There is an aloe on a windowsill in almost every flat in Riga, Tallinn and Vilnius, and almost nobody waters it. It sits in a pot too small for it, leaning towards the glass, forgotten for weeks at a time, and it does not die. Someone burns a finger on a pan, snaps off a leaf tip, and a clear, cool gel runs out — more water than the plant has been given all month. Then they put the pot back and call it a cactus.

It is not a cactus. It is something stranger and, for anyone who drinks the gel, more interesting: a lily's distant cousin that learned to keep water in the desert, and has been carried along human trade routes for at least two thousand years because of what it keeps. This is the story of what is actually inside that leaf — the water, the sugar chain, the bitter layer we remove — and of what the trials have found when people drank it.

A succulent, not a cactus

Botanists settled the question a while ago. In the current classification of flowering plants, the genus Aloe sits in the family Asphodelaceae, in the order Asparagales — the same order as asparagus and the onion [1]. Cacti belong to an entirely different branch, the Caryophyllales, where their nearest relatives on a dinner table are beetroot and spinach. Aloe and cactus look alike for the reason a dolphin looks like a shark: the same problem, solved twice. The problem is drought.

There are more than 500 species of Aloe, spread across Africa, the Arabian Peninsula, Madagascar and the islands of the western Indian Ocean [2] [3]. The one on the windowsill, Aloe vera, is the northern outlier: when a team led by Olwen Grace reconstructed the genus's family tree in 2015, its origins traced to the Arabian Peninsula, at the northernmost edge of the group's range [2]. The same study asked why, out of 500 relatives, this one conquered the world. Part of the answer is in the leaf: of the 81 aloe species with a documented medicinal use, 98 per cent have thick, succulent leaves, and the big water-storing leaf is the single best predictor of whether a species was ever used. The other part is geography. Aloe vera grew near the great historic trade routes and was brought into cultivation early, and that, the authors conclude, rather than any unique chemistry, is why it is the aloe in every shop [2].

What the leaf is keeping

A succulent is a plant that has solved drought with storage, and Aloe vera solves it in two ways at once. The first is a trick of timing. Most plants open the pores in their leaves by day to take in carbon dioxide, and lose water through them while the sun is up. Aloe does the opposite. It uses a form of photosynthesis called crassulacean acid metabolism, or CAM, in which the pores open at night, when the air is cool and humid, and close through the heat of the day [4]. Carbon dioxide gathered in the dark is stored as acid and spent in the light. Under drought the plant simply keeps its pores shut longer, and even under severe water stress it still opens them at night [4].

The second solution is the one we drink. Inside the leaf, beneath the green skin, the cells are enormous and almost entirely vacuole — a tissue botanists call hydrenchyma, which is to say water tissue [4]. That is the gel. When the plant is analysed, the inner leaf comes out at 98 ± 1 grams of water in every 100 grams, and the clear mucilage at about 99.5 per cent [5] [6]. Everything that makes aloe aloe lives in the remaining one to two per cent, which is why almost every impressive-sounding figure about the plant — so many per cent polysaccharide, so many per cent fibre — turns out to have been measured on dried material and quietly multiplied by a hundred. The honest number is small. The leaf is a reservoir, and what it holds in solution is a modest amount of something specific.

It is worth knowing what is not there. The most detailed analysis of the fresh plant to date, published in 2019, found no detectable vitamin C in the inner gel or the mucilage; the little the leaf makes is in the green rind [6]. Aloe is not a vitamin food. It was never the reason anyone drank it.

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Two materials in one leaf

Cut an aloe leaf across and you see three layers: the tough green rind; just inside it, a ring of vascular bundles; and the clear inner parenchyma, the fillet [5]. From the cells beside those bundles seeps a bitter yellow sap. It has nothing to do with the gel. The gel is water and sugar chains; the yellow sap is a concentrate of anthraquinones — aloin above all — and it is a laxative, which is exactly why it was the first part of the plant that people traded [5] [7].

