Ultraviolet Exposure of Insects and Vitamin D Synthesis. An intriguing study from 1934 is answered in 2018

Earlier in the year I was looking up articles in old magazines when I my eye shot towards a tiny snippet at the foot of a page:

VIOLET RAYS FOR MEALWORMS:—Reptile fanciers may be interested to learn that, according to a report in “The News Chronicle,” experiments have shown that live mealworms on which many of the Zoo animals are fed, prove far more beneficial if reared in artificial sunshine. (The Aquarist, March-April 1934, page 14)

The reason for my interest was that the finding implied that the mealworms kept under ultraviolet light were making Vitamin D. But insects do not synthesise Vitamin D; or do they?

I searched the scientific literature but could find no further information. The article in The Aquarist appeared on a page devoted to news from the Zoological Society’s Aquarium and ‘the Zoo” referred to is clearly London. I had no access to News Chronicle archives and put a copy of the intriguing report aside as ‘unsolved’.

For reasons that will become obvious further into this article, a few days ago I realised that the news from London Zoo would probably have appeared in a press release from the Zoo early in 1934 and that other newspapers could have carried the same story. The Zoo bombarded the newspapers of the day with stories about the animals, the keepers and the visitors. Some newspapers even had designated zoo correspondents such was the interest of the public in the Zoo and all its works. I searched the online British Newspapers Archive not expecting a great deal since its coverage is less extensive than was promised at its launch. However, my gloomy prediction was wrong. An article from the Daily Herald (12 January 1934) appeared on my screen:

VIOLET RAY MEALS AT THE ZOO

     Several of the animals at the Zoo are now having their food treated by artificial sunshine. Milk treated in this way is being given to the two orphan nilghaie antelopes recently born at the zoo. They are fed from the bottle twice daily in the presence of visitors. Experiments have shown that even live mealworms, on which many of the Zoo animals are fed, prove far more beneficial if irradiated. Monkeys fed on mealworms kept under ordinary sunless conditions were found to be more liable to rickets than those given mealworms reared in artificial sunshine.

In the 1930s Vitamin D and ultraviolet radiation were hot topics in research. They are also in the public eye. Rickets were a scourge and even in the 1960s a walk in the industrial cities of the north of England soon revealed reminders of its effect on the skeleton in the form of the bent legs of old ladies who, in those days, would not be seen dead in trousers.

The news report from London Zoo showed aspects of the research of the previous 20 years*. Ultraviolet irradiation of some human foods produces vitamin D, milk, for example. That would be the reason for exposing milk to UV lamps for feeding to young animals like the Nilgai. Was it supposed at the Zoo that the vegetable matter on which mealworms feed were producing Vitamin D and that the mealworms then ingested and retained the vitamin? Or did they think that the mealworms themselves were synthesising Vitamin D, like mammals exposed to sunlight?

I do not know then answer to these questions. A search of the reports in the archives of ZSL may provide some answers along with identifying whose idea it was to try ultraviolet irradiation of mealworms. My guess is that it was a continuation of the work of Sir Peter Chalmers Mitchell FRS and Joan Procter (who had died in 1931) who were interested in the practical benefits of ultraviolet light in improving the health of animals in the Zoo.

Mealworms, the larvae of Tenebrio molitor

The importance of ultraviolet and/or Vitamin D for reptiles in captivity has been recognised for decades. They, like mammals, can obtain their Vitamin D from their food or from synthesis in the skin exposed to ultraviolet. However, it was soon established that the common insects farmed for live food for reptiles, amphibians, birds, mammals and some fish had as well as low calcium a very low Vitamin D content. Supplements for reptiles have been provided by, for example, shaking a powder on their live insect prey or by putting minerals and vitamins into the insect’s food, thereby ‘gut loading” the soon-to-be-eaten insect. But, even then, special ultraviolet-emitting lamps still appear essential for the health of many species.

Having put the report in The Aquarist aside, I was astonished to see a paper published in July reporting that mealworms and other insects do synthesise Vitamin D when exposed to ultraviolet rays. In four insect species farmed in the absence of ultraviolet radiation, exposure to ultraviolet dramatically increased their vitamin D content. The effect varied between species but it is interesting to note that increases in both Vitamin D₃ and Vitamin D₂ (again with variation between species) were involved. The synthesis of Vitamin D₃ is characteristic of vertebrates; D₂ of plants, yeasts and fungi.

From Oonincx et al. 2018

The effect of ultraviolet was rapid, the concentration of Vitamin D₃ in mealworms (the larvae of a flour beetle, Tenebrio molitor) rising steadily during 8 hours of continuous exposure.

From Oonincx et al. 2018

The work of the Dutch-led team who did this research^† shows the practical benefit of ultraviolet irradiation of farmed insects for feeding wild animals in captivity as well as explaining reports of high Vitamin D₃ in some species of insects collected in the wild, i.e. exposed to sunlight. However, it raises as many questions as it answers. Previously, it has been argued that Vitamin D₃ is something special to vertebrates, closely involved in the metabolism of calcium and phosphorus needed for a bony skeleton. So what is it doing in insects? One sure bet is that it isn’t being produced for the benefit of vertebrate predators. And if it can be synthesised by insects what about other invertebrates?

Vitamin D₃ is just the precursor of the active molecule calcitriol, the synthesis of which includes several stages and different organs. Another key question is whether insects have the biochemical pathways to produce the molecule that is active in vertebrates. With that information it might be possible to infer whether Vitamin D in insects may be involved in those processes not concerned with calcium and bone in which it has been implicated, such as programmed cell death and innate immunity, in vertebrates. A whole new field of comparative endocrinology (the physiologically active form of Vitamin D is usually considered nowadays as a hormone) could be opening up.

Whatever the answers it is interesting that the work on ultraviolet irradiation of mealworms at London Zoo reported in the British press in 1934 was forgotten and only 84 years later has the question been raised again—and answered.

*Rajakumar K, Greenspan SL, Thomas SB, Holick MF. 2007. Solar. Ultraviolet Radiation and Vitamin D. A historical perspective. American Journal of Public Health 97, 1746-1754.

†Oonincx DGAB, Keulen P van, Finke MD, Baines FM, Vermeulen M, Bosch G. 2018. Evidence of vitamin D synthesis in insects exposed to UVb light. Scientific Reports 8:10807. DOI:10.1038/s41598-018-29232-w 1