Vitamin C - sample variablitiy

Had a big clean up of office, threw out lots of paper and uncovered some very interesting articles that would have been good in my book..... such is the issue with having so many paper files :) So will be a few posts from these papers over the next we while.

The paper was published almost 20 years ago in 1991, in the first paragraph on the introduction they state:

It has long been known that the vitamin C content in a given food can vary appreciably depending on growing conditions, stage or maturity, regional differences, and seasons of the year. variations within a given fruit or vegetable also significant. 

It has long been known????!!! When did you find out about the variation in vitamin C?! I know I didn't know about it until the 21st Century, 10 years after it was written. Hence you can see the massive gulf between what researches know and what the general public know.

Let me pick out a few results:

• Oranges, which people"know" are vitamin C foods range from 52-78 mg/100 g. That is approximately a 50% increase.
• Grapefruit, another citrus varies from 21-31mg/100g. That is again a 50% increase
• Potatoes, which in the Western diet significantly contribute to vitamin C intake, could have levels of 11mg/100g or 27 mg/100g depending on cultivator. That is more than a 300% increase. 

Due to this variation they conclude about human studies:

The large range of values for the vitamin content in a given food suggests further than in human diet studies, when the major sources of vitamin C are from a few foods, daily analysis are required for the necessary precision.

I haven't specifically gone looking for vitamin C studies based on food, that said this is the first time I have come across a recommendation that the exact food eaten on that day should be measured for the vitamin C content. Undertaking vitamin C measuring is complex and somewhat pricey so every study I have read uses tables cause this is simpler and faster. 

Reference: Vanderslice and Higgs Vitamin C content of foods: sample variability. Am J Clin Nutr 1991;54:1323s-7s.  Picture credit.