Dioscorides, the Greek army physician whose De materia medica remained Europe's standard pharmacy text for more than a thousand years, describes aloe in Book III. The leaf, he writes, is thick and curls back on itself like a squill's; the plant grows mostly in India, from which its juice is exported, and also in Arabia; the juice comes in two grades; and it is astringent, drying and purges the bowel [7]. That juice was the dried bitter sap — the drug the Middle Ages called "aloes" — and it is still an industry today. In South Africa the sap of Aloe ferox is wild-harvested for export, as is Aloe secundiflora in Kenya; the gel industry, by contrast, works from Aloe vera plantations in Mexico, the United States and South America [3]. Two products, two trades, two different parts of one leaf. Reviewers were still complaining in 1999 that the literature constantly confuses them [8].

For a drink, the distinction is everything, and it is why the processing of aloe begins with a knife. The gel is filleted out of the leaf and the rind and the bitter layer are discarded. The numbers show why. When the United States National Toxicology Program analysed a non-decolourised whole-leaf extract, it contained 14.1 to 15.9 milligrams of aloin A per gram; the decolourised version of the same material contained 0.06 to 0.2 — roughly a hundred times less [9]. The aloe industry's own certification standard for a drinkable product is stricter still: no more than 10 parts per million of aloin [10]. A proper aloe juice is the inside of the leaf, and only the inside. Nobody wants Dioscorides' purge in a breakfast glass.

Acemannan: the sugar chain with the fragile coat

So what is in the one or two per cent that is not water? The characteristic molecule of aloe gel is acemannan, a long chain of mannose sugars linked end to end — 84.9 per cent mannose, with a little glucose and galactose — and dressed, on average, with about one acetyl group per sugar unit [11]. Those acetyl groups are the signature of the fresh plant. They are also the fragile part. Fresh gel has been measured at 107 to 139 milligrams of acemannan per gram of dry matter [12], and how much of it survives into a bottle, and in what condition, depends almost entirely on what is done to it between the field and the glass.

The research here is unusually clear, and it is research about processing rather than about health. In a study of four industrial drying methods — spray-drying, freeze-drying and two others — every one of them cut the acemannan yield by about 40 per cent and stripped more than 60 per cent of the acetyl groups from the mannose chain [13]. Dry heat does the same more slowly: after 15 to 20 hours at 60, 80 and 100 °C, the gel kept 86, 53.5 and 39.3 per cent of its acetylation, and a commercially produced inner-leaf product tested alongside had kept 34.9 per cent [14]. Pasteurisation at 65 to 85 °C removes acetyl groups and shears off the galactose side chains [12]. The molecule that leaves the leaf is not always the molecule that reaches you.

In 2023 a Spanish group bought fifteen aloe beverages off the shelf and measured them [15]. Of the eight labelled as more than 99 per cent aloe, only three contained more than 160 milligrams of acemannan per 100 grams, and four contained less than 35. The seven flavoured drinks, made with 30 to 77 per cent aloe, all fell below 30 milligrams and in most cases had lost their acetyl groups entirely; across the whole set, all but one sample had less than 35 per cent of its acetylation left [15]. The label said aloe. The chemistry, in many cases, said water with a memory of aloe. This is the reason we care so much about how little we do to a juice — we wrote about heat and what it costs in Cold-Pressed vs Pasteurized Juice — and why, with aloe in particular, the first question to ask of any bottle is not what it promises but how far it has travelled from the leaf.

What the trials found: the gut

Aloe has been drunk for the stomach for as long as it has been drunk at all, and the gut is where modern trials have used something closest to a juice. In 2018 a Korean team pooled the three randomised, placebo-controlled trials of aloe in irritable bowel syndrome — 151 people in all. Symptom scores improved by 0.41 standard deviations more on aloe than on placebo (95% confidence interval 0.07 to 0.75), the chance of responding was 1.69 times higher (1.05 to 2.73), the three trials agreed with one another, and no adverse events were attributed to the aloe [16]. That is a real, if modest, pooled effect in a condition that medicine finds notoriously hard to shift.

