymes. Fatty acids can be lengthened or shortened, hydrogen can be added or removed to saturate or de-saturated a fatty acid, and the “head” of the lipid can be exchanged to turn triglycerides into phospholipids, glycolipids or other lipids. These transformations require tremendous energy and resources and place a strain on larval organisms. When given a suboptimal diet, larval organisms can quickly deplete their enzyme reserves and lose their ability to generate proper cell membranes.

Many organisms lack the ability to modify some fatty acids. These fatty acids are highly unsaturated long chain fatty acids (HUFAs) such as DHA and EPA, and are considered Essential Fatty Acids for marine fish – fatty acids that must be supplied though feed and cannot be synthesized by marine fish larvae.

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