When ultraviolet B (UVB) light from the sun shines on the skin, humans synthesize vitamin D3. But the major source of this vitamin for most people is fish and dairy products. Plant-based diets tend to be less abundant in vitamin D3, and approximately 1 billion people worldwide suffer from deficiency. A better vegetable source of vitamin D3 would help reduce the global incidence of its deficiency.
Tomatoes grow worldwide and have a fully sequenced genome. Although tomato flesh and leaves contain the precursor to vitamin D3, 7-dehydrocholesterol (7-DHC), it’s not usually bioavailable when tomatoes are consumed because it acts as an intermediate for other specialized biological processes. The tomato’s natural level of vitamin D3 is low compared to other dietary sources.
Scientists can manipulate the tomato genome without altering important biological processes in the fruit. To accomplish this, a worldwide consortium of researchers from the United Kingdom, Italy, Chile, and Cuba developed a method for increasing 7-DHC levels, and subsequently vitamin D3 levels, when tomatoes are exposed to UVB light (2). Fortifying tomatoes with higher levels of 7-DHC will hopefully boost dietary consumption of vitamin D3 around the world.
“Vitamin D deficiency and insufficiency are hugely important, much more important than people realize because they're major risk factors for a number of chronic diseases that you really don't want to get,” said Cathie Martin, one of the authors of the study and a plant geneticist at the John Innes Center. Chronically low levels of vitamin D3 associate with cancer, Parkinson’s disease, and dementia.
Plants in the nightshade family, which include tomatoes, have an enzyme called 7-DHC reductase that converts 7-DHC to cholesterol. Using CRISPR-Cas9 gene editing, Martin and her team knocked out the activity of this enzyme, which led to higher levels of 7-DHC in the tomato fruit and leaves. Using mass spectrometry, the researchers observed increased levels of 7-DHC in the leaves and the peel compared to the non-mutated fruit.
Biofortified ripe tomato fruit exposed to UVB light for one hour produced vitamin D3 in amounts comparable to two medium-sized eggs or a quarter can of tuna. The leaves alone yielded 200 µg/g dry weight of vitamin D3, meaning that less than half a gram of the gene-edited tomato leaf could produce one entirely plant-based vitamin D3 supplement.
“We definitely need to have this option for consumers to take vegan friendly or plant-based vitamin D3. We're thinking it's just the beginning,” said Jie Li, the study’s lead author and a postdoctoral researcher.
Eating one whole fortified tomato would provide approximately 20% of the recommended daily vitamin D3. The researchers may increase the vitamin D3 levels by extending UVB light exposure in processes such as sun-drying. The high levels of vitamin D3 in the tomato leaves may make this vegetative waste product usable for making a vitamin D3 supplement suitable for vegans.
“People tend to think of GMOs as something where maybe there's glyphosate involved or there's some kind of dependency on a chemical or something like this. And here's a great example where none of that is true,” said Kathleen Hefferon, leader of Forte Protein, a biotech company producing meat, fish, and dairy proteins without animals, who was not involved in this study.
Martin and her team are working on other strategies to fortify tomatoes, such as introducing antioxidants found in blueberries and blackberries.
Reference
- R.P. Heaney, R.R. Recker, J. Grote, R.L. Horst, L.A.G. Armas, Vitamin D3 Is More Potent Than Vitamin D2 in Humans, J Clin Endocrinol Metabolism 96 E447–E452 (2011). https://doi.org/10.1210/jc.2010-2230.
- J. Li, A. Scarano, N.M. Gonzalez, F. D’Orso, Y. Yue, K. Nemeth, G. Saalbach, L. Hill, C. de O. Martins, R. Moran, A. Santino, C. Martin, Biofortified tomatoes provide a new route to vitamin D sufficiency, Nat Plants 1–6 (2022). https://doi.org/10.1038/s41477-022-01154-6.
