Vitamin A is an essential nutrient for healthy skin and mucous membranes, for differentiation of cells particularly for the developing embryo, it helps maintain the immune system and is essential for the function of the eyes (the visual process). Vitamin A is called retinol and we can make it from the beta-carotene (and a few other carotenoids) found in highly coloured fruits and vegetables. However, fruit and vegetable consumption in many countries is very low and, therefore, the intake of beta-carotene is low, too. Retinol occurs naturally in liver and liver products, dairy products, some fish and in cod liver oil. Cod liver oil is one of nature's richer sources of retinol as well as vitamin D. We are unable to comment on the amounts of A and D in cod liver oils marketed in the United States which is where the authors of the paper reside.
What happened to cod liver oil in the USA or how it is marketed may be different from the practice in other countries. In the UK cod liver oil the oil has for many decades contained A and D in an approximate ratio of 10:1 based on the amounts in International Units (I.U.). There is a British pharmacopoeial standard for the oil which has specified for many years no less than 600 I.U A and no less than 85 I.U. D per gram. The European Pharmacopoeia specifies, per gram of oil, no less than 600 I.U. and no more than 2500 I.U. of A and no less than 60 I.U. but no more than 250 I.U. vitamin D. Both reflect the ratio of A:D in natural cod liver oil of around 10:1('as nature designed it').
Vitamin D is essential for the absorption of calcium from food and, consequently, it is required for development and maintenance of healthy, strong bones and teeth. Recent research is suggesting that the vitamin may have more and important actions. Very few natural foods contain vitamin D and most of the body's vitamin D comes from the action of sunlight on the skin. The vitamin is fat-soluble (as is A) and it can, therefore, be stored. This is important because the excess made beyond our immediate requirement could be stored for use at a later date. However, the fact that we rely on sunlight has a downside in that the sunlight needs to be of useful strength in order for us to make vitamin D. At higher latitudes, the time and the amount of useful sunlight are short and low, respectively, during late autumn, winter and early spring. This means that body vitamin D status may be compromised and it is the authors' personal view that this is a public health problem that should be addressed urgently.
We also feel and agree that vitamin D nutrition and, therefore, status should be improved. However, we cannot comment on the view of the authors on how they see that this should be achieved. It is possible that A may interfere with D at the level of the cell but this needs to be investigated meticulously before conclusions can be drawn and recommendations can be made by Government departments or agencies. Retinol has been around in nature for a very, very long time and cod liver oil was given to many people in several countries as the first supplement of vitamins D and A. In the UK, in the early days of the second World War, cod liver oil was given away in clinics to pregnant women, nursing mothers and children under the age of five years because of food restrictions and the fact that many women would need to work in factories and other places and would, therefore, be less exposed to sunlight. This continued until the late 1950s/early 1960s.
Cod liver oil has been taken for decades by many generations of people and without apparent adverse effects. Before suggesting that cod liver oil is anything but beneficial, more quality research needs to be done to find out what any interaction between these vitamins means. Only with sound scientific evidence on the actual interaction and the levels of each vitamin that may cause this interaction can authorities make (dietary) recommendations with reference to the intakes of these vitamins and to any maximum level of A (and D) in supplements and foods.