It is remarkably widely accepted that fruit and vegetables are good for you. Three a day, five a day, nothing but fruit and vegetables all day..... The problem is that all of the evidence of benefit is epidemiological, and this never proves causation, merely association.
Where does hard science take us? This was the first study I stumbled across, about Polish cyclists.
Effects of a low carbohydrate diet and graded exercise during the follicular and luteal phases on the blood antioxidant status in healthy women.
I only have the abstract of this paper so there is no information as to exactly what comprised the 5% of energy intake which was derived from carbohydrate. Whether it was pure sucrose, apples or bananas, there wasn't a lot there in total. So minimal plant based antioxidants to speak of. End result?
"The 3 days of the L-CHO diet, which had been preceded by glycogen-depleting exercise, resulted in a stimulation of the blood antioxidant defence system in young eumenorrhoeic women both at rest and during the graded cycling exercise to maximal oxygen uptake."
Not bad for three days of dumping the fruit and veg.
The next study, which is an excellent piece of work, was this one:
Green tea extract only affects markers of oxidative status postprandially: lasting antioxidant effect of flavonoid-free diet.
Never mind the green tea bit, that turned out to be irrelevant. It's what happened when almost all fruit and vegetables were removed from the diet of the volunteers for 10 weeks that's interesting. The result being:
"The overall effect of the 10-week period without dietary fruits and vegetables was a decrease in oxidative damage to DNA, blood proteins, and plasma lipids, concomitantly with marked changes in antioxidative defence."
This later study was NOT a low carb study. Potatoes, bread and cake were all included in the sample menus provided in the full text. Please note the DECREASE in oxidative damage to your genes, your protein structure and your lipids. Great stuff fruit, when you put it in the bin.
The conclusion seems to be that there is something nasty in fruit and vegetables, something pro-oxidant. You just have to ask yourself why a plant should manufacture an antioxidant in the first place. You can bet your bottom dollar that it was not for the benefit of herbivores! No, plants hate herbivores and negotiate with substances like strychnine rather than antioxidants.
The most likely candidate for a generic pro-oxidant toxin, produced by plants, is fructose. Plants outside of domestication contain relatively little fructose and appear to use it as a lure, to get their seeds eaten, then use the fruit eater as a transport method. They protect themselves from the fructose with antioxidants. Plants in domestication have been selected, by ourselves, to produce quite unreasonable amounts of fructose (plus glucose and sucrose). Just compare the average Granny Smith to a wild crab apple. A bag of sugar.
Mammals do exactly the same when presented with fructose. They make an antioxidant, in the case of mammals it's uric acid. Uric acid improves the total plasma antioxidant capacity after fruit ingestion. The mechanism is rather well summarised here:
Consumption of flavonoid-rich foods and increased plasma antioxidant capacity in humans: Cause, consequence, or epiphenomenon?
"We conclude that the large increase in plasma total antioxidant capacity observed after the consumption of flavonoid-rich foods is not caused by the flavonoids themselves, but is likely the consequence of increased uric acid levels."
Small studies using intervention strategies show this clearly. Don't forget those marvelous fruits and vegetables in this intervention study too. Oxidised LDL cholesterol anyone?
Or the Dutch study.
So in summary plants produce fructose which is both attractive and damaging to mammals. They protect themselves as best they can with antioxidants.
I don't see any causality between fruit and vegetable consumption and improved health.