The trials behind the pooled number deserve to be read on their own terms. The British one gave 58 people 50 ml of aloe gel drink four times a day for a month; 35 per cent responded on aloe against 22 per cent on placebo, a difference that did not reach significance overall, with a trend towards benefit in those whose main symptom was diarrhoea [17]. The strongest single result comes from a different condition altogether. In 2004, 44 outpatients with mildly to moderately active ulcerative colitis drank 100 ml of inner-leaf aloe gel or a placebo twice daily for four weeks. On aloe, 30 per cent reached remission and a further 37 per cent improved — a clinical response in 47 per cent, against 14 per cent on placebo; the response difference was significant, the remission difference on its own was not, and the lining of the bowel looked the same under the endoscope in both groups [18]. The authors called the benefit possible and modest and asked for a larger trial. Twenty years on, nobody has run it — which tells you more about who funds trials of a plant nobody can patent than about the plant. We wrote about the gut's own night shift in Sleep and the Second Brain; aloe is one of the oldest things people have poured into it to keep the peace.

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What the trials found: blood sugar

The other body of human research on oral aloe concerns blood sugar, and here the striking thing is how many independent analyses point the same way. In 2016 alone, three separate meta-analyses were published. One pooled nine trials in 283 people and found fasting glucose 46.6 mg/dL lower on aloe, and, across five of those trials in 89 people, HbA1c — the three-month average — 1.05 percentage points lower [19]. A second, stricter analysis of five randomised trials in 415 people with prediabetes or early, untreated diabetes found fasting glucose 30.05 mg/dL lower and HbA1c 0.41 points lower, the HbA1c result with no disagreement at all between trials [20]. A third, from Thailand, found HbA1c 0.99 points lower across the five trials that measured it, in 235 people [21].

When a 2021 overview in Frontiers in Pharmacology set the meta-analyses for eighteen medicinal plants side by side, aloe gel had the largest effect on HbA1c of any of them — 0.99 percentage points, ahead of psyllium and fenugreek [22]. The authors add the caveat that belongs with the number: some of the underlying trials ran for less than eight weeks, too short to see the full effect on a three-month average, and they ask for trials of at least three months [22]. Two more honest footnotes. The first analysis found signs that small negative studies may be missing from the literature [19]; and most of these trials used aloe capsules or dried gel powder, not a drink [20]. The direction is consistent across every analysis we could find; the size of the effect, and whether a glass of juice delivers it, is the open question. If the afternoon slump is your particular problem, the mechanics are in The 3 p.m. Collapse.

Two thousand years of the same plant

Step back from the trials and the longer record is remarkable in its own right. Dioscorides wrote his entry in the first century [7]. A living Aloe vera was being shipped along the Red Sea and Indian Ocean routes before there was a Europe to ship it to, and when the genus's history was finally reconstructed with DNA, its place on those routes — not its chemistry — turned out to be the best explanation for its fame [2]. Today every aloe species on earth is protected under the CITES convention on trade in endangered plants, with one exception: Aloe vera, which has been cultivated for so long and so widely that it is no longer really a wild plant at all [3].

The clinical record is younger and slower. When two Exeter researchers systematically reviewed every controlled trial of aloe in 1999, they found the evidence promising and thin [23]. A quarter of a century later it is still thin, and still promising — a handful of small trials in the gut, a cluster of meta-analyses on blood sugar, and a plant that has outlived every empire that traded it. We find that an honest position to drink from. Not a miracle; a reservoir, with a long memory and a short list of things it demonstrably does.

What to do this week

1. Read the label before the claims. With aloe, the percentage of aloe and the absence of flavouring tell you more than any promise on the front. The 2023 shelf study found the flavoured drinks had almost nothing of the plant's signature molecule left [15].
2. Drink it cold and early in the bottle. The acetyl groups that mark fresh gel are lost to heat and time [13] [14]. Keep it in the fridge, and treat an open bottle like a cut leaf.
3. Make it a small daily glass, not a cure. The gut trials used 100 to 200 ml a day over four weeks [17] [18]. A morning glass of Hello Aloe, neat or in water, is the honest size of the habit.
4. Pair it with what you already know works. Aloe with ginger and turmeric is a classic kitchen combination; Fire Bloom puts it in one box, and the Set for Digestive System Balance pairs a litre of aloe with a bottle of pressed ginger and a jar of berry honey.
5. Keep the windowsill plant. It is the same species as the bottle, and the best reminder on the shelf of what the gel is for: a leaf keeping water in a dry room.

