7 juin 2025

Green smoothies and immunity: what the evidence actually shows

A green smoothie does not boost your immune system. It helps you eat vegetables you would otherwise have skipped, which is a different claim but still a useful one. Two numbers frame everything else: the NHS counts 150 ml of smoothie as a single portion of fruit and vegetables per day no matter how much you drink, and the 2013 Cochrane review covering 11,306 participants found no reduction in the risk of catching a cold from regular vitamin C supplementation in the general population.

This article starts from there. What follows is not a pitch for a wellness potion but five weighed recipes that hold up nutritionally, plus the handful of cautions most smoothie articles leave out, including one that put a 54-year-old man on permanent dialysis.

What vitamin C does, and what it does not

The idea that vitamin C protects against colds dates from the 1970s and has been tested thoroughly since. The Cochrane review by Hemilä and Chalker, published in 2013, pooled 29 comparisons covering 11,306 participants taking at least 0.2 g of vitamin C daily on a regular basis. The relative risk of developing a cold was 0.97, with a 95% confidence interval running from 0.94 to 1.00. In other words: nothing, or close to it.

Two findings do stand out. Among 598 marathon runners, skiers and soldiers on subarctic exercises, the relative risk dropped to 0.48 (0.35 to 0.64), a halving. And on episode duration, regular supplementation shortened colds by 8% in adults (3 to 12%) and by 14% in children (7 to 21%). On a seven-day cold, 8% works out at roughly thirteen hours. Real, measured, and modest.

The point that matters most for a smoothie: taken once symptoms have started, vitamin C showed no consistent effect across seven comparisons and 3,249 episodes. The Tuesday morning smoothie you make because you already feel rough will not shorten your cold.

Feeding a system is not the same as revving it up

None of this makes vitamin C a placebo. The review by Carr and Maggini in Nutrients in 2017 sets out its role: it accumulates in phagocytic cells such as neutrophils, where it supports chemotaxis, phagocytosis and microbial killing, and it contributes to epithelial barrier function. Those authors estimate that prevention requires intakes of 100 to 200 mg a day to saturate plasma and tissues.

The official requirement sits well below that. The NHS puts the recommended intake at 40 mg a day for adults aged 19 to 64, and notes that the body cannot store vitamin C, so you need it every day. A smoothie with one kiwi, half a lemon and a handful of spinach clears 40 mg comfortably. What you cannot do is bank a reserve: past saturation, the surplus goes out in urine. And above 1,000 mg a day from supplements, the NHS lists stomach pain, diarrhoea and flatulence.

The honest phrasing is therefore this: a green smoothie helps cover a daily requirement, and covering that requirement is part of how immunity works normally. It does not boost anything. A properly supplied immune system does not work harder if you give it more.

How many portions a smoothie actually counts for

This is the part almost every smoothie article skips, and it is the one that changes most about how you fit them into a day.

The NHS is explicit: 150 ml of fruit juice, vegetable juice or smoothie counts as a maximum of one portion out of the daily five, and that ceiling applies to the whole day. Drink a glass of orange juice and a smoothie on the same day and you are still at one portion, having exceeded the volume recommendation. The reasoning is not arbitrary: blending releases the sugars held inside the structure of the fruit, which raises the risk of tooth decay, hence the advice to drink juices and smoothies with a meal rather than between meals.

The Swiss FSVO takes a different route to the same place. It recommends five portions a day, three of vegetables and two of fruit, with one portion set at 120 grams. The Swiss nutrition recommendations allow you to swap one portion occasionally for 2 dl of 100% juice with no added sugar, but cap that at four portions a week, explaining that juices no longer contain all the fibre and are not very filling.

Both authorities say the same thing with different numbers: a smoothie does not stand in for a plate of vegetables. It replaces one portion, not three.

The free sugars problem

The NHS explicitly classes the sugars in unsweetened smoothies as free sugars, alongside honey and agave syrup. The recommended ceiling for an adult is 30 g of free sugars a day. The WHO sets the same limit as a percentage: under 10% of total energy intake, with additional benefit expected below 5%.

That does not condemn smoothies, but it does disqualify a common habit: the two-banana, one-mango, apple-juice smoothie in which the spinach mostly provides colour. A blend like that can approach 40 g of sugars, the entire daily budget, before breakfast has properly started.

The countermeasure fits in one line: one fruit per glass, the rest vegetables, unsweetened liquid and some fat.

What blending does to blood glucose

A crossover trial published in Nutrients in 2022 compared the glycaemic response of twenty young adults to the same amount of gala apple and blackberries, eaten whole or blended. The result runs against intuition: glucose peak and area under the curve were significantly lower with the blended fruit (p < 0.05), with 60-minute glucose marginally lower (p = 0.057). The authors suggest that ground blackberry seeds release compounds that slow absorption.

