Q: Does Seasonality affect Nutrient Content?

foods in season and nutrient content

I was recently asked this great question: Is there a nutritional difference in foods when they are in season vs. out of season? The short answer is yes — foods are more nutritional when in season. More foods need to be tested until we have exact details on which foods differ in what kind of nutrients and to what extent. However, I believe that in this case the details do not matter that much. The important question is: “Should I eat seasonally because of this?” Let’s look at some things that we do know and figure out the answer to this question.

Looking at the facts

The nutritional difference between seasonal and non-seasonal produce is due to two factors, both of which we will now examine in detail.

1. Out of season food is often not local

Eating in season means eating what is in season around you —it is always “summer” somewhere! Therefore eating seasonally generally means eating (more) locally. Produce starts to lose nutrients and flavor as soon as it is harvested, so when food has to travel hundreds of miles before arriving at the supermarket, a lot of nutrients and flavor are lost.

Furthermore, since manufacturers want the food to look fresh and ripe when it hits the shelves, produce is often harvested too early and then later sprayed with plant hormones to speed up ripening. Since the ripening occurs so quickly and often in some crowded box, the plant cannot accumulate as many nutrients and as much flavor as it would if it slowly ripened on its own. This explains why a strawberry can have a beautiful deep red color but taste like nothing.

Useful Information

Eating locally keeps business in the community and helps local farmers, who are generally less aggressive with their use of chemicals and protect biodiversity by producing a larger variety of plants. Try a farmer’s market or sign up for a CSA program in order to get fresher and tastier produce!

A study on Vitamin C content in broccoli

In this study researchers compared the Vitamin C content of supermarket broccoli in May (in season) and supermarket broccoli in the Fall (shipped from another country).

The result: the out-of-season broccoli had only half the vitamin C of the seasonal broccoli. They also looked at the vitamin C difference between organic and conventionally grown broccoli, but there was not a statistically significant difference between the two.

Vitamin C is less stable than other vitamins

It is important to note that vitamin C is a particularly fragile molecule. It breaks down much more easily than all other vitamins. It is therefore very likely that the content of other vitamins is not affected when eaten out of season as much as that of vitamin C. Minerals in general are more stable than vitamins and therefore degrade even less over time. The mineral content of produce is mostly determined by the soil quality in which the food was grown.

However, the nutritional value of a food does not only depend on vitamin and mineral content. There are hundreds of different phytonutrients (more are being discovered all the time), some of which are also affected by seasonality.

What are phytonutrients?

(phyto=plant) Phytonutrients are nutrients that occur naturally in plants, but are not essential nutrients such as the established vitamins and minerals. This means they are not needed for survival. However, there is strong evidence that many of them offer great health benefits (cancer-protection, anti-aging, bone health etc.) This is one reason why popping a vitamin pill cannot replace eating fruits, vegetables, whole grains and legumes.

2. Weather differences can affect nutrients directly

Plants create phytonutrients in order to protect themselves from damaging UV-rays (sun), pests, and pollution. Since our own biochemistry is not all that different from a plant’s, many phytonutrients can protect us as well.

In many cases, radiation from sunlight acts as the trigger that puts the biochemical processes in motion which ultimately create these phytonutrients, similar to when your skin produces more melanin (brown pigment) in response to sunlight in order to protect itself. Chilling is another possible environmental trigger that positively affects phytonutrient content in plants.

Since the amount of sunlight (and other possible triggers) is tightly linked to different seasons, phytonutrient content is too. A lot of phytonutrients are responsible for the color and flavor of a food, so it follows that they get compromised as well.

If taste is affected, nutrition most likely is too

Even without any direct evidence, we could have arrived at a similar conclusion. You only have to compare the flavor of a fresh vs. seasonal tomato and you know they are different. As stated above, both flavor and nutritional content comes down to a plant’s biochemistry. Biological systems are so complex and interconnected that when a chemical difference causes a change in flavor, it likely also changes the nutritional value.

Discussing the implications

Should you eat more seasonally because of this?

While there are definitely many good reasons to eat more seasonally (some are discussed below), I am not sure I would count nutritional differences as one of the main ones. Not only are most nutrients only minimally affected, but there are still a lot of benefits in eating fruit and vegetables, even when they are out of season.

Produce contains a lot of fiber (healthy and filling), is mostly very low-calorie and even with some nutrient loss still contains a lot more nutrients than most foods. Generally, it is always better to eat produce out of season than to not eat it at all! And quite frankly, most people have much more pressing dietary problems than not getting enough nutrients because the produce they eat is out of season…

You can help nutrient loss

If you are concerned about nutrient loss, start in your kitchen. Nutrient content of food is much more affected by the way you cook (process) it than by any seasonal differences discovered so far. In comparison: vitamin C in broccoli was reduced by 50% due to the difference in season, but it is possible to lose up to 80% of vitamin C by preparing broccoli incorrectly! Also, food still loses nutrients even when in the fridge (rate is slowed down however), so try to eat it sooner rather than later.

