Raisins, Blood Sugar and Teeth

Table of Contents

1. What a Raisin Actually Is
2. The Sugar Arithmetic, Done Properly
3. The Glycaemic Index of Raisins
4. Why Dried Fruit Behaves Better Than Its Sugar Suggests
5. The Longer Trials: Twelve Weeks and Twenty-Four
6. If You Have Diabetes
7. Teeth: The Conventional Warning
8. Teeth: What Was Actually Measured
9. Practical Dental Rules
10. Fresh Grapes, Raisins and Juice Compared
11. Practical: Portions, Pairing and Buying
12. Key Research Papers
13. Connections
14. Featured Videos

What a Raisin Actually Is

Take a grape, which is roughly 80% water, and dry it until it holds around 15%. Everything that was dissolved in that water stays behind, four times as concentrated by weight. That is the whole of what happens, and it explains both the good and the bad.

The names describe the grape and the process, not different fruits. Raisins in most of the English-speaking world are dried dark grapes, historically and still largely Thompson Seedless, sun-dried until they darken. Sultanas (called golden raisins in the United States) are typically dried from the same or similar seedless white grapes, often treated with sulfur dioxide to keep them pale and dried mechanically. Currants in the dried-fruit sense are not the berry currant at all but small dried grapes of the Black Corinth variety — the "Corinthian raisins" that appear in Greek research, and the reason the word exists.

What survives drying: sugars, potassium, iron, fibre, and a substantial fraction of the grape's polyphenols, including anthocyanins from dark-skinned varieties. What is lost: most of the vitamin C, which is heat- and light-sensitive, and the water that made the fresh fruit filling. What is added, in sulfured products: sulfur dioxide, which matters to a minority of people and is discussed below.

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The Sugar Arithmetic, Done Properly

Here is the mistake almost everyone makes, in both directions.

By weight, raisins are far more sugary than grapes. Gram for gram there is no argument: remove 80% of the mass as water and what remains is about four times as concentrated. This is why raisins look alarming on a nutrition label.

By serving, the difference is much smaller than the label suggests — because nobody eats a raisin the way they eat a grape. A small box of raisins is a modest handful. To get the same weight in fresh grapes you would need a large bowl. The fair comparison is not 100 grams of one against 100 grams of the other; it is the portion a person actually eats, and there the gap narrows considerably.

But the ease of overeating is real and is the genuine risk. A raisin takes almost no chewing, gives almost no sense of fullness, and is designed to be eaten by the handful from a bag. Fresh grapes come with their own brake: the water fills you up and slows you down. Dried fruit removes the brake. If you eat raisins out of a large bag while doing something else, you will eat a great deal more sugar than you intended, and this — not the glycaemic index — is the practical problem.

The sugars themselves are the grape's own: roughly equal parts glucose and fructose, with very little sucrose. That composition matters for the next section.

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The Glycaemic Index of Raisins

Glycaemic index measures how much a fixed amount of a food's available carbohydrate raises blood glucose over two hours, compared with pure glucose set at 100. Below 55 is conventionally "low", 56 to 69 "medium", 70 and above "high".

Two well-conducted trials have measured raisins directly.

Viguiliouk and colleagues, publishing in Nutrition & Diabetes in 2018, ran an acute randomised multiple-crossover trial in 10 healthy adults following the ISO standard method for determining glycaemic index. Against white bread with a glycaemic index of 71, they measured raisins at 55, sultanas at 51 and dried apricots at 42. All the dried fruits tested came in below white bread; raisins, sultanas and apricots did so significantly. The researchers also tested whether dried fruit could lower the glycaemic response of a starchy meal by displacing half of its carbohydrate: all the dried fruits lowered it, though only dried apricots reached statistical significance in this small study. A third hypothesis — that a small "catalytic" dose of fructose would improve glucose handling — showed no benefit for any of them.

Zhu and colleagues, publishing in Nutrients in 2018, tested four dried fruits in 11 healthy subjects and measured raisins at 56, dried jujubes at 55, dried apricots at 56 and dried apples at 43 — strikingly close agreement with the Canadian numbers for raisins. This study went further and tested mixed meals. Combining dried fruit with white rice reduced the glycaemic response compared with rice alone (rice at 81 against dried apple and rice at 65), and adding 30 grams of almonds lowered the glycaemic index further in nearly every combination. The authors also found that the ratio of fructose to glucose in a test meal was inversely correlated with its glycaemic index.