They add the limitation that matters straight away: commercially sold smoothies run from 450 to 828 ml, up to 81% larger than the portion tested, so the finding cannot be transferred to commercial formats. Blending one fruit is not the problem. Blending four into half a litre is.

Oxalates: the risk nobody mentions

Spinach and kale are not interchangeable, and the difference is not a matter of taste.

Spinach is high in oxalate. A trial published in the British Journal of Nutrition in 2004 measured magnesium absorption in nine adults from a meal served either with 300 g of spinach (6.6 mmol oxalate) or with 300 g of kale (0.1 mmol). Fractional absorption fell to 26.7% with spinach against 36.5% with kale (p = 0.01). Kale carries roughly sixty-six times less oxalate than spinach for the same weight.

For most people eating spinach now and again this has no consequence. The problem shows up when a daily spinach smoothie becomes a routine in someone whose kidneys are already fragile.

A documented case

Cureus published a case in 2024 of a 54-year-old man with type 2 diabetes and stage 3A chronic kidney disease. His creatinine went from 1.7 to 8.6 mg/dl and his estimated glomerular filtration rate from 45 to 6.8 ml/min/1.73 m². Kidney biopsy showed acute tubular injury with abundant calcium oxalate deposits. The dietary history turned up regular consumption of spinach and kale smoothies, which he drank believing they were good for his diabetes. Haemodialysis was started, he did not recover kidney function, and he remained dialysis-dependent.

It is not an isolated case. A review in Clinical Kidney Journal in 2022 lists several oxalate nephropathies linked to vegetable juicing, including one at two cups of spinach a day, with oxalate intake estimated at around 1,300 mg a day in two of those reports. The authors offer an explanation that points straight at smoothies: oxalate from blended vegetables may be absorbed more effectively, via the paracellular route and because added water dilutes the calcium that would normally bind it in the gut. They also note that very high-dose vitamin C is a non-enzymatic precursor of oxalate, which applies to supplements rather than fruit.

Hyperoxaluria is defined by urinary excretion above 40 to 45 mg per 24 hours. The practical guidance comes down to three points:

If your kidneys work normally and you vary your greens, a spinach smoothie is not a problem.
If you drink a green smoothie every day, rotate the leaves: kale, lamb’s lettuce, rocket, romaine, radish tops, watercress. Spinach once or twice a week, not seven times.
If you have chronic kidney disease, a history of stones, or bariatric surgery in your past, talk to your doctor before making this a daily habit.

Vitamin K and anticoagulants

Raw spinach contains 482.9 µg of vitamin K per 100 g according to the USDA FoodData Central database. A 40 g handful already delivers close to 200 µg, well above what an ordinary day contains.

The NHS page on warfarin, updated in February 2026, states that foods high in vitamin K such as broccoli, spinach and other green leafy vegetables affect how the medicine works. The exact wording matters: you can carry on eating them, but you should talk to your warfarin clinic. What destabilises the INR is not vitamin K itself, it is an abrupt change in intake. Starting a daily green smoothie after years without one, or stopping it overnight, produces exactly that kind of change.

The same page rules out two drinks often added to smoothies while on warfarin: cranberry juice and cranberry products, and grapefruit juice.

The five combos, weighed

Each recipe below makes roughly a 250 ml glass. The fruit quantities are deliberately low, for the reason set out above.

1. Spinach, pineapple and ginger

40 g fresh spinach
100 g fresh pineapple, diced
5 g fresh ginger, grated
150 ml cold water
10 g shelled hemp seeds

Rinse the spinach and blend everything for about forty seconds. Ginger takes over quickly: five grams is plenty, go to eight if you like heat. The hemp seeds add fat, which improves absorption of the carotenoids in the spinach, plus a little protein so the glass is not forgotten twenty minutes later.

This version suits people who do not drink a smoothie every day. Otherwise, alternate it with the ones below.

2. Kale, banana and lemon

50 g kale, central rib removed
80 g banana, roughly half a medium one
Juice of half a lemon
150 ml unsweetened plant drink
5 g ground flaxseed

Raw kale is tough: blend it on its own with the liquid for thirty seconds before adding anything else. The lemon is not decorative, its acidity cuts through the bitterness of the glucosinolates and makes the blend drinkable.

This is the recipe to favour if you drink a green smoothie daily, for the oxalate reason above.

