Mikkipedia

In this Mini Mikkipedia episode, Mikki explores whether fruit and vegetables really “boost” immunity in the way we often hear during winter. While produce absolutely supports health, the clinical evidence suggests its immune benefits are more specific than the marketing implies, especially for those starting from a low intake. Mikki unpacks research on fruit and vegetable intake, vaccine response, vitamin C loss in stored produce, and why frozen vegetables deserve more respect. She then broadens the conversation into what actually moves the needle for immune resilience: adequate energy intake, sufficient protein, correcting true vitamin D and zinc deficiencies, and supporting gut health through fermented foods and dietary variety. The takeaway? Your immune system does not need hype; it needs enough fuel, enough protein, and the right nutrients. Tiny old-school truth bomb, really.
 
Highlights / Topics Covered:
  •  Why “boosting immunity” is an oversimplified claim 
  •  What human trials show about fruit, vegetables, and immune function 
  •  How storage affects vitamin C in fresh produce 
  •  Why frozen vegetables can be a smart nutritional choice 
  •  The hierarchy of immune support: food, protein, vitamin D, zinc, and gut health

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Creators and Guests

Host
Mikki Williden

What is Mikkipedia?

Mikkipedia is an exploration in all things health, well being, fitness, food and nutrition. I sit down with scientists, doctors, professors, practitioners and people who have a wealth of experience and have a conversation that takes a deep dive into their area of expertise. I love translating science into a language that people understand, so while some of the conversations will be pretty in-depth, you will come away with some practical tips that can be instigated into your everyday life. I hope you enjoy the show!

00:02
Hey everybody, it's Mikki here. You're listening to Mini Mikkipedia on a Monday. And today I want to chat about your immune system and what actually moves the needle when it comes to ensuring good immunity, particularly across the winter months. Now, the reason this sort of sparked my interest was I was listening to the radio the other day, as I love to do during the day. It's like company for me when you're back from home.

00:27
And an ad came on and they were talking about getting your five plus a day of fruits and vegetables and how this helps build immunity over winter. Now, I love fruits and veg, as you know, huge fan. Follow me on social media. You will see the copious amounts of both vegetables and fruit that I eat to keep me satisfied, to give me good fiber because I love the taste. And also, of course, because they do have

00:55
vitamins and some minerals that do obviously help support physiology. However, I did wonder, do vegetables actually help boost our immune system? Are these actually helping keeping us well over the winter months? Or are there other things at play when it comes to diet? You think that I would know the answer to this question given the fact that I've been in the nutrition industry for, what, 28 years at this point, but...

01:23
couldn't remember seeing a human clinical trial actually assessing this. I just never looked. And so I jumped on Claude and it's connected to Consensus, which is an excellent AI app that helps you delve into research. It's very good, very accurate. And I said, bring up these human clinical trials that show that vegetables help boost our immune system. And so that's what I'm going to talk about today.

01:52
what I actually found. So when I looked at what consensus pulled up for me is that in fact yes vegetables do help the immune system but it's in a much narrower and more specific way than what we've been led to believe. When researchers actually have run trials where they feed people more fruits and vegetables and then measure their immune function the results are far more mixed than you'd expect.

02:18
There is a really nice study out of the UK called the Aging and Dietary Intervention Trial. They took 80 odd older adults who were eating two or fewer servings of fruit and veg a day, nothing, and they randomized half of them to bump it up to five plus servings for 16 weeks. I actually think five servings a day is pretty light. But anyway, then partway through they gave everyone this

02:43
pneumococcal vaccine and a tetanus vaccine and they measured how well their bodies responded to the vaccines. And the higher VEG group mounted a significantly better antibody response to the pneumococcal vaccine than the control group. Now this does matter because that is actually a real functional immune outcome. It's not a blood biomarker that might mean something. They're specifically looking at the antibody response.

03:12
And that's the actual question of can your body respond to a pathogen when it meets one? Interestingly, there was no difference in the tetanus response. So even within one good study, it wasn't a clean sort of across the board win. However, when you look at studies done in people who are already well nourished, i.e. they're already eating five plus a day, and then you measure the actual machinery of the immune system, the natural killer cells, the lymphocytes, the cytokines, they mostly don't budge.

03:41
There are trials where people went up to eight serves of fruit and veg a day, sort of where I might sit, if you like, and they were beautifully documented and their blood levels of vitamin C and carotenoids shot up exactly as you'd hope, proving that they did actually eat the vegetables. What they found was that the measures of antioxidant capacity and DNA damage didn't meaningfully change. In other trials in healthy adults, what shifts is inflammation?

04:09
So things like C-reactive protein, which is a marker of systemic inflammation, does come down, but it's not that the immune cells themselves actually change and become more capable. So the picture that emerges from the human data is pretty consistent then. More vegetables help most when you're eating very little to begin with, and the benefit tends to show up in inflammation and in functional responses like vaccination, but not in some general supercharging of your defenses.