So the answer is: raisins sit around the low-to-medium boundary, roughly 55, and are consistently lower than white bread. That is a genuinely surprising result for a food that is a quarter sugar by weight, and it needs an explanation.

One caution belongs here. The dried-fruit glycaemic literature has substantial industry involvement — the Canadian trial discloses funding relationships with dried fruit trade bodies, and much raisin research has been supported by growers. The two studies above agree closely with each other and were run by different groups on different continents, which is reassuring, but the pattern of who funds this work is worth knowing.

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Why Dried Fruit Behaves Better Than Its Sugar Suggests

Four things work together, and understanding them lets you predict how other foods will behave.

1. Fructose does not raise blood glucose directly. Roughly half the sugar in a raisin is fructose, which is absorbed and taken to the liver rather than raising circulating glucose the way glucose does. That halves the immediate glycaemic load compared with a starch of the same carbohydrate weight, which digests almost entirely to glucose. This is exactly the fructose-to-glucose relationship the Nutrients study measured. It is not an argument that fructose is harmless — large amounts from added sugars and sweetened drinks are a separate and well-founded concern — but within whole fruit, at fruit-sized doses, it is why the glycaemic response is blunted.
2. The fibre and cell-wall structure are still there. Drying removes water, not fibre. Raisins retain soluble and insoluble fibre in an intact plant matrix, and the sugars are held inside cells that must be broken down before absorption. This is the single largest difference between raisins and grape juice, where the cell walls have been destroyed and the sugar is in free solution.
3. The polyphenols themselves slow digestion. Grape polyphenols inhibit alpha-amylase and alpha-glucosidase, the enzymes that break starch into absorbable glucose — the same enzyme family targeted by a class of diabetes medication. This is one plausible mechanism for the displacement effect: put raisins alongside a starch and some of the starch is digested more slowly.
4. The physical form is dense and chewy. Chewing takes time, gastric emptying is slower for a dense chewy food than for a liquid or a soft refined starch, and glucose therefore arrives more gradually.

The single most useful practical consequence is the one the almond finding illustrates: what you eat raisins with changes the result more than the raisins do. Raisins with nuts, or stirred into plain full-fat yogurt, or scattered on oatmeal or brown rice, produce a far flatter glucose curve than the same raisins eaten alone from a bag.

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The Longer Trials: Twelve Weeks and Twenty-Four

Glycaemic index measures a single meal. Two controlled trials have asked what happens over months.

Anderson and colleagues ran a 12-week randomised controlled trial published in Postgraduate Medicine in 2014, comparing raisins eaten three times a day against conventional processed snacks. Fasting glucose was unchanged in both groups. In the raisin group, post-prandial glucose fell by 13.1 mg/dL by week 12 (significant against baseline and against the snack group), and HbA1c fell by 0.12% (significant against baseline and greater than the snack group's change). Systolic blood pressure fell significantly in the raisin group at 4, 8 and 12 weeks; the snack group showed no significant blood-pressure change. Body weight did not change in either group.

Read that honestly. An HbA1c reduction of 0.12% is very small — a fraction of what a single diabetes medication achieves. The trial was open-label (you cannot blind someone to raisins), the groups were unbalanced at 31 raisin-eaters against 15 snack-eaters, and the comparator was processed snack food, which is a low bar. What the trial genuinely shows is that swapping processed snacks for raisins did not worsen anything and moved several markers slightly in the right direction — which, for a food widely feared as a sugar bomb, is the useful finding.

Kanellos and colleagues ran a 24-week randomised controlled trial in Greece, published in Nutrition in 2014, in 48 people with well-controlled type 2 diabetes who were eating fewer than the recommended five daily servings of fruit and vegetables. The intervention group ate 36 grams of Corinthian raisins a day — two fruit servings — in place of snacks of similar energy density. After 24 weeks, body weight, glycaemic control and lipid profile were unchanged in both arms. The raisin group reduced diastolic blood pressure and increased total antioxidant potential significantly against both baseline and the control group, and showed a rise in circulating p-hydroxybenzoic acid, a polyphenol metabolite that confirms the raisins were actually