3. Rocket, green apple and cucumber

30 g rocket
100 g green apple, unpeeled
100 g cucumber
100 ml water
Juice of half a lime

Rocket’s pepperiness climbs during blending. Thirty grams is already pronounced. Keep the apple skin, it holds most of the fibre and polyphenols. This is the least sugary of the five, at around 12 g of sugars for the whole glass.

4. Lamb’s lettuce, kiwi and pumpkin seeds

50 g lamb’s lettuce
150 g kiwi, roughly two fruit
15 g pumpkin seeds
150 ml water

Kiwi is among the densest fruit for vitamin C per gram, and two of them comfortably cover the 40 mg daily recommendation. The lamb’s lettuce takes the place spirulina usually holds in green smoothies, for a reason set out below. Pumpkin seeds bring zinc, a mineral vegetarian diets sometimes cover only narrowly.

Blend the seeds first, dry, if your blender is not powerful.

5. Parsley, pear and lime

20 g flat-leaf parsley, leaves and tender stems
120 g ripe pear
Juice of half a lime
150 ml water
10 g almond butter

Twenty grams of parsley is a decent bunch, and that is quite enough: beyond it the glass turns herbaceous to the point of being unpleasant. The almond butter rounds everything off and supplies the fat the other four lack if you skip the seeds.

Seasonality: what belongs in the glass, and when

A green smoothie built from imported baby spinach and a January mango is not nutritionally wrong, but it is expensive and hard to justify environmentally. The Swiss FSVO explicitly recommends seasonal, regional fruit and vegetables from sustainable production, and advises against air-freighted produce.

In practice, for a temperate climate: from October to March, kale, lamb’s lettuce and winter purslane carry the green recipes, alongside stored apples and pears that stay available into spring. April to June brings spinach, rocket, watercress and wild garlic, with the first strawberries from May. July to September opens up cucumber, chard, berries and plums. Citrus and pineapple remain imports whatever you do, but they travel by ship, which makes them far lighter on emissions than air-freighted out-of-season berries.

Frozen berries are a serious option here rather than a fallback. Processed shortly after harvest, available year round, they give a thicker texture without ice and usually cost less than fresh fruit out of season. Check for packs with no added sugar, which is standard for plain berry mixes but not guaranteed for ready-made smoothie blends.

One note on the herbs: parsley, mint and lemon balm from a pot on the balcony are the cheapest smoothie ingredient there is, because you only ever need twenty grams at a time and a shop-bought bunch usually spoils in the fridge before you finish it.

What a green smoothie does not fix

Three nutrients deserve naming, because no quantity of blended greens supplies them.

Vitamin B12. The Swiss FSVO notes that it is found almost exclusively in foods of animal origin. A balanced ovo-lacto-vegetarian diet provides enough, but a vegan diet requires supplementation through tablets or fortified products. No green vegetable contains it in a usable form. Our article on vitamin B12 for vegetarians covers the forms and doses.

Iodine. The FSVO lists it among the critical nutrients of a vegan diet. Iodised salt and, carefully, certain seaweeds are the realistic levers. See our guide to balancing iodine on a plant-based diet.

Protein. A typical green smoothie holds 3 to 8 g of protein, against the 60 g of dried pulses or 120 g of tofu the FSVO counts as one daily protein portion. If you replace breakfast with a smoothie, add a protein source: seeds, nut butter, or a yoghurt alongside. Our plant protein guide sets out the equivalences.

Iron deserves a nuance rather than a box. Raw spinach holds 2.71 mg of iron per 100 g, but it is non-haem iron, poorly absorbed and further hindered by the oxalate sitting in the same leaf. Vitamin C from kiwi or lemon improves that absorption, which makes the greens-plus-citrus pairing about more than taste. Our article on optimising plant iron absorption goes into the detail.

Spirulina: why it is not in these recipes

Green smoothie recipes routinely call for a teaspoon of spirulina powder. A study in Foods in 2022, which analysed spirulina supplements sold on the Slovenian market by ICP-MS and GC-MS, found that 86.7% of products carried inaccurate elemental declarations relative to their actual content. Declared iron ranged from 7.64 to 316% of reference intakes, but real bioavailability was far lower. The authors also point to documented contamination with cadmium, mercury, arsenic and lead, and to adulteration with cheap flours, concluding that stricter control is needed.

This does not mean all spirulina is bad. It means quality depends entirely on the producer, which makes it a poor fit for a generic recipe. If you want to use it, our microalgae buying guide explains what to check with a supplier.

Building your own: the ratio, not the recipe

Once you understand the five recipes above you do not need more of them. What matters is the ratio, and it fits in four lines.

Per glass, count on 30 to 50 g of leafy greens, one fruit of around 100 g, 100 to 150 ml of unsweetened liquid, 10 to 15 g of fat from seeds or nut butter, and an acid note. The fruit is the ceiling, not the starting point: as soon as two fruits go in, the free sugars arithmetic shifts noticeably.