04:38
or that you're absolutely not going to get sick this winter because you ate your vegetables. So the boost your immunity framing is just not what these trials support. It's more about repletion and you're topping up someone who was running low. You're not turbo charging someone who is already full. So I do think that vegetables, as much as I love them and fruit, are likely overstated when it comes to their ability for you to stay 100 % well over winter.

05:05
The other thing which I think we need to consider is that even though in that human clinical trial where we found people were able to sort of mount this antibody response to a pathogen is what is actually going in your trolley. And this is a bit of a reality check. So the trials that I've mentioned, they use pretty decent quality produce. And that's not necessarily what is sitting in your fridge right now.

05:30
because the nutrient we tend to care most about, is vitamin C, it's fragile. It's water soluble and it's oxygen sensitive. And it degrades with time, with warmth, with water loss, with bruising, cutting and cooking. And leafy greens are pretty much the worst of them all because they lose water fast and that drives the vitamin C straight down with it.

05:54
So if we look at actual numbers, there's research showing that refrigerated green beans, for example, lost more than 90 % of their vitamin C after about 16 days of cold storage. Other work tracking raw spinach and broccoli, one of my faves, then they lost around a third of their vitamin C after just one day in the fridge and were down by roughly two thirds to 90 % by day seven. So that bag of spinach has been picked, trucked, set in a distribution center,

06:22
sat on the supermarket shelf and then sat in your fridge. And the vitamin C number you see in a food composition database reflects produce at harvest, not produce on day seven of its retail life. And there is a nice study that actually built in a fresh stored category, five days in the fridge to mimic how we really shop. And it showed exactly why that assumption catches people out. So that is just vitamin C, however.

06:48
The minerals, the fiber, and most of the carotenoids from the precursor to vitamin A are pretty stable through storage. They don't fall off the way that vitamin C does. So a slightly tired vegetable is still a genuinely good food, but it really is just those fragile vitamins that you're losing, and it's not the whole package. One thing I think is worth mentioning here is just the value of frozen vegetables, actually.

07:15
they're often nutritionally equal to and sometimes better than the fresh ones in your supermarket. One study compared eight fruits and vegetables fresh versus frozen and found the vitamin content of the frozen produce was comparable to and occasionally higher than its fresh counterpart for most of the vitamins they measured. And this comes back to what I was talking about with the age of the vegetable. Frozen vegetables are often

07:42
blanched and frozen within hours of harvest so the nutrients get locked in while the fresh stuff keeps reducing in their quantity of vitamin C every day it sits around. Once it's frozen it's actually remarkably stable holding well for about a year or more. Of course there's almost always a trade-off right because actually better carotene does drop quite a bit with freezing in some vegetables so it is food specific and it's not a blanket win.

08:10
But I do think that the headline pretty much stands. If you've been feeling like frozen vegetables, like your frozen peas or your frozen spinach or broccoli or cauliflower doesn't cut the mustard, do think again, actually, because they are a great option. They're cheaper. There's less waste. And nutritionally, they're absolutely fine, if not better than some of those other options. For what it's worth, I love a frozen Brussels sprout on a tray bake.

08:37
You know I love frozen cauliflower rice and my smoothies, if you follow me on social media. And I'm not opposed to having those frozen broccoli and cauliflower packets because sometimes they're just so much more cheaper than the actual fresh stuff, which may not be so fresh. So if we think more broadly about diet, what does the evidence actually support with regards to immune system? And I just want to give you a hierarchy in order of how much each thing actually matters. So number one is

09:06
Eat enough food and hit your protein. So this is actually a big one. Protein energy malnutrition is described in the literature as the single most common cause of secondary immune deficiency in the entire world. And before you tune out thinking this is just a developing world problem that has nothing to do with you, actually it's the mechanism which I want to talk about. So when the body is short on protein and energy, i.e. calories, it does some dramatic things.

09:33
It shrinks the thymus, which is the gland that trains your T cells. It drops the number of T cells you have, especially the helper T cells. It weakens your antibody response, your secretory IgA, the function of your neutrophils, which is your complement system. We've known this for decades, and it's been confirmed again and again since. And the cause and effect is pretty clear. In the controlled study, this is a preclinical trial.

10:02
mice put on a very low protein diet had worse influenza, lower virus specific antibodies, and fewer of the CD8 T cells that actually clear the virus. And critically, when the researchers switched those animals back onto adequate protein, their immunity recovered. They cleared the virus better and they were protected on the next exposure. There's also a really sobering finding actually that energy restricted underweight.

10:29
animals had higher mortality from a primary flu infection because they simply couldn't meet the energy cost of mounting a response. And that's the part that people miss. Running an immune response is metabolically expensive. Fever alone cranks up your energy and protein needs. If you're running on empty, you don't have the raw material to fight back. This is where some of you may be sitting, which is why I don't want to let this one go. The endurance athlete who's training hard and under-fuelling without even realizing it

10:59
chronically a bit low on energy availability and then wondering why they catch every bug that goes around and take ages to shake it off. This is the problem. And for those of you who are in your 40s and 50s, who've been told to eat less for so long by so many people, and you've been under eating protein every single day while your body composition shifts and your recovery slips, if that is you, the single highest yield thing you can do for your immune system isn't actually a supplement.

