Research Review: To every food there is a season

Research Review:
To every food there is a season

By Helen Kollias


So you’ve decided to turn over a new leaf and eat healthy. Great! Off you go to the supermarket with your grocery list.

Oh wait, maybe you should go to the special organic grocery store. Or the local farmers market, the farm… or how about you just become a farmer yourself.

Now that you’ve decided where to go, should you worry about other stuff?

Like if that vegetable is in season or not? Is it fresh, frozen or canned? And do these things even matter?

You just want to eat “healthy” and you thought the hard part was figuring out which foods were healthy, but how the food was produced may be as important.

Last week I reviewed an article that compared the nutrient content of organic and “conventional” foods over 50 years. While they didn’t find a difference between the production methods, I mentioned that I would rather see a study that compared a single nutrient from a single crop, and compared organic and “conventional.”

Why? Because too many other factors come into play when you compare multiple crops and multiple nutrients over a long period of time.

Another part (some argue the main part) of the organic and conventional debate is the question of safety: does one have less pesticides or less harmful pesticides than the other? This is a whole other can of worms that I’ll tackle in a different review, but for this one I’ll look at nutrient content.

This week’s study looks at vitamin C levels in broccoli, comparing organic versus conventional and seasonality. But before I get into that study I thought I’d give you a bit of background on other studies that looked at nutritional value and food.

What matters to the nutritional value of food?

While most people focus on whether organic or conventional is more nutritious, there are other factors that affect food nutrition. Factors that change nutritional value are (1):

  1. Variety of cultivar (specific cultivated plant)
  2. Weather/climate the crop was exposed to during growth
  3. Past harvesting – the more you harvest the less nutritious the food

Even pesticide use may affect vitamin C levels. In one study (2) they found that application of pesticides to peaches and pears decreased vitamin C levels.

Why? One theory is that vitamins and other phytonutrients are in the plant as a defense. Many of these plant chemicals have antibacterial, antifungal, and antiviral properties, so this makes sense. Fruit without pesticides had to build up its defences. No hand holding for this fruit!

Type of plant matters

It’s not a huge surprise that the variety of cultivar (aka the type of plant) would affect the amount of vitamin C (ascorbic acid). Figure 1 (from ref 3) compares five green vegetables (the difference between organic and conventional are also graphed).

Figure 1 – Vitamin C content in five fresh vegetables (from ref 3)

You can see that not only does each vegetable have a different amount of vitamin C, but growing methods (organic or conventional) affect the vitamin C levels differently.

Thus the effect of growing methods seems to vary with the type of plant. Organic lettuce has over 5 times more vitamin C than conventional, but Chinese kale shows no difference – in fact organic may have a bit less vitamin C than conventional.


Yup, the weather matters nutritionally. Most sun exposure means more antioxidants in the plant. This make sense when you think that antioxidants are to protect the plant from oxidative damage that the sun can cause. Drought and pathogens also change antioxidant levels (1).

Research question

Yes. If you compared USDA nutrient data (4) of 43 vegetables from 1950 to 1999 you would see a decrease in vitamin C levels. In 1955, broccoli had 118 mg vitamin C per 100g of broccoli, but by 1999 broccoli had 93.2 mg of vitamin C (per 100g). That’s a 20% drop in vitamin C levels over about 50 years.

This week’s review looks at how seasonality as well as production method affect vitamin C in broccoli.

Wunderlich SM, Feldman C, Kane S, Hazhin T. Nutritional quality of organic, conventional, and seasonally grown broccoli using vitamin C as a marker. Int J Food Sci Nutr. 2008 Feb;59(1):34-45.


Last week I said I would feel more comfortable with a study that went to the grocery store and picked up some organic broccoli and some conventional broccoli, then compared the nutritional content of each.

This week’s study did just that. Using random shoppers, they collected fresh heads of conventional and organic broccoli in May and in the fall (between October and December) from the local supermarket (in this case northern New Jersey, USA).

The broccoli selected was at the same stage of ripeness and harvest.

To add to the comparison, frozen broccoli was also collected (i.e. bought) from the local grocery store.


Once the broccoli was collected it was taken to the Food and Nutrition Laboratory (I wonder if they eat the leftovers) and assayed for Vitamin C levels. For those interested, they used the 2,6-dichlorophenolindophenol method.

That’s it. They bought broccoli from the grocery and checked the vitamin C levels. Simple enough.


Though they found slightly higher levels of vitamin C in the organic broccoli (both in May and the fall) there wasn’t enough of a difference to be considered “statistically” significant – less than 10%. But many would argue that this difference may still matter biologically – a little more vitamin C over the course of a lifetime could make a difference in overall health.

While organic versus conventional didn’t have a big difference, seasonality did. The broccoli in May (out of growing season for broccoli in the USA) had about half the amount of vitamin C than the in-season broccoli (fall). Yup, seasonal differences in vitamin C levels mattered more than how the broccoli was grown.

Something else that surprised me was that frozen broccoli had about 30% less vitamin C than the organic broccoli – in May. This means frozen broccoli had less vitamin C than out-of-season broccoli.

What the...? Has this dude been scamming us?
Has this dude been scamming us?


Okay, so this study didn’t definitively answer your nutrition and organic food questions, but I hope it did give you some insight into the complexity of food and nutrient content. Not only does it matter how your food was grown, but when it was grown (compared to when you’re buying).

Some other things to remember: First, vitamin C breaks down quickly compared to other nutrients. Thus, vitamin C is one of the nutrients most sensitive to storage.

Second, each plant nutrient profile is affected differently. So while this study shows that seasonality affects vitamin C in broccoli, other studies looking at other nutrients and plants need to be done.

Yes, it’s nice to be able to buy peaches while snow falls, but chances are you’re getting less than what you would’ve got in the summer: fewer nutrients, less taste and less value (I’m sure your winter peach is a wee bit more expensive than your summer peach). So do you really need a peach in the winter or would an in-season much tastier, more nutritious, cheaper orange be better?

Bottom line

While the debate about nutritional value of organic and conventional continues, this study shows very clearly that seasonality does matter. Yes, seasonality probably matters more so with vitamin C than with other nutrients, but chances are that not only are seasonal foods more nutritious, they are tastier.


Click here to view the information sources referenced in this article.

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