Portland-- For some time now researchers have been aware of low levels of a particular type of fatty acid called DHA (docosahaenoic acid) in the retina of retinitis pigmentosa (RP) patients. Better than 50% of the fatty lining around the night-vision (rod) cells is DHA. Animals deprived of omega-3 fatty acids, which contain or convert to DHA, develop visual impairment from retinal disorders. Researchers have tested red blood cell concentrations of DHA among RP patients but they have not been remarkably different from individuals with normal sight. Blood levels of nutrients may only reflect recent consumption of a particular nutrient and not the levels in the involved tissue, in this case, the retina.
In men, sperm is the only other tissue (or fluid) in the body that contains a very high level of DHA, about 20 percent of total fatty acids. Monkeys fed a diet lacking in omega-3 fatty acids also show decreased DHA levels in their retinas and sperm. Sperm in male RP patients (Usher's and X-Linked Recessive types) exhibit abnormal shape and reduced motility (swimming action). Over a decade ago it was reported that the sperm of RP patients have a bizarre structure.
A recent test of Usher's Type II patients revealed strikingly low DHA levels in sperm (4.7% of total fatty acids were DHA) compared to normal individuals (DHA was 19.2% of fatty acids), a 76% difference. The sperm of non-Usher's RP patients was comparably higher (12.8%) but still not the 19.2% of normals. The semen of RP-males had a lower concentration of sperm (70 million sperm cells per milliliter vs. 155 million sperm cells in normal-sighted males). There were far fewer abnormalities in the sperm cells of normal individuals then RP patients. Upon microscopic examination researchers found that 1.5% to 2.5% of the sperm cells of RP patients did not have a head; none of the sperm from normal-sighted males exhibited this abnormality.
D.R. Hoffman, a leading DHA researcher, has shown that some RP patients are unable to convert omega-3 fatty acids in their diet into DHA, a problem that may occur in the liver or in the retina (RPE layer) itself. Another possibility is that some noxious process such as oxidation destroys the DHA. When the rate of oxidation is greater than the supply of antioxidants (oxidation in the retina is accelerated by unfiltered sunlight, autoimmune reactions, intra-retinal viruses, lack of antioxidants such as vitamin E) the fatty acid membranes which surround retinal light-receptor cells may deteriorate.
A number of other factors may be responsible for abnormally low retinal DHA levels: (1) poor absorption of omega-3 fatty acids in the digestive tract, (2) diminished transportation of DHA from liver to the retina via cholesterol particles in the blood stream; (3) inability to transfer DHA across the retinal pigment epithelial (RPE) layer due to calcifications or (4) the inability to convert omega-3 fatty acids in the RPE to DHA, are some of factors being studied.
Researchers at the Oregon Health Sciences University who conducted this study now say that "dietary supplements of DHA from fish oil should be studied further as a possible intervention in the inevitable downhill course of this disease." By measuring sperm levels of DHA it's possible that clinicians would be able to determine if retinal DHA levels are being replenished. The full report of their study can be found in the current issue of Investigative Ophthalmology (Volume 38, pages 2619-28, 1997).
if RP is caused by some problem at the back of the eyes, for instance a blockage in the passage of DHA across the retinal pigment epithelium (RPE) barrier that separates the blood stream (choroid layer) from the photoreceptors (rod and cone cells); or if there is a problem in converting omega-3 fatty acids within the RPE into DHA, then the sperm test would not serve to adequately measure whether retinal levels of DHA were being replenished. None the less, this is an important advance in the progress of understanding RP.
Some common metabolic factor is responsible for the low DHA levels in sperm and retina. If this factor is found and corrected, a significant breakthrough in the treatment of RP would be expected. Both DHA, vitamin A, vitamin E and lutein are fat-soluble nutrients. All may be in short supply in the retina of RP patients. Is RP, in part, a disease of fat malabsorption in the digestive tract or abnormal fat metabolism in the liver?
Another recent study, published in last month's Investigative Ophthalmology (volume 38, pages 2387-2407, 1997), showed that dogs with a form of retinal disease (rod-cone cell degeneration) who were given DHA supplements did not exhibit higher retinal levels of DHA despite increased DHA levels in their blood stream. Furthermore, when DHA is directly administered to retinas of animals with rod-cone degeneration without having to pass through the digestive tract, liver and bloodstream, there is no difference in their retinal fatty acid levels than those of normal animals. Thus researchers surmise there is a metabolic defect outside of the retina "presumably in the biosynthesis of DHA in the liver."
The liver is the body's storage house for fatty vitamins such as vitamin A and where fatty vitamins are attached to cholesterol particles to transport to tissues such as the retina. DHA supplementation leads to a greater buildup of this fat in the liver of dogs with rod-cone disease than among normal dogs.
Food supplements may help to resolve liver disorders. Some digestive enzyme supplements provide bile, which is the juice secreted by the liver to aid in dissolving fats and fatty acids. Nutrients that improve fatty acid metabolism are lecithin granules, vitamin B6, biotin, zinc and magnesium. Food supplements which are known to boost liver function and the production of glutathione (a major antioxidant) in the liver are the herb milk thistle, N- acetyl cysteine and alpha lipoic acid. Digestive enzymes such as hydrochloric acid and the pancreatic enzyme lipase are known to improve the absorption of fats such as DHA in the digestive tract. Future clinical studies may prove that food and enzyme supplements may be of value in overcoming DHA metabolic problems.
Many RP patients are already beginning to increase their consumption of DHA. While omega-3 fatty acids from flax seed slowly convert to DHA within the body, this conversion may be interfered with by adrenal (stress) hormones. DHA-rich fish oils provide the already-formed DHA and are the preferred source of DHA. The commercially available source of DHA from algae is provided in a base of omega-6 fatty acids (sunflower oil) which interferes with DHA metabolism and is thus undesirable for RP patients.
Readers should be made aware there are different types of RP and the above information may not apply to all. However, DHA abnormalities in retinal disorders appear to be common if not universal. It would be wise for individuals with healthy eyes to consume DHA-rich fish oils for maintenance of their visual system. Increased amounts of vitamin E are suggested when supplementing the diet with fats such as DHA as they are vulnerable to oxidation.
Bill Sardi, reporter at large for FOCUS.
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www.focusnewsletter.org/rpsperm.htm
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