The greens decide how much correction is needed. Mild leaves such as lamb’s lettuce, spinach and romaine need almost none. Bitter and peppery leaves such as kale, rocket and watercress need acid, which is why lemon or lime appears in the recipes above. Herbs like parsley and mint behave like spices: twenty grams flavours an entire glass.

The fat is not an afterthought. Carotenoids in green leaves are fat-soluble, and a glass with no lipids at all leaves some of them less available. Ten grams of nut butter or a handful of seeds covers it.

The liquid decides texture and nothing else. Water is the neutral choice. An unsweetened plant drink makes the result creamier and, in fortified versions, adds some calcium. Fruit juice used as the liquid is the most common hidden sugar source in smoothie recipes, and it can nearly always be swapped for water without anyone noticing.

Preparation, storage, hygiene

A smoothie is a raw blended food, so it is a friendly environment for bacteria if you leave it standing.

Wash the leaves thoroughly, including bagged ones labelled as washed. Scrub the skin of any fruit you are not peeling.

Drink it within the hour if it sits at room temperature. In the fridge, in a closed container, twenty-four hours is the limit. A smoothie browns before it becomes unsafe, but colour is not a reliable safety indicator in either direction.

Do not leave it in a rucksack or a warm car. That is the classic case of the morning smoothie drunk at noon.

Clean the blender immediately. Dried plant residue around the seal is the least accessible and most frequently forgotten spot in the kitchen.

On storing the ingredients themselves, our article on proper storage of fruit and vegetables gives the timings by family.

Six common mistakes

Counting the smoothie as three portions. It counts as one, in both frameworks, NHS and FSVO alike.

Drinking it between meals. The NHS specifically recommends taking it with a meal to limit teeth exposure to the sugars released by blending.

Stacking fruit to hide the vegetables. That is the fastest route to 40 g of free sugars in a glass. One fruit, not three.

Using spinach every day. See the oxalate section. Rotate your greens.

Adding honey or agave syrup. The NHS classes them as free sugars, exactly like white sugar. If your smoothie needs sweetening, the usual cause is too much bitter green, not too little sugar.

Making a week’s worth on Sunday. Twenty-four hours refrigerated is the reasonable limit. Freeze portioned ingredients instead and blend in the morning.

Who should be careful

People with chronic kidney disease, a history of calcium oxalate stones, or previous bariatric surgery, for the reasons set out above.

People on warfarin or another vitamin K antagonist, not to avoid green vegetables but to keep intake steady and tell their clinic about the change.

People treated for hypothyroidism, who are better off eating brassicas such as kale cooked rather than raw, and discussing it with their doctor.

People with hyperkalaemia, in the context of kidney failure for instance, because of the potassium load in leafy greens and bananas.

In all these cases the call belongs to a clinician who knows the file, not to an article.

Frequently asked questions

Can a green smoothie replace a meal? Not as it stands. A 250 ml glass supplies 100 to 200 kcal and little protein. To make it a genuine breakfast you need a protein source and some complex carbohydrate. Our article on a balanced vegetarian breakfast gives full compositions.

Is it better to blend vegetables or eat them whole? Whole, if the choice is real. The smoothie wins when the alternative is eating no vegetables at all, which describes plenty of mornings. It is a gain by compromise, not an absolute gain.

Do frozen fruit and vegetables work? Yes. The FSVO notes that because they are processed just after harvest, they keep most of their vitamins and minerals. They are often more practical and give a thicker texture without ice.

Do I need a powerful blender? For spinach and lamb’s lettuce, no. For raw kale, whole seeds and frozen fruit, a weak machine leaves lumps and warms the mix. One trick: blend the leaves with the liquid alone first, then add the rest.

Does a smoothie lose its vitamins if it sits? Vitamin C does oxidise over time, but the scale of the loss over a few hours in the fridge is modest next to the natural variation between two batches of fruit. The real reason to drink it promptly is microbiological, not nutritional.

Does a smoothie help during an infection? Nothing in the data shows that. Trials of vitamin C started at symptom onset produced no consistent effect. What a smoothie offers during an infection is fluid and easy calories when appetite is missing, which is not nothing.

Going further

If you are after other combinations, our article on balanced smoothies combining fruit, vegetables and protein covers the proportions, the one on local smoothies without exotic fruit works from produce available in Switzerland, and the piece on therapeutic smoothies examines the health claims one by one. For the cold season, root vegetables are a less obvious but denser option, and our guide to adapting meals to the seasons explains what belongs in a glass in February.