Sharks do not get tumours. That little nonsense, about which a book was even published, turned out to be incorrect. However, alkylglycerols, which are most often obtained from shark liver, do show an interesting effect on the hematopoietic organ, bone marrow, and still have a place in the micronutrients of patients being treated for tumours.
The action of alkylglycerol
In the chemistry of oils, it was said that alkylglycerols (ether lipids) are almost absent in vegetable oils, and are the dominant ingredients in oils obtained from fish and sharks. Shark liver oils are mainly present in dietary supplements.
Unlike acyl-glycerols (triglycerides) which act as fatty acids produced by the enzymatic breakdown of triglycerides in the body, alkylglycerols achieve their biological effects as whole molecules, without enzymatic breakdown to the free alcohols that make up alkylglycerols. Our body creates acylglycerols. Examples are the compounds plasmalogens, to which platelet activating factor (PAF) belongs, and some endocannabinoids, such as 2-arachidonyl-glycerol-ether, which has an important function in the control of intraocular pressure, signal transmission in the nervous system, and regeneration of the nervous system. Endocannabinoids are even known outside of scientific circles because they bind to cannabinoid receptors to which some of the active compounds from marijuana also bind.
Alkylglycerols from shark and fish oils have a different structure than those produced by the human body. They aroused scientific interest precisely because of the sharks themselves. Namely, sharks are a species that originated, as far as we know, more than 400 million years ago and have successfully survived four or five major extinctions on the planet. Their high resistance to infections makes them interesting. There is a myth that sharks cannot get tumours. In 1992, I. Williams Lane published the book Sharks Don’t Get Cancer. The book became a real bestseller, and people started looking en masse for a magic substance or substances that protect sharks from tumours. Shark cartilage preparations and shark liver oil experienced an exponential rise in sales. Ostrander et al. have been critical of these claims and very well documented showed that sharks get tumours, although at a lower incidence rate compared to humans. The myth has been shattered, but that does not mean that a lie does not hide a grain of truth. Substances that act against tumours in in vitro and in vivo models have indeed been isolated from the liver and cartilage of sharks. Among these substances are alkylglycerols.
Folk customs of shark oil treatment have been studied in the fishing nations of northern Europe. As early as 1959, Linnman et al. published a study on the effect of shark liver oil components on stimulating blood cell production (hematopoiesis), and in 1978 Brohoult et al. published the first meaningful clinical study of the impact of a shark’s liver on cervical cancer. However, the use of shark oil has always stood aside and preparations containing it have remained in the domain of dietary supplements. Deniau et al. published an excellent review paper on the biological action of alkylglycerol. There is a relatively large amount of preclinical research in experimental animals. Increased sperm motility, immunomodulatory and antitumour activity are the most interesting features of the biological effects of alkylglycerol. However, the number of clinical studies is very modest. The authors agree that it is difficult to explain all the effects with alkylglycerol alone, because shark liver oil also contains ω-3 acids and squalene, which also affect overall biological activity. The chemical composition of alkylglycerol determines the antitumour activity. Alkylglycerols of monounsaturated alcohols of 16 and 18 carbon atoms (16:1 and 18:1) are the most effective, while saturated alcohols of 18 carbon atoms (18:0) in tumour models even stimulate tumour growth. So it is apparent that the chemical composition of alkylglycerol is very important for its action.