Want to know more?

If you want to learn more about how to prevent nutrient loss or how to prepare food in order to maximize flavor and nutrients, stay tuned for later posts! There is definitely a lot to be said on the subject.

The benefits of eating more seasonally

There are definitely many benefits to eating more seasonally beyond nutritional differences. First of all, produce is often cheaper when in season. Also, there is absolutely no doubt that seasonal and fresh food tastes much better than what you usually find in supermarkets out of season. This can definitely help some people to eat more produce. A friend of mine converted from a complete tomato hater to a tomato lover when he tried a ripe fresh tomato for the first time.

Focusing on the food that is available during each season also helps you to eat a wider variety of foods. Every food has its very own nutrient composition (different phytonutrients) and eating different types of foods helps you reap a wide variety of health benefits. A lot of people have their standard fruits and vegetables that they consume all year round. While this is certainly good, eating different types of vegetables and fruits is almost as important as eating them in the first place.

Want to try it?

Remember that the world is not black and white. Every step you take towards a better diet (no matter how small) is a great step! Try a fruit or vegetable that you have never tried that is currently in season. Don’t expect to like everything right away, taste is mostly acquired. But chances are you will like it a lot more when it does not taste like cardboard.

Every season has amazing tasting food and I strongly believe that you should not eat things when they do not taste good. Life is too short to not enjoy food! Of course, if you enjoy the wrong kinds of food… life is even shorter.

Special challenge

If you already eat a lot of produce, but want to eat a wider variety, try this challenge: Include at least one seasonal fruit or vegetable in your diet everyday. Make sure they are different ones on all days of the week. If you wonder what is in season right now, check out this guides: Fruit chart and Vegetable chart. Have fun exploring!

If you have any more questions for me feel free to ask me in the comments below or send me a private message. Let me know if you tried anything and how it was!

– Christina

Image: FreeDigitalPhotos.net

13 comments

  1. Mark H. says:

    What about frozen veggies vs “fresh, but not local”? Any idea about C and micro-nutrient content preservation, and is there less ripening chemicals used on these products?

    • Another great question!

      Vitamin C content is most likely even lower in frozen produce than in “fresh, but not local”, because Vitamin C content is reduced by about 30% by the actual process of freezing.

      This vitamin is an exception however, freezing itself causes a loss of about 5% of: Vitmain A (and carotenoids, lycopene etc.), thiamine, folate and calcium, a 10% loss of potassium and cooper and has no effect on iron, magnesium, phosphorus, zinc, vitamin B12, vitamin B6, niacin and riboflavin.

      The real question is: “how soon after harvest was the produce frozen and how long will you keep it in the freezer?” Nutrients are still lost at a small rate even when frozen. So if buy frozen vegetables but don’t use them for a long time, you are probably better off buying them fresh when you need them.

      Of course, if there is time between the time of harvest and freezing, they also loose nutrients. In a study, the Vitamin C content of frozen broccoli was compared to seasonal broccoli and “fresh, but not local” broccoli. The Vitamin C content of the frozen broccoli was 30% lower than the out of season one. This suggest that there was in fact some nutrient loss between the time of harvest and time of freezing. However, this probably depends on the manufacturer. You could probably call them and ask about this.

      Overall, the two are probably pretty similar in nutrient content (frozen vs out of season). So choose whatever makes you eat them more (prize/flavor varies)! But including some frozen vegetables into your diet —Ones that are generally not available fresh— can increase the variety of produce you eat and is therefore a good idea (peas, snap peas, edamame etc.)

      One way to make sure that fruits/vegetables were not chemically ripened is to buy them organic. These also contain less pesticides and other contaminants. So if you can afford it and are concerned about this, choose whatever is available organic (fresh or frozen).

      Another great idea is to buy produce local when in season (and cheap and tasty) and freezing some of it yourself. This is probably the best way to ensure maximum nutrient content in out of season produce.

      • Mark H. says:

        Why is Vitamin C being reduced, and does it turn into something harmful? Is it working as an anti-oxidant veggie preservative keeping the food safe by binding to harmful radicals? Which leads to another question…as nutrient value declines what kind of harmful “anti” nutrients increase?

    • @Mark H.

      I love how your questions dig into the details! I am going to try to answer your question as well as I can without going into any specific details of biochemistry. If you want to know even more details I suggest you pick up a biochemistry and organic chemistry textbook. ☺

      Here are some important things to realize: nutrients in plants are ultimately there because the plant needs them for itself. As stated before, cells (and their underlining chemistry) are pretty similar in plants and animals. A big difference is what kind of nutrients we can create ourselves and what has to be taken in through diet. Plants need oxygen/carbon from the air, 16 different minerals from the soil and sunlight in order to produce all kinds of vitamins/phytonutrients, which we ultimately take from them.

