Letter To The Editor: “A reply to “Comment on ‘Formation of methanol and formate in Wister rats after oral administration of methylated rapeseed oil: A fuel for lamps’””

To the Editor:

We thank Dr. Noseworthy et al. for the comments on our article. The primary goal of the study was to show what we could prove if methanol is released after ingestion of methylated rapeseed oil. Secondarily, we wanted to discuss whether the released amounts could be dangerous for humans. In theory, if the oil only contained methylated oleic acid, a maximum of 0.1 g methanol could be released from one milliliter of oil (1). A child weighing 10 kg would have to ingest 50 ml to reach a minimal lethal dose of methanol of 0.5 g/kg; an adult of 70 kg 350 ml of lamp oil. These high doses are unlikely to occur because the oil tastes bitter. The minimal lethal dose of 0.5 g/kg b.w. corresponds to a peak serum concentration of 650 mg/l (volume of distribution (Vd) in humans approximately 0.77 ml/kg). Symptoms can be excluded at concentrations lower than 200 mg/l or 0.144 g of ingested methanol per kg b.w. (2). In practice, five out of 12 rats that received 4 ml/kg of lamp oil had methanol levels above 200 mg/l so that in any given case, the harmlessness of methanol ingestion can be assumed. However, these amounts can only be achieved by intentional ingestion.

We agree with Noseworthy et al. that it is unlikely that all of the oil would undergo hydrolysis. As mentioned above, a theoretical maximum of 0.1 g methanol could be released from one milliliter lamp oil. After resorption and hydrolysis of 1 ml/kg lamp oil, we could expect a maximum of 130 mg/l in the serum (Vd = 0.77 ml/kg). Rats administered 1 ml/kg of lamp oil had peak levels up to 82.3 mg/l after one hour. This would correspond to a maximal hydrolysis of 66%.

The concentration of methanol in the lamp oils themselves, however, is low. The methanol concentration in the rapeseed oil we used for our experiments was 68.4 mg/l. This is less than 1% of the amount expected after maximal hydrolysis, and it is a concentration that is found, for example, in fruit juices (3). In fruit schnapps the concentration is much higher (>1000 mg/l). Therefore, methanol, as a remnant or from spontaneous release from methylated rapeseed oil, seems to be negligible.