Organic, Conventional, or Homegrown: What the Research Really Shows About Nutrition - Ep. 313

[00:00:00] I have said for years that homegrown vegetables are more nutritious than what we buy at the store, and I have said that with a lot of confidence. I also have a firm belief that organic produce is more nutritious than conventional, but maybe not for the reasons that you might think. And after digging deep last week into why I trust university extension resources in terms of peer-reviewed research and the expectations behind that,

[00:00:27] I thought that maybe I should put my money where my mouth is, so to speak, and see if there is real published science behind what it is that I believe. And my hypothesis has always been this. Food is more nutritious when it's consumed as close as possible to where it was grown, and the soil health affects the level of nutrition available in that food.

[00:00:51] So if an organic producer is adhering to healthy soil practices, then the food will be more nutritious to start with. And I wanted to prove or disprove that theory once and for all so that I can speak with true authority on it. Today's episode is a result of me truly digging into the research because it has changed over the years, and me just trying to come up with the truth.

[00:01:19] Strap in my gardening friends because it's a wild ride. Welcome back to Just Grow Something. I'm Karin Velez, specialty crop farmer and garden educator here to keep gardening grounded in good science. We are getting down and nerdy again this week, but stick with me. I went through the peer-reviewed literature on this question, and I'm going to walk you through what it says, the parts that support the case for growing your own food, and the parts that may be complicated.

[00:01:49] Let's dig in. So shout out this week goes to the number of people who are brand new here and who have messaged that you are going back and starting with episode one. This is happening more and more frequently lately, and I decided that maybe I too should go back and listen to a few of the very early episodes from seasons one and two for myself.

[00:02:14] And after giggling a little bit at my very over-the-top intros and outros and gasping at some of the shady audio quality over the years, I did extract one very important tidbit. I have been incredibly consistent about mentioning three very specific things from the very beginning on this podcast. University Extension, which was mentioned as early as episode two.

[00:02:44] Garden Journals, as early as episode five. And Mulch, beginning, I believe, in episode eight. Add to that the number of times I've talked about my coffee habit, and I think you can say I've got a pretty consistent theme running through all six seasons of this show. Good, bad, or indifferent. So whether you've stuck around since the beginning or you jumped in mid-season, I appreciate you being here either way.

[00:03:08] If you are new and you aren't following the show yet in your podcast player or subscribed on YouTube, hit that button now so you don't miss anything. I mean, you don't want to miss my next rant about mulch, do you? So this idea that consuming food closer to where it's grown is more nutritious for you has been something that I have talked about as early as episode three of this podcast,

[00:03:35] which I realized as I was doing that little revisit of the early days of this show. I've also written blog posts about it. And we've always grown things in an organic and sustainable manner on our farm, and that started back when I was simply a backyard gardener growing for my family. Plus, being in both the food-growing space but also the recreational athletics space,

[00:04:02] I am super invested in nutrition and I even got a certification in nutrition coaching. So I also get blog posts and videos and articles from registered dietitians who tell people not to bother with organic, and I get small certified organic growers maintaining that, no, you absolutely should buy organic. So if you've spent any time reading anything about organic food,

[00:04:30] you have likely run into this headline in one form or another. Organic produce is no more nutritious than conventional produce. It shows up constantly and it usually traces back to one specific study. So let's start there and we'll give it a fair shake. In 2012, a team at Stanford published a systematic review in the Annals of Internal Medicine.

[00:04:55] They pulled together like 223 studies comparing nutrient and contaminant levels in organic and conventional foods, which at the time made this the largest review of its kind. Their conclusion, in their own words, was that the published literature lacks strong evidence that organic foods are significantly more nutritious than conventional foods.

[00:05:21] They found no meaningful differences in vitamin C, vitamin A, calcium, potassium, magnesium, or iron. The one nutrient difference they did find was slightly more phosphorus in organic food, but that's not exactly a headline grabber, right? So where the Stanford review did find a real consistent difference was pesticide residue.

[00:05:45] Conventional produce had detectable pesticide residue about a third of the time compared to only 7% for organic. Even looking outside of produce, they found organic chicken and pork also carried lower rates of antibiotic resistant bacteria. So the takeaway from that study was never that organic and conventional foods were identical.

