Marine phospholipids: new food supplements from the sea.
Marine lipids mainly comprise triglycerides, cholesterol and phospholipids. Although the nutritional value of the polyunsaturated fatty acids EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) has been subjected to intensive scientific evaluation during the last 30 years, phospholipids have only recently attracted attention as a result of being promoted as the "missing link" between eating fish and taking fish oil supplements. This story has been driven by the promotion of krill oil as a food supplement by new companies on the market. Krill contain very little fat (triglycerides) but a substantial amount of phospholipids. The aim of this article is to discuss the possible health benefits of marine phospholipids, not only from krill but also from marine sources in general.
Structure and Function
The study of phospholipids (PLs) is a vast scientific field, but the focus has largely been on synthesis, their effects on tissues and their metabolism. Little attention has been paid to the possible effects of PLs derived from the diet. The number of PLs detected in marine species is large, and our knowledge about these PLs is continuously expanding. PLs are important building bricks for cells walls and intracellular organelles; they form a scaffold for membrane proteins that are engaged in biochemical reactions and/or signal transduction, in response to external or internal stimuli, and they also serve as precursors for the synthesis of other bioactive molecules. The reason for the heterogeneity of marine PLs must be related to the individual requirements of the marine species that exist in very different environmental situations, needs that have developed during millions of years. The PLs necessary for normal human cell function can be synthesized in several tissues; an eventual need for an external supply of PLs in the diet has therefore not been defined. Components of the PLs in our diet are certainly utilized in humans, and some of them--the omega-3 fatty acids (FAs) and choline-containing PLs in particular--are essential and have to be provided by external sources.
[FIGURE 1 OMITTED]
Chemically, like triglycerides, PLs are constructed with a backbone of glycerol (although some have sphingomyelin instead). One OH-group is linked to a phosphate group, which, in turn, is esterified with a charged head, usually an aminoalcohol such as choline, ethanolamine, inositol or serine, and the remaining two OH-groups are esterified with fatty acids (Figure 1). The FA tails of PL molecules are hydrophobic whereas the head is hydrophilic. This spectacular construction within one molecule (called amphiphilic) gives the PL molecule very specific abilities that are important for the formation of permeable cell membranes and intracellular organelles. PLs from food or food supplements are attacked by saliva and, in particular, by phospholipase A1 and A2 from the pancreatic gland, which thereby liberates one of the FAs from the sn-1 or sn-2 position, respectively. The resulting PL metabolite, containing only one FA, is called lyso-PL (Figure 2).
Using separate isotope tagging, the individual fate of the PL molecule's aminoalcohol head and FA tails have been followed after intake. (1,2) The main portion is hydrolysed to lyso-PL, which is directly absorbed by the intestinal cells. After absorption, the head part of the molecule is sequestered into HDL-cholesterol particles that are synthesized in the intestinal wall. The remaining FAs in the intestinal fluid have to be baked into micelles, together with bile acids and monoglycerides from the triglyceride content of the meal. The intestinal uptake of FAs is therefore a complicated and time-consuming process. When absorbed within the intestinal cells, the FAs are re-esterified to triglycerides and integrated into lipid transport particles called chylomicrons, together with cholesterol. These particles are released into the lymph in the thoracic duct, finally ending up in the liver where the chylomicron particle is disintegrated and the content is used in the synthesis of non-polar lipids such as triglycerides and cholesteryl esters. These are the main components of the so-called very-low-density (VLDL)-particles that are later converted into LDL-particles in the circulation. The HDL-particles and the LDL-particles are equipped with different proteins, so-called lipoproteins, which serve as keys to cell receptors. By means of this targeting process, these particles will end up in different tissues carrying different parts of the PL molecule: the head and the FA tail to the target tissues. The biological impact of this effect is, however, poorly understood.
[FIGURE 2 OMITTED]
Marine PLs are mainly phosphatidylcholine (PC) and phosphatidylethanolamine (PE), but some phosphatidylserine (PS) and phosphatidylinositol (PI) may also be present. The total content of PLs in fish and crustaceans is only about 5%, significantly less than the triglyceride content of oily fish, a fact that manifests itself in the cost of extracting marine PLs. Therefore, to be of nutritional and commercial value, marine PLs would either have to contain essential components that are not present in fish oil or the availability of these components, such as EPA and DHA, would have to be significantly better. Krill oil producers often make the latter claim.
