AAn issue that has been prominent in the current debate on the health risks of genetically modified (GM) foods is whether there are adequate methods of testing for the safety of these foods. One view is that the safety assessments of these foods are not as rigorous as those for new chemicals or drugs. Today 's Lancet carries two Research Letters reporting work on the potential risks to human health of the lectin Galanthus nivalisagglutinin (GNA ) , a compound that may be useful in protecting food plants from attacks by insects. These letters raise issues about the design of studies on safety.Stanley Ewen and Arpad Pusztai report that , when fed to rats , GM potatoes containing the GNA lectin have proliferative and antiproliferative effects on the gut.  They suggest that several of these effects are due to alterations in the composition of the transgenic potatoes , rather than to the newly expressed gene product.  However, data on the composition of the different diets are not reported in the letter.  Pusztai has released some of these details on the internet (http://www.rri.sari.ac.uk/gmo/ajp.htm).  These details indicate that the content of starch, glucosepolymers, lectin, and trypsin and chymotrypsin inhibitors in GM potatoes differed from that of the parental line.  Unfortunately, these differences have not been examined further by analysis of an extended range of lines, for evidence on whether these differences are attributable to the genetic modification or to natural variations.  Another shortcoming of the study is that the diets were protein deficient; they contained only 6% protein by weight.  There is convincing evidence that short-term protein stress and starvation impair the growth rate, development, hepatic metabolism, and immune function of rats.(1, 2)Ewen and Pusztai say that the significant differences between diet groups in variables such as mucosal thickness or crypt length are evidence of the biological effects of the GM foods.  Such a claim is easy to make but difficult to prove, because no consistent patterns of changes were observed in the study.  Ingestion of potatoes may be associated with several adaptive changes in the gut because of the low digestibility of raw or partly refined potato starch.  In rats caecal hypertrophy is a common response to short -term feeding of various poorly digestible carbohydrates, such as raw potato starch (3 , 4).  A physiological response of this nature is probably of little toxicological significance.  Dose -response studies would have helped in the assessment of consistency of response.