[00:06:11] It was that if you're choosing organic strictly for a nutrition boost, then that review didn't back that up. If you're choosing it to reduce pesticide exposure, that evidence absolutely supported you. That's a big, well-designed study. And it's the one that most articles cite when they tell you organic is a waste of money. But it's not the only study, and it's not even the most comprehensive one.

[00:06:40] So two years after the Stanford review, a much larger research team published a meta-analysis of all of this information in the British Journal of Nutrition. That one looked at 343 peer-reviewed publications, which is over 100 more studies than the Stanford team reviewed. And some of those additional studies just hadn't been published yet in the two years prior.

[00:07:05] So that team found something that the Stanford review wasn't really designed to catch. In the research, organic crops had substantially higher concentrations of a group of compounds known as polyphenolics, which include things like flavonoids and flavanols and anthocyanins, okay? So depending on the specific compound, organic crops ran somewhere between 19% and 69% higher.

[00:07:34] The researchers estimated that someone eating an organic diet would take in 20% to 60% more of these compounds than someone eating conventionally grown food. Now the same analysis found conventional crops carried roughly double the cadmium, which is a toxic heavy metal, likely because of how certain phosphorus fertilizers make cadmium more available for plant roots to absorb.

[00:07:59] So why did two well-conducted reviews reach such different headlines? A big part of the answer is definitional, right? Vitamins, minerals, and protein are nutrients in the very classic sense, things that your body needs in specific amounts to survive and to function. Polyphenols and the other phytochemicals aren't classified as essential nutrients

[00:08:27] because you won't develop a deficiency disease if you don't eat them. But there is a growing body of research that connects these compounds to reduced inflammation and lower risk of chronic disease and cancer prevention. So when a review only measures classic nutrients, organic and conventional food look nearly identical. When a review also measures phytochemicals, a real and repeated difference shows up.

[00:08:57] And this pattern holds across several other reviews too. There was one in 2001 that looked like 41 comparison studies and found organic crops averaged about a fifth to a third more vitamin C, iron, and magnesium than conventional crops, along with a significantly less amount of nitrate. A different 2011 review took a more controlled approach and they only included studies that compared the same crop variety grown in similar soil conditions.

[00:09:27] And under those tighter controls, the organic advantage in micronutrients like beta-carotene, vitamin C, boron, copper, and zinc ran even higher in some comparisons approaching 50%. Now none of this means organic food is a guaranteed nutritional upgrade every single time you buy it. Study results vary by crop and by growing region and by which specific compound that you're measuring.

[00:09:50] What it does mean is that the flat, very confident claim that there's no nutritional difference between organic and conventional food doesn't hold up once you look at the phytochemical data, which is not just the vitamin and mineral data. Here's where the research gets really interesting and where my own hypothesis kind of started to shift a little bit while I was reading it.

[00:10:14] If we assume going into this that organic versus conventional would be the deciding line, then the strongest recent review on the subject argues that we're actually drawing the wrong line. All this talk about organic versus conventional truly hits home for me as someone who has not only gardened organically for over 20 years, but whose farm is committed to sustainable practices that go beyond just not using synthetic pesticides or fertilizers.

[00:10:42] That's why I love the commitment that Larry's coffee has made, not just in terms of supplying organic and fair trade coffees, but even in the way they operate their own facilities. They have solar water heated floors and a massive clear story on their building that provides passive solar, saves energy and provides natural day lighting, which I think is inherently better for their workers than fluorescents or LEDs. They use zone heating, which saves energy by only using it where folks are working.

[00:11:11] All of their food waste is composted using vermicomposting, go worms. They have their own gardens and they have four tanks that hold 2,500 gallons of rainwater that they collect and use to run their bathrooms and to water those gardens. If I lived in Raleigh, North Carolina, I guarantee you I would be at Larry's Coffee multiple times per week. But since I don't, I'll have to settle for having their coffee shipped to me

[00:11:37] and I just received a fresh batch this week to keep me fully stocked up. You can support a coffee company that's doing so much to support the environment and the people working in it, both domestically and abroad, by heading to larryscoffee.com slash justgrow. That link supports this show and you get a free fair trade gift with your first purchase. Support your coffee habit, support this podcast, and support a company that does more than pay lip service to sustainability efforts.

[00:12:06] Larryscoffee.com slash justgrow is where you go. The link is in the show notes. So in 2021, a review was published in the journal Frontiers in Sustainable Food Systems that reframes this whole debate around soil health instead of the organic label.