Interestingly, it is not only the intake of marine omega-3 fatty acids that has decreased during the last 100 years, but also the intake of PLs in general. (3) Some even claim that a low intake of choline could cause degenerative diseases such as Alzheimer's. Choline acts as a precursor in the synthesis of the neurotransmitter, acetylcholine, which is active in both the peripheral and central nervous system. Choline is an essential nutrient and the supplementation of PLs containing choline could be an argument for increasing the intake of PLs in general. PC contains mainly saturated and mono-unsaturated FAs. By contrast, PE is the marine PL with the highest content of polyunsaturated FAs, mainly the omega-3 fatty acids EPA and DHA. Herring PLs contains about 25% PE, with DHA being the main omega-3 fatty acid. Blue whiting contains about the same amount of PL and significantly more DHA than EPA. Krill, by contrast, mainly contains PC and PI, with EPA as the main omega-3 FA. The high content of cholesterol in krill and other crustaceans is a matter of concern. Are food supplements that contain marine PLs a better source of omega-3 fatty acids than fish oil and what are the health benefits of getting PL products from herring or krill?
Research and Analysis
Recently, a series of pharmacokinetic studies has been completed in which animals and healthy individuals were given marine PLs: uptake levels and distribution were evaluated. Unpublished results from feeding studies in rodents given a PL concentrate extracted from herring meal (EPAX AS, Norway) have demonstrated significantly better EPA/DHA absorption compared with animals given EPA/DHA in triglyceride form. By means of a new spectrophotometric analysis technique, called Time-of-Flight Secondary Ion Mass Spectroscopy (TOF-SIMS), tissue specimens can be analysed directly for different lipid fractions. The TOF-SIMS analysis showed that animals given the PL concentrate had a higher DHA content in the liver, muscle and adipose tissue compared with controls. Also, PE was increased in the same tissues, indicating that PE followed the same pattern as DHA. These results are new and extremely interesting, as they follow marine lipids from supplementation to the target tissues.
The question is whether dietary PLs not only increase omega-3 fatty acid levels but also raise the tissue content of PC, PE and other PLs. It can now be clearly demonstrated that marine omega-3 fatty acids, and in particular DHA from herring meal PLs, have better bioavailability compared with DHA given in the form of triglycerides. This means that food supplements containing EPAX PL could be useful for the prevention of Alzheimer's disease, age-related macular degeneration and even Type 2 diabetes. Furthermore, it seems that it is not just intestinal uptake that is improved with this marine PL product, but also DHA tissue distribution as well. It remains to be investigated whether marine dietary PLs are carried to tissues via lipoprotein particles, or whether the increased intake of polyunsaturated fatty acids induces PL synthesis, possibly by gene interaction.
Introducing a completely new food supplement of marine origin to the market immediately raises the question of the sustainability of the raw material. Krill catching has a long history in Japan where they enjoy the crustacean for its own sake, a product called Okiami. However, if we are going to use krill for the extraction of tiny amounts of lipids, the need for raw materials will be much higher than the traditional catch. Organizations engaged in environmental issues are therefore keeping a watchful eye on this new trade. The harvesting of herring and blue whiting go back a long way in the countries that fish in the North Atlantic region, and the control of quotas is strictly regulated and enforced. In addition to human consumption, the production of oil and meal has been the main commercial outlets for these fisheries. Extracting PLs from these species would only increase the value of this raw material. The extraction of PLs from herring, blue whiting and other fish belonging to the group of industrially fished species would upgrade the trade, and converts parts of the industrial catch into high-quality food supplements for the benefit of human health.
(1.) O. Zierenberg and S.M. Grundy, "Intestinal Absorption of Polyene Phosphatidylcholine in Man," J. Lipid Res. 23, 1136-1142 (1982).
(2.) D. Lekim and H. Betzing, "Intestinal Absorption of Polyunsaturated Phosphatidylcholine in the Rat," Hoppe-Zeiler's Z. Physiol. Chem. 357, 1321-1331 (1976).
(3.) T. Wang, "Chemical Structure and Biological Function," in F.D. Gunstone, Phospholipid Technology and Applications (Oily Press, P.J, Barnes & Associates, Bridgwater, UK, 2008).
For more information
PO Box 2047 NO-6028 Aalesund, Norway.
Tel. +47 7013 5960
Morten Bryhn, MD, PhD
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|Title Annotation:||essential fatty acids|
|Publication:||Nutraceutical Business & Technology|
|Date:||May 1, 2009|
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