Q. "Do genetically enhanced animals and/or foods lose or gain any nutritional value?"

James Murray developed a herd of genetically modified goats at the UC Davis dairy goat barn. (Photo/UCD)

A. The answer to this question is not straight forward, or to put it another way "it depends." When we put a new gene in, or knock an existing gene out, we may change the composition of the protein in an animal. However, whether this affects the nutritional value of that food depends on a number of factors. First, is the gene, and therefore the protein, expressed in the part of the animal being consumed for food? The switches we use to control the expression of a transgene can be very specific as to what tissue or cell expresses the gene, how much of the product is made, when the product is made, and whether the product remains in the cell or is exported out of the cell, say into the milk.

Using our dairy goats as an example the gene is only expressed in the mammary gland epithelial cells during lactation, so if you are eating the meat then there is no effect at all that we have seen, even if the meat came from a lactating doe.
 
Second, the effect on the nutritional value would depend on the amino acid composition of the protein and the amount of the protein in relation to all the other proteins in the tissue you consume. Thus, even if we were to transfer in a protein that contained a large increase in an essential amino acid, if it was expressed at a low level there would probably be no significant increase in the dietary value of the food.
 
Third, it is possible that the gene produces a protein that in turn functions to change one or more components of the cell or tissue. Thus, when growth hormone was put in it resulted in increased growth or efficiency of feed utilization, but does not result in a change in the nutritional value of the animal when consumed by humans. However, when we put a gene in to change the composition of the fat in milk, we were able to show that we made a milk, which may be better for humans to consume with respect to cardiovascular disease.
 
So, going back to the beginning, whether the addition or removal of a gene causes a food to lose or gain nutritional value depends on the gene product itself and also on the pattern of expression. Thus the answer to this question really depends on which gene, as it must be answered on a case-by-case basis for each one.

James D. Murray is a professor in the UC Davis Department of Animal Science and the Department of Population Health and Reproduction.

Due to the volume of Ask it! submissions, not all questions can be answered online.

Read the Ask it! archive.