In response to a question in a Barbados Newspaper on whether or not cats should be fed canned tuna, the expert -a local veterinarian (vet)- had this to say: “Unfortunately, canned tuna contains an enzyme that destroys a very important vitamin for cats: Vitamin B1. If you feed your cat lots of tuna on a regular basis it can develop a vitamin B1 deficiency.â€
Robert D. Lucas. Ph.D. reacted on this claim by the local vet as follows: As an owner of a ten-year old cat and a person who has taught courses in sea-foods and also as someone who has actually canned foods and made sausages from sea-food surimi, I have some comments to make on the above statement, as well as the role of the local vets in food-safety programmes under the World trade Organisation (WTO) aegis’.
In letters to the Advocate in the past, I have always stated that persons should always stick to their areas of technical competence, as I will show below.
Thiamine (B1) is a water-soluble vitamin. Its absence in the diet can result in persons developing Beri-Beri. It acts as a co-enzyme in the metabolisation of carbohydrates in the body. It is found in most plants and meats. There has been a report in the literature in 1968 that, the enzyme thiaminase was found in tuna. This enzyme is responsible for the destruction of thiamine. It is interesting to note that fresh-water fish have high thiaminase activity, as do tissue from clams, shrimps and mussels. Thiaminase activity is low in most salt-water fish (Halver, John. E., University of Washington, Seattle, 1979). The concentration of thiamine is adversely affected by oxygen, sulphur dioxide and water.
Distilled water results in greater retention of thiamine in canned produce than pipe water. This is, however, immaterial. Once tuna have been caught, they are rapidly chilled down to under 40ºF (4.4ºC); gutted and washed to remove blood and kidneys. The chilled fish then under-go one or two processes. They are either held frozen at (-6.6ºC) -20º F for further processing or are given a pre-cook.
In the latter case, the heat from the cooking destroys all enzymes. This is to be expected, since enzymes are proteins and on heating become denatured or lose their structural integrity. Following pre-cooking, the tuna is then canned and subjected to processing at 250ºF (121ºC) for varying lengths of times depending on the size of the can used.
During thermal processing, about 50 to 75 per cent of the original vitamin B1 found in the food is destroyed. This percentage can be reduced by increasing the acidity of the packing medium (water/ brine/ oil) used. It has been found that, when the packing media are neutral or alkaline there are greater reductions in the concentration of thiamine in canned foods.
If tuna are held at -20ºF before further processing, there is still some enzymatic reaction taking place. However, the reaction rate is very slow and there is little loss in the total amount of thiamine present.
Although there is a reduction in the value of thiamine as a result of thermal processing; what is of interest, is the actual daily requirement needed by the cat for good health.
For example, there is the cumulative effect on thiamine intake if the cat is fed a whole 6.5-ounce can of tuna, which the expert has not taken into account.
A cat may require, let us assume, 70 milligrams of thiamine daily, and there is a residual level of 25 milligrams of thiamine per ounce of canned tuna. Feeding a cat one 6.5-ounce can of tuna is more than adequate in meeting the daily requirements of thiamine for the cat.