[00:12:31] Their argument, backed by decades of studies, is that a crop's mineral and phytochemical content depends less on whether a farm is certified organic than on the biological condition of its soil. It's the biological activity of the soil that that crop is grown in, specifically the abundance of diversity of bacteria, fungi, and earthworms, doing the work of making nutrients available to the plant roots.

[00:13:00] Now, organic and conventional are both umbrella terms that cover a wide range of practices. You might not think that in terms of organics, but it's true. A conventional farm using no-till methods and cover crops can actually still build excellent soil biology.

[00:13:19] An organic farm that relies on heavy tillage for weed control can degrade its soil life just as much as a conventional farm could. So lumping every farm into one of two buckets kind of blurs what's happening underground, which really seems to be the most important part. And this is where my hypothesis sort of fell apart.

[00:13:47] I was making the assumption that the organic label was being adhered to in a way that I see my local farmers and myself practicing organic, being concerned about the soil health just as much as we're concerned about not using synthetic pesticides or fertilizers. Unfortunately, the requirements for this under the certified organic labeling standards have fallen way off.

[00:14:13] And on a commercial level, and I'm talking large scale commercial farming, they don't seem to be practiced as strictly as one might hope. And this is just one more problem with the government owning a trademark on something that farmers originally came up with. But that's a whole other story. The mechanism connecting soil life to plant nutrition is something that we've talked about before. The soil fungi that you generally cannot see. Mycorrhizal fungi, they form those partnerships with the plant roots.

[00:14:42] That extends them far beyond what the roots could reach on their own. And that helps deliver minerals like zinc and phosphorus back to the plant in exchange for the sugars. So a 2014 meta-analysis pulled data from 104 separate studies and found that when mycorrhizal fungi were present and active, zinc concentrations in crops increased by close to a third.

[00:15:08] Synthetic nitrogen fertilizer applied at the rates that are typical of conventional agriculture tends to reduce how many of these fungal partnerships a plant bothers to form. When nitrogen is super abundant and really easily absorbed directly by the plant, the plant just has less incentive to invest its energy in feeding those little fungal partners. And so it recruits fewer of them and the mineral uptake through that particular pathway drops.

[00:15:38] Nitrogen fertilizer also affects phytochemical production through an entirely different mechanism. Plants produce defensive compounds, a lot of which are the same polyphenols that we talked about a few minutes ago, partly in response to stress and scarcity. So when nitrogen is scarce, plants tend to invest more energy into these defensive compounds. When nitrogen is abundant, plants put that energy into growth instead, right?

[00:16:08] We've talked about the excessive leafy green growth that happens when we have too much nitrogen present, and that comes at the detriment to other activities, most notably fruit production, but apparently also in these phytochemicals and compounds too. So this pattern has actually been documented since the 1940s, when a study found that heavy nitrogen fertilization cut the calcium content of turnip greens by more than a third across several field trials.

[00:16:36] And it lowered their vitamin C levels too. And that's something being grown specifically for the leafy green growth. And this pattern shows up across crop after crop and decade after decade. And long-term field trials back this up. The Rodale Institute has run side-by-side organic and conventional plots on the same land since 1981.

[00:16:57] By 2018, researchers had reported that oats grown in the organic system averaged roughly a third higher mineral content across the board, with a 23% bump in iron and 40% more zinc. Vegetables from the same trial showed similar patterns. Organically grown tomatoes and jalapeno peppers ran 18 to 36% higher in vitamin C, and carrots averaged 29% higher in total antioxidants.

[00:17:25] A separate 2010 study out of California compared 13 paired organic and conventional strawberry farms, and found the organic strawberries had significantly more polyphenols, vitamin C, and antioxidant capacity, alongside measurably higher soil carbon, microbial biomass, and plant-available iron and zinc in the organic soil itself.

[00:17:48] And a 2016 study of wheat farms in central India found organic wheat carrying 20% more zinc than conventional wheat, even though the soil's total zinc supply wasn't any different between the two, which points straight back to biology rather than just the raw mineral availability. So the conclusion is actually a layered one. Farming practices absolutely change the nutrient density of the crops.

[00:18:18] But the organic versus conventional label is just a rough representation for the practices that actually matter, like tillage frequency, synthetic nitrogen use, cover cropping, and just basically how much living biology is present in the soil. Building healthy soil, whichever certification is on the label, is what actually makes the difference.