      As the plant lives its life, it respires and therefore constantly uses up nutrients and creates more. However, when nutrients get produced they are not immediately used up. There are always some nutrients “floating around” inside and outside the cells. When we harvest a plant, the plant can obviously no longer get minerals from the soil (cannot create many more nutrients), but the cells keep respiring. Therefore these nutrient reserves slowly decrease. That is why fresh is better!

      You may wonder why we cannot still use the nutrients when they are in essence still in the plant! It is important to understand that a chemical’s properties —and therefore function in the body— are due to its “shape”. Everyone who took basic chemistry knows that two “things” can react to form a completely new “thing”, because the shape of chemical changes when they react. This means vitamin C is only “vitamin C” when its molecules have a certain shape. The shape of vitamin C is fragile (contains bonds that are easily broken), so that is one reason why it degrades quickly. Heat and certain chemicals can break these bonds easily. Which is one reason why vitamin C is lost readily during cooking.

      The other reason is that Vitamin C is, like you said, an antioxidant and is needed quite a bit by the plant’s still respiring cells. Especially when you bruise (cut) the plant. Even though I do not have specific sources on that, I believe that vitamin C levels degrade more in frozen produce than other vitamins do, because frozen foods are often chopped. This also exposes more surface area to air, which increases oxidation. So the plants use their vitamin C, which acts as a reducing agent to reduce the oxidation. This answers one of your questions, vitamin C does not turn into anything harmful. (as far as we know, of course) ☺

      And yes, that is exactly why it is used as a preservative. When you cut an apple it turns brown (due to oxidation). If you put lemon juice on the slices however (lots of vitamin C) the slices don’t turn brown (at least not as quickly).

      So in a way you could say that nutrient value decreases and “anti”nutrients (free radicals mostly) increase. But not because one converts into the other. Bottom line:
      – eat your produce when the plant has not already used up all the nutrients itself.
      – keep produce cold to slow respiration rate (with some exceptions)
      – don’t cut fruit and veggies a long time before using them (with some exceptions)

      But in general, as long as you do not eat rotten produce, I do not think the “anti” nutrients (I like that word, thanks!) in still somewhat fresh produce should concern anybody. We do have our own defenses against them, plus some that are still in the plant. What I am much more concerned about is the free radicals that get created by cooking foods the wrong way! More about that later.

  2. Rapunzel says:

    What an amazing & informative post, a lot to “digest.” I am guilty of not buying locally as often as I should, simply out of laziness. It’s easier to pick up fresh veggies at the supermarket rather than making the extra trip to the farmers market. *blush* Thank you for the reminder to do better!

    • Thank you!
      Well, I guess it all depends on how close the nearest farmer’s market is. At least here in Iowa, there is often some local produce in the supermarket as well. It is specifically labeled as such. Maybe you could ask if they have something similar in your store!

      When it is nice out, it is amazing to go pick up some produce with a bicycle. The fresh air and scenery is so beautiful and it makes the whole process feel extra healthy (and it is!). A lot of times the bicycling itself is motivation enough for me to go get fresh food. We all struggle with the same things no matter how good the motivation/reasons. We are all human… The key is to find small tricks/strategies that work for YOU. Taking the bike works for me.

      As I said in the post. Do not worry too much about the nutrients. Cooking it the right way is much more important when it comes to preserving nutrients. Focus on the taste and maybe that will be motivation enough to buy more fresh produce. Can’t wait for spring/summer!

  3. Stef says:

    amen! so glad you wrote this post, a lot of people are totally unfamiliar with the concept or in season/out of season because things appear yearround in the stores. the only reason i know the diff is that i have the luxury of shopping at the farmer’s market :)

  4. Kendra says:

    A little confused…. The chart you linked to shows broccoli as being in-season all year round.

    • Yeah you are right, Kendra. This table says when you can find it at the market and people harvest it. Not necessarily when it would be best to harvest them… broccoli is a fall vegetable. I am sure there are other tables out there. Maybe it would be best to talk to a local farmer about seasonality, since it also depends on where you live.

  5. Kate says:

    These posts are very helpful, but I was wondering if there was a way to cite them. Do you have any sources where you got this information from? I’m trying to write a paper for a 400-level nutrition class.

    Thanks!
    Kate

    • Hi Kate,

      I linked to some sources in this article (research paper), but the rest of the article I just wrote from the top of my head. You could cite me, but I doubt that is very credible. :) Sorry! Feel free to email me and tell me about your paper, maybe I can help you out with some sources.

  6. […] turn supporting Australian farming families and our local economy.  Out of season produce is often not locally sourced and in some cases is sourced from overseas. Check out a few of our Australian suppliers […]

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