Aloe vera, in short

Aloe vera is a succulent in the asparagus order, not a cactus; its inner gel is about 98–99% water, and its signature molecule is acemannan, a mannose chain whose acetyl groups are destroyed by drying and heat. Three pooled trials in 151 people found a modest benefit in irritable bowel syndrome, and several meta-analyses report lower fasting glucose and HbA1c, mostly with capsules.

One caution: oral aloe is not recommended during pregnancy or breastfeeding, or for anyone with unexplained abdominal pain.

Back to the windowsill

Go and look at the plant you have been neglecting. Turn the pot. Notice how the leaves have thickened in the window's heat and how the plant has leaned, patiently, towards the one thing it was short of. It is not a cactus and it does not need you to understand it; it has been keeping water in dry places since before there were people to forget to water it. Everything we have tried to do with aloe — the filleting, the cold, the short road from field to bottle — comes down to one ambition: to hand you that leaf's reservoir with as little lost on the way as we can manage. Snap off a tip tonight. The gel is the point. It always was.

Frequently asked questions

Is aloe vera a cactus?

No. Aloe vera belongs to the family Asphodelaceae in the order Asparagales, alongside asparagus and onions; cacti are in the order Caryophyllales, related to beetroot and spinach [1]. The two look alike because both store water in thick tissue to survive drought — the same problem solved independently.

What is actually in aloe vera gel?

Mostly water: 98 ± 1 g in every 100 g [6]. The remaining one to two per cent is dominated by acemannan, a chain of mannose sugars carrying acetyl groups, with small amounts of other sugars and minerals [11]. No vitamin C was detected in the inner gel in the most detailed analysis to date [6].

Does aloe vera juice help the gut?

The evidence is modest but consistent. Three randomised trials pooled in 2018, with 151 people, found irritable bowel symptoms improved more on aloe than on placebo, with a response rate 1.69 times higher [16]. In ulcerative colitis, a four-week trial of 100 ml twice daily found a clinical response in 47 per cent on aloe against 14 per cent on placebo [18].

Why is the yellow part of the leaf removed?

The yellow sap under the rind is a concentrate of anthraquinones such as aloin, a laxative that was the plant's first traded product [5] [7]. A non-decolourised whole-leaf extract has been measured at 14.1–15.9 mg of aloin A per gram, about a hundred times more than a decolourised one [9]; drinkable aloe is made from the filleted inner gel and is certified to no more than 10 ppm of aloin [10].

