Recommendation: First, identify where your focus should be when it comes to healthy eating. If you’re eating veggies every day without much effort, research clearly suggests you may benefit from thinking more about how that healthy food gets from the farm to your plate. However, if there are obstacles preventing you from getting healthy food consistently, you have more to gain by simply thinking about how to get any healthy food on your plate, regardless of how it got there.
Don’t make the perfect the enemy of the good
The fact that fresh veggies are healthier may not matter to some people. For example, consider healthy living on a scale from 0 to 100. Going from “0” to “1” is not like going from eating cookies for dinner to eating the freshest, local spinach. Likewise, going from “99” to “100” is not where quitting smoking happens; that is, quitting smoking is part of the first steps along this process–a.k.a. from “0” to “1”. Therefore, it’s effective to have an idea of where we are on the scale before determining where to spend time and effort in changing our habits. For this reason, even if spinach loses a lot of its nutrition after 2 weeks out of the ground, the person who’s just beginning their journey to improve health shouldn’t burden herself with only eating spinach from this week’s farmer’s market. In essence, the difficulty of getting “perfect” spinach would make it the enemy of the “good” spinach which would be a great improvement from eating cookies. The underlying benefit of first making easier, yet still effective choices is that once we are far down the path of healthy living, we’ve got routines in place which make seeking that farmer’s market spinach easier and less annoying, for lack of a better word. While the science makes a great case for everyone to exclusively shop at farmer’s markets, it doesn’t necessarily outweigh the great, legitimate reasons for not doing so (including convenience, habit, and preference). Further, in my opinion there’s a lack of data in one big way; while we know nutrients drop rapidly and significantly, I don’t see research showing that this drop brings the level of nutrients below the level I need in order to “be healthy”. For example, fresh broccoli was shown to lose about 50% of its vitamin C (ascorbic acid) within a week out of the ground, but it still retained enough to provide the daily recommended intake in one serving.1 If my goal is to get vitamin C from broccoli, the conventionally-grown broccoli in the grocery store is fine.
Don’t let the “good” replace the “perfect” as a target
Certainly, classic academic/nutritionist advice for healthy eating reflects the “moderation/balance/variety” concept. This guidance is of course sound for those seeking to reduce chances of illness. But is it sound advice for those seeking to maximize health or performance (the ones who’ve already incorporated moderation and have time/effort to spend tweaking their habits further)? The business world teaches that avoiding failure isn’t the same as seeking success; is it different with health? If each person respects their own limits of what they can and want to do, why shouldn’t the guy who’s happy to spend all day reading research do just that? For him, the “moderation/balance/variety” would be simply “good” health, whereas he’s willing to seek “perfect” health (knowing it’s not possible)–it’s the same wisdom as setting goals just out of reach. It’s this person who truly benefits from knowing what the research on this topic shows, which is that “avoiding illness” is definitely not the same as “maximizing health”.
The message is clear: the fresher, the better
As shown in the tiny summary of research below, it’s almost guaranteed that there’s a vast difference in the nutrient content of seemingly identical veggies and fruits available to us. Before even getting onto the shelf, several factors determine how healthy your veggies will be when you eat them. Nutrient concentrations can depend on vegetable type, maturity at harvest, genetic variations, pre-harvest conditions, storage conditions, processing, and preparation.
It should be noted that some veggies are naturally high in certain nutrients which are stable (e.g. beta carotene in carrots), but low in the nutrients that disappear over time (e.g. folate). While potatoes are a good source of folic acid, carrots are not. Therefore, it’s not effective to worry about folate loss in carrots, even though we know that folic acid levels decline during storage. Taken broadly, the idea is to avoid eating foods which are supposed to be high in a nutrient if that nutrient is lost while sitting on the shelf. Don’t spend a ton of money on two-week old conventionally grown strawberries thinking you’re getting the vitamin C you need.2 The good news is that we don’t have to memorize every vegetable and the nutrients in them that deteriorates over time; there seems to be enough evidence to justify efforts to get certain groups of produce soon after harvest more than others (e.g. leafy greens and berries).
Most of the research I found was related to the level of anti-oxidants in foods after harvest. For this reason, it’s important to remember there isn’t yet proof that eating anti-oxidants has disease-preventing effects in humans. For that, we need to see more in vivo (in the body) research with humans. Still, the anti-oxidant activity of these foods is clearly shown in vitro (in petri dishes) and ex vivo (in body-like environments outside the body). In my opinion, the other benefits of most fruits and veggies are enough reason to eat them, but anti-oxidant activity is likely an added quality which justifies eating more of the foods that contain them.
One meta-analysis reports several findings,
“There are few reports on vitamin B1 (thiamine) and niacin loss during storage. Potato tubers had insignificant loss of vitamin B1 and niacin after 30 weeks at 5°C and a slight loss at 10°C. Small losses are found during storage of green beans, peaches and sweet potatoes. Carotene content (vitamin A) shows little loss in sweet potato during 4 months of storage at 24°C. Significant losses of β-carotene occurs in kale (17%) collards (30%), turnip greens and grape held at 10°C instead of 0°C. Oddly, carrots show an increase in carotene during the first months of storage even allowing for water loss and storage at different temperatures, and there is a steady increase in lycopene and other carotenes during tomato ripening at 15°C and 30°C, while at less than 1°C and above 30°C, no lycopene synthesis takes place. Folic acid losses of up to 40% can occur in potatoes stored at 7.5°C for 8 months.”3
Spinach, stored in perforated bags for up to 7 days at 10 °C (50 F), apparently retains flavonoid content, but not anti-oxidant capability due to the type of Vitamin C changing while still measuring as “Vitamin C”.4 The stability of these flavonoid compounds contrasts with that of hydroxycinnamic acids (anti-oxidants in tea leaves, coffee, red wine, various fruits (especially red ones), vegetables and whole grains)5 and anthocyanins (anti-oxidants in red and purple berries) (Gil et al., 1998b).
Other than time, it seems the most important factor of post-harvest nutritional decline is temperature. Depending on where the food naturally grows, this could mean either heat or cold is harmful. Tropical sweet basil, for example, is harmed by refrigeration (even under 15 C/ 59 F)6 while leafy vegetables held at 6°C (42 F) lose 10% of their AA content in 6 days and those held at room temperature lose 20% in only 2 days.7 Same goes for spinach, cabbage, snap beans, and citrus (though all vary in rate of decline, partially due to higher stability of Vitamin C in acidic (citrus) environments).
Processing and farming method also affect nutrients. There is evidence that freezing berries and corn preserves anti-oxidants significantly more than freeze-dried or air-dried processing. The same research shows that neither conventional nor organic farming resulted in as high anti-oxidant levels as sustainable farming (defined in the study as “meeting the needs of consumers without compromising the ability of future generations to meet their own needs”).8
What’s the best way to eat?
To absolutely maximize health, it seems that we should grow our own food on land which is sustainably farmed; this means crop rotation and also raising animals with the intent to develop a soil-enhancing ecosystem. Basically, it’s all about good dirt, proper sunlight, water, a little hormetic stress, full-ripening, and consumption as soon as possible after harvest. That’s why home-gardeners enjoy way better food.