11:29
it's probably not vegetables. It is eating enough total food to meet your calorie requirements and hitting your protein. And what I will say on the back of that is that if you are in a calorie deficit, that's fine. And I work with hundreds of women in this category. Making sure that it's not an aggressive across the board deficit is essential, particularly if you do struggle with your immune system. So the number two thing I will say beyond

11:58
protein and energy requirements is fixed in actual deficiency and not just an imaginary one. Vitamin D and zinc lead here. Vitamin D is the most rigorously studied vitamin to help with the immune system. There was a landmark 2017 meta-analysis which pulled 25 trials in over 11,000 people and found that supplementing modestly reduced the risk of respiratory infections.

12:25
the number needed to treat was about 33. So it is a real but a modest effect. Just to clarify, number needed to treat is one of the most useful ways to express how a treatment actually does in the real world because it translates a statistical effect into a plain count of people. So how many people do you have to give the treatment to for one of them to get the benefit you care about? So a number needed to treat of 33 for vitamin D means that you have to supplement 33 people

12:54
through the study period for one additional person to avoid a respiratory infection they otherwise would have caught. The other 32 either didn't benefit or were never going to get sick anyway or got sick despite supplementing. And I know that sounds weird, but actually one in 33 got the actual win, which is attributable to vitamin D. And it's actually a statistically significant and real response. However, I will say that

13:22
super supplementing with vitamin D isn't going to stop you getting sick. And particularly here in New Zealand and other parts of the world during the winter months where a lot of people are deficient, actually getting your vitamin C to within that optimal range of 100 to 150 nanomoles per liter is definitely something that we need to work on. Zinc is another one that tells that it's pretty similar actually. It doesn't reliably stop you catching things, but it does shorten how long you're sick.

13:52
and the effect on duration can be substantial. In zinc deficient older adults, a year of supplementation lowered the incidence of infections and brought down inflammatory and oxidative markers. So again, it's most useful in people who are deficient and in children. So what we really want to be doing is correcting that shortfall. And if you listened to my podcast with Rachel Arthur, we discussed zinc on that podcast and she talked about

14:20
dosing it. So it's not about mega dosing zinc every day because that actually desensitizes the receptors. It's about making sure that you have sufficient intake and it might be that you're dosing three times a week with a pretty modest dose of zinc to ensure adequacy, to help for when if you do get sick, you're not sick for too long. So those are the two nutrients that I

14:45
to micronutrients that I definitely wanted to mention, vitamin D and zinc, not gonna find them in vegetables by the way. So number three is feeding your gut. Now, there is some evidence to show that taking a probiotic helps reduce both the prevalence of getting an upper respiratory infection and the duration of that infection. And what they also found in meta-analyses, looking at all of the research,

15:13
was that when that probiotic was delivered in the form of food like kefir, yoghurt, kimchi, there was a bigger effect than taking it in a capsule. So I think that that is super interesting. One caveat with this, where the evidence is right now though with probiotics, I often talk about the evidence to support taking a probiotic if you're an athlete and you're traveling overseas.

15:37
for any particular reason, for a competition or whatever. And I still stand by that. But for general sort of immunity, probiotics didn't seem to move the needle for physically active people the way that they do for people who are sedentary. And we do know that the gut microbiota is different for someone who is physically active versus people who are not. So I think that really is a bit of a watch this space. Having said that though, honestly, most people could benefit from having some fermented food in their diet.

16:06
just in general. mean, I'm just talking about immunity here, but we know that gut health is a super important part of overall health. you're certainly not going to, it's certainly not going to be harmful to getting in more of those probiotics and prebiotics. So let me land the plane on this one. Your immune system is not sitting there wanting to be boosted necessarily, right?

16:32
It is a finely tuned, metabolically expensive system that requires adequate fuel, adequate protein, vitamin C, vitamin D, zinc, probiotics, and actually just in general, an excellent diet. Under-fuelling chronically is one of the biggest drivers of immune dysfunction, and it's probably one most of you could be tripping over without realizing. So have that in the back of your mind when you're reviewing your diet.

17:02
You also want to fix a genuine vitamin D or zinc deficiency with a sensible dose across a week. You want to feed your gut with fermented foods, plenty of fiber and a varied diet. And also recognize that for someone who's already well-nourished, stacking high dose single nutrients on top of that almost always just doesn't do a ton more for your immunity. It might be great from a placebo perspective, however. So.

17:29
That's your mini micropedia for the week. Hopefully you enjoyed that and I'd love to hear what you thought. I am over on Instagram threads and X @mikkiwilliden Facebook @mikkiwillidennutrition or head to my website, mikkiwilliden.com scroll right down to the bottom, pop your email in the little box that you find there and jump on my weekly email list. And I talk all about things like this and more each week on that email.

17:58
Alright team, you have the best week. See you later.