References

1. The Angiosperm Phylogeny Group (Chase MW, Christenhusz MJM, Fay MF, et al.). An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG IV. Botanical Journal of the Linnean Society. 2016;181(1):1–20. doi:10.1111/boj.12385
2. Grace OM, Buerki S, Symonds MRE, et al. Evolutionary history and leaf succulence as explanations for medicinal use in aloes and the global popularity of Aloe vera. BMC Evolutionary Biology. 2015;15:29. doi:10.1186/s12862-015-0291-7
3. Grace OM. Current perspectives on the economic botany of the genus Aloe L. (Xanthorrhoeaceae). South African Journal of Botany. 2011;77(4):980–987. doi:10.1016/j.sajb.2011.07.002
4. Silva H, Sagardia S, Ortiz M, et al. Relationships between leaf anatomy, morphology, and water use efficiency in Aloe vera (L) Burm f. as a function of water availability. Revista Chilena de Historia Natural. 2014;87:13. doi:10.1186/s40693-014-0013-3
5. Hamman JH. Composition and applications of Aloe vera leaf gel. Molecules. 2008;13(8):1599–1616. doi:10.3390/molecules13081599
6. Añibarro-Ortega M, Pinela J, Barros L, et al. Compositional features and bioactive properties of Aloe vera leaf (fillet, mucilage, and rind) and flower. Antioxidants. 2019;8(10):444. doi:10.3390/antiox8100444
7. Dioscorides. De materia medica, Book III, chapter 22 (aloe). Translated by Beck LY. Pedanius Dioscorides of Anazarbus: De materia medica. Hildesheim: Olms-Weidmann; 2005.
8. Reynolds T, Dweck AC. Aloe vera leaf gel: a review update. Journal of Ethnopharmacology. 1999;68(1–3):3–37. doi:10.1016/S0378-8741(99)00085-9
9. National Toxicology Program. Toxicology and carcinogenesis studies of a nondecolorized whole leaf extract of Aloe barbadensis Miller (Aloe vera) in F344/N rats and B6C3F1 mice (drinking water study). NTP Technical Report Series. 2013;(577).
10. International Aloe Science Council. Processing methods for Aloe vera leaf: position statement on decolorization. 31 May 2016.
11. Qian W, Wang H, Shan D, et al. Extraction, purification, structural characteristics, biological activities and pharmacological applications of acemannan, a polysaccharide from Aloe vera: a review. Molecules. 2019;24(8):1554. doi:10.3390/molecules24081554
12. Rodríguez-González VM, Femenia A, González-Laredo RF, et al. Effects of pasteurization on bioactive polysaccharide acemannan and cell wall polymers from Aloe barbadensis Miller. Carbohydrate Polymers. 2011;86(4):1675–1683. doi:10.1016/j.carbpol.2011.06.084
13. Minjares-Fuentes R, Rodríguez-González VM, González-Laredo RF, Eim V, González-Centeno MR, Femenia A. Effect of different drying procedures on the bioactive polysaccharide acemannan from Aloe vera (Aloe barbadensis Miller). Carbohydrate Polymers. 2017;168:327–336. doi:10.1016/j.carbpol.2017.03.087
14. López Z, Femenia A, Núñez-Jinez G, et al. Dry but not humid thermal processing of Aloe vera gel promotes cytotoxicity on human intestinal cells HT-29. Foods. 2022;11(5):745. doi:10.3390/foods11050745
15. Comas-Serra F, Estrada P, Minjares-Fuentes R, Femenia A. Evaluation of acemannan in different commercial beverages containing Aloe vera gel. Gels. 2023;9(7):552. doi:10.3390/gels9070552
16. Hong SW, Chun J, Park S, Lee HJ, Im JP, Kim JS. Aloe vera is effective and safe in short-term treatment of irritable bowel syndrome: a systematic review and meta-analysis. Journal of Neurogastroenterology and Motility. 2018;24(4):528–535. doi:10.5056/jnm18077
17. Davis K, Philpott S, Kumar D, Mendall M. Randomised double-blind placebo-controlled trial of aloe vera for irritable bowel syndrome. International Journal of Clinical Practice. 2006;60(9):1080–1086. doi:10.1111/j.1742-1241.2006.00980.x
18. Langmead L, Feakins RM, Goldthorpe S, et al. Randomized, double-blind, placebo-controlled trial of oral aloe vera gel for active ulcerative colitis. Alimentary Pharmacology & Therapeutics. 2004;19(7):739–747. doi:10.1111/j.1365-2036.2004.01902.x
19. Dick WR, Fletcher EA, Shah SA. Reduction of fasting blood glucose and hemoglobin A1c using oral aloe vera: a meta-analysis. Journal of Alternative and Complementary Medicine. 2016;22(6):450–457. doi:10.1089/acm.2015.0122
20. Zhang Y, Liu W, Liu D, Zhao T, Tian H. Efficacy of aloe vera supplementation on prediabetes and early non-treated diabetic patients: a systematic review and meta-analysis of randomized controlled trials. Nutrients. 2016;8(7):388. doi:10.3390/nu8070388
21. Suksomboon N, Poolsup N, Punthanitisarn S. Effect of Aloe vera on glycaemic control in prediabetes and type 2 diabetes: a systematic review and meta-analysis. Journal of Clinical Pharmacy and Therapeutics. 2016;41(2):180–188. doi:10.1111/jcpt.12382
22. Willcox ML, Elugbaju C, Al-Anbaki M, Lown M, Graz B. Effectiveness of medicinal plants for glycaemic control in type 2 diabetes: an overview of meta-analyses of clinical trials. Frontiers in Pharmacology. 2021;12:777561. doi:10.3389/fphar.2021.777561
23. Vogler BK, Ernst E. Aloe vera: a systematic review of its clinical effectiveness. British Journal of General Practice. 1999;49(447):823–828.

This article is for education and is not medical advice. If you have a diagnosed condition, are pregnant or take regular medication, talk to your doctor or pharmacist before changing your diet.