A Case Study on the Genetic Basis of Nutritional Variability of Food
How much vitamin C is in 100g (about two thirds of a cup) of blueberries? Well, that depends. My trusty kitchen companion “The Food Guide” reckons 13mg, my nutrition textbook “Understanding Nutrition” counts 7mg, and the USDA food database has measured 9.7mg in their release 28. What the online USDA food list does not mention is that this value was based on 4 data points, the minimum amount measured was 7.4mg and the maximum was 11.5mg (based on the Access database for release 28). There is a lot of variation in the vitamin content of blueberries, it seems. Some of the reasons for this were examined recently by Liu and co-workers, who investigated the vitamin C content of blueberries in two different cultivars at different stages of ripening, and linked this to changes in gene expression.
The authors used two commercial varieties of blueberries: Bluecrop and Berkeley. They harvested the berries at six different stages of ripeness, from when the berries were very small, hard and dark green, up until the fruits had reached their largest size, and were blue and soft. The fruits from both varieties both showed the same pattern of ascorbic acid (vitamin C) content: vitamin C levels were around 4mg per 100g fruit during the first stage, but then rose quickly to around 8mg per 100g fruit by the third stage, then dropped again as the fruit neared harvest size. In the last stage of ripeness, Bluecrop variety contained about 5.5 mg vitamin C/100 g fruit, while Berkeley variety had a lower vitamin C content (3mg/100g fruit).
The authors then looked at the genes involved in ascorbic acid production in both varieties to see whether there were genetic differences that could cause the difference in vitamin C. There are 10 enzymes that are assumed to be involved in ascorbic acid accumulation in blueberries and other plants:
1. GDP-d-mannose pyrophosphorylase (GMP)
2. GDP-d-mannose-3’,5’-epimerase (GME)
3. GDP-l-galactose phosphorylase (GGP)
4. l-galactose-1-phosphate phosphatase (GPP)
5. l-galactose dehydrogenase (GDH)
6. l-galactono-1,4-lactone dehydrogenase (GLDH)
7. ascorbate oxidase (AO)
8. ascorbate peroxidase (APX)
9. monodehydroascorbate (MDHA)
10. dehydroascorbate (DHA)
Enzymes 1 to 6 are involved in the production of ascorbic acid from the sugar D-mannose, while 7 to 10 are a recycling system. The authors cloned these genes and ran a search through a gene database to find similar genes that had been described in the literature. Genes from the rock cress (a small brassica), apple, kiwifruit, cacao, and sesame were comparable to those in the two blueberry cultivars, and the two blueberry cultivars shared 99.1-100% similarity. However, differences in genetic code are only part of the story. Gene expression is also important, and very small differences in the genetic code can lead to large differences in the expression of particular genes. The relative expression level was similar for the genes coding for enzymes GMP, GPP and GDH (number 1, 4 and 5 in the list above) over the six stages of fruit ripeness for both cultivars. However, there were differences in genes for enzymes GME, GGP and GLDH over the second half of the fruit development phases. The gene expression of the Bluecrop cultivar, with the higher vitamin C content, was elevated for these three genes. In addition, for the four enzymes involved in vitamin C recycling, the gene expressive for enzyme DHA (which catalyzes dehydroascorbate back to ascorbic acid) was also elevated for Bluecrop.
So it seems that differences in gene expression are correlated with differences in the vitamin C content of blueberries. In the Bluecrop cultivar, higher gene expression along the vitamin C synthesis and recycling pathways seemed to result in greater vitamin C accumulation in the fruit.
Even so, the vitamin C content in blueberries is rather low. My sources cited levels between 3mg and 13 mg per 100g fruit. Adult requirements vary from 75mg to 120mg per day depending on age, gender and life stage. That’s a lot of blueberries! Better sources include orange, mandarin, kiwifruit, papaya, strawberry, guava, brassica vegetables like broccoli, and bell peppers, or your trusty vitamin C supplement. Biochemical data indicate that vitamin C deficiency is not uncommon: 7.1% of US adults had clinical deficiency according to a representative sample from 2003-2004 (Schleicher et al.). Don’t rely on your blueberry muffin for vitamin C, whatever the cultivar.
Liu F, Wang L, Gu L, Zhao W, Su H, Cheng X. Higher transcription levels in ascorbic acid biosynthetic and recycling genes were associated with higher ascorbic acid accumulation in blueberry. Food Chemistry 2015;188:399-405. http://dx.doi.org/10.1016/j.foodchem.2015.05.036
D’Amico, S (ed.). The Complete Food Guide. 1999. Könemann Verlagsgesellschaft mbH, Cologne, Germany.
Schleicher RL, Carroll MD, Ford ES, Lacher DA. Serum vitamin C and the prevalence of vitamin C deficiency in the United States: 2003-2004 National Health and Nutrition Examination Survey (NHANES). Am J Clin Nutr. 2009 Nov;90(5):1252-63. doi: 10.3945/ajcn.2008.27016. Epub 2009 Aug 12. http://www.ncbi.nlm.nih.gov/pubmed/19675106
Whitney, E and Rolfes, SR. Understanding Nutrition. Eleventh Edition. 2008. Thomson Wadsworth, USA.