[00:18:43] And then there's one more thread that's worth pulling before we kind of leave the growing side of this and move to what happens after the harvest, because you've probably also heard some version of this claim, that a carrot, for example, today has far less nutritional value than a carrot from like our grandparents' era. The most cited source behind that claim is a 2004 study from the University of Texas,

[00:19:10] and that was published in the Journal of the American College of Nutrition. They compared USDA food composition records from 1950 and 1999 across 43 different garden crops, and they reported apparent declines in protein, calcium, phosphorus, iron, riboflavin, and vitamin C, with individual nutrients dropping anywhere from 5% to almost 40%, depending on the crop.

[00:19:39] And I have quoted this study before when talking about soil health decline and the nutritional content of our foods. But I want to give you the complication here, because essentially what I've done with this episode is a sort of mini meta-analysis, and a good analysis doesn't cherry-pick the studies that support the tidiest story.

[00:20:02] A 2017 critical review in the Journal of Food Composition and Analysis went back through this same historical data, and it raised some serious methodological concerns. Comparing food composition tables that are collected 50 years apart runs into a pile of confusing variables, different crop varieties, different testing labs, different analytical models,

[00:20:32] different regions of origin, and even differences in how ripe the produce was when it was tested. So that review found that once you account for these factors, the case for soil depletion as the driving cause is actually a lot weaker than what we thought it was. Where real historical declines do show up consistently across studies,

[00:20:58] calcium down 16%, phosphorus down 9%, iron down 15%. The leading explanation isn't necessarily depleted soil. It's decades of crop breeding that has prioritized size and yield and shelf life over mineral density. So we think about this logically.

[00:21:22] Higher-yielding varieties tend to produce more plant tissue without a matching increase in nutrient uptake, which spreads the same amount of minerals across more plant material. This is the dilution effect, and it's a breeding story as much as it is a soil health story. So where does that leave us?

[00:21:47] The historical decline numbers are real enough to take seriously, I think, but the usual explanation that we hear, which is that we have mined the nutrients out of American soil, oversimplifies it a bit. I think it's a much more twisted picture that involves plant breeding and testing methodologies, and yes, soil biology too.

[00:22:11] So now we get to the part of this analysis that I think matters most for us as home gardeners, because this is where the growing method stops being the main character and time takes over. So vitamin C is the nutrient that scientists use most often as a marker for nutrient decay,

[00:22:39] because it's water-soluble, it's sensitive to oxygen, and it breaks down very predictably, so much so that we can track it very easily. So a 1998 study published in Food Chemistry is still one of the clearest looks at this timeline, even now, 28 years later. The study tracked vitamin C in peas, broccoli, green beans, carrots, and spinach,

[00:23:05] from the moment of harvest all the way through commercial distribution and home storage, comparing all of it against the same vegetables commercially frozen shortly after picking. So if you've listened for a long time, you've likely heard me talk about this. Fresh peas and broccoli lost more than half of their vitamin C within just 24 to 48 hours of being picked.

[00:23:30] They did hold their quality reasonably well for up to about 14 days if they were chilled immediately and kept cold consistently the entire time. But produce that went through typical market distribution and then home refrigeration did not get that ideal treatment, and it absolutely showed in the studies. For example, the data found that frozen spinach, quick frozen right after harvest,

[00:23:58] and stored for up to a year retained more vitamin C than fresh spinach that had spent a week sitting in a home refrigerator after being shipped in. And the nutrient status of frozen whole green beans and frozen carrots had virtually no loss when it was frozen, but faced the same degradation when not held at temperature after being harvested. And that result surprises people, I think.

[00:24:28] But it doesn't mean that frozen vegetables are inherently better than fresh ones. It means the fresh vegetables in that comparison had already lost most of their nutritional advantage by the time a week had passed because vitamin C just doesn't wait around. It starts degrading the moment the vegetable is separated from the plant. And that rate of degradation depends heavily on temperature and time.

[00:24:54] And we don't always know how it's been handled or at what temperature it's been held when it's been traveling from the field to the store or how it's being held at the store before we buy it and then we bring it home, okay? A 2007 review backs this up, but from a different angle. This review concluded that nutrient loss in produce marketed as fresh is routinely underestimated by consumers

[00:25:18] precisely because appearance is a poor stand-in for nutrient content, okay? A vegetable can look crisp and vibrant and may have already lost a very substantial share of its vitamin C. Nothing about its outward appearance is going to give us a clue as to the level of nutrient loss. And this isn't limited to vitamin C either. Broccoli offers one of the more dramatic examples, and it's probably the one that I've talked about the most.

[00:25:45] So in addition to the loss of vitamin C that we've already talked about, broccoli has a cancer-protective compound called sulforaphane, and it comes from a precursor called glucoraphanin. A 2003 study found that broccoli stored at room temperature lost 55% of that compound within just the first three days after harvest.

[00:26:06] Even refrigerated storage in a sealed bag only slowed that loss to about the same 56% by day seven. That's more than half of one of broccoli's signature health compounds gone within about a week, even if it was held at just the right temperature, regardless of how it was grown or whether it was organic or conventional.

[00:26:32] And then one more study tracked a range of common vegetables through refrigerated storage and found that the total phenolic compounds, anthocyanins, and vitamin C all declined the longer the produce sat in the refrigerator, even though the vegetables looked perfectly fine the entire time. So if you put all those studies together, then you get a pattern that has nothing to do with organic certification. The nutrient content of a vegetable is the highest at the moment of harvest,

[00:27:01] and it declines from there very rapidly, sometimes very sharply within days. Grocery store produce, by the nature of how our food distribution system works, has almost always been separated from the plant for much longer than, say, a garden vegetable that you pick and eat the same day, or something you can get at the farmer's market that was picked the day before. That gap in time, not the presence or absence of a synthetic fertilizer,

[00:27:28] is where a very meaningful chunk of the homegrown advantage comes from. So let's go back to the hypothesis that I had at the start of this and be honest about which parts the research supports and which part that it maybe complicates. Does soil health affect the nutrient content of what we eat? Yes, absolutely it does. The evidence here is broad and it is consistent,

[00:27:51] running from 1940s fertilizer trials all the way through modern meta-analysis of mycorrhizal fungi. Soil biology changes how much mineral and phytochemical content ends up in the harvestable part of the plant. Does organic production make a difference? Well, sometimes. And the honest answer here is that where organic is a representation of soil building practices, it has a benefit.

[00:28:21] But the overall requirements of the certification itself is not the direct cause. Where organic farms build genuinely healthy soil, and the Rodale trial and the California strawberry study are good examples, the nutrient differences are real and measurable. Where an organic farm still relies on heavy tillage, or a conventional farm has adopted no-till and cover cropping,

[00:28:47] then the organic versus conventional line stops predicting much of anything. The clearest, most consistent difference between the two labels is that the conventional produce carries more pesticide residue, and organic produce carries more phytochemicals, and both of those facts stand on pretty solid ground. Is homegrown superior to store-bought because of less nutrient degradation?

[00:29:12] And this is the part of my hypothesis that the evidence absolutely supports most strongly, and it doesn't depend very much on how the food was grown. It depends on time. A vegetable harvested and eaten within a day holds on to vitamin C and other beneficial compounds that a vegetable sitting in distribution and storage for a week or more has already lost, regardless of whether either one of them came from an organic farm, a conventional farm, or your own raised bed.

[00:29:39] Taken together, this little analysis mostly confirms what I've been telling you, but for a more layered set of reasons than I originally would have given you. Homegrown food has a real nutritional edge, and the biggest part of that edge comes from the short distance between your garden bed and your dinner plate, not just from the fact that maybe it wasn't sprayed with anything. If you also build healthy, biologically active soil in that garden

[00:30:08] through compost and minimal tillage and diverse plantings, you're stacking a second advantage on top of the first one. So, the takeaway for the home garden is to harvest what you're eating that day as close to mealtime as your schedule allows, rather than picking a big batch and refrigerating it for a week if you can manage it. For anything you do need to store, get it cold quickly,

[00:30:35] because temperature is the biggest thing that you control after harvest. And keep building your soil with compost and cover crops, because that's the piece of this puzzle that compounds year over year. And if you can shop for growers that you trust at the farmer's market or the local co-op, and you can learn about their growing practices, you'll likely get more nutritional bang for your buck than anything you can find at the grocery store.

[00:31:01] And in a world where a lot of our foods are lacking in nutritional content, I think that's kind of important. Until next time, my gardening friends, keep on cultivating that dream garden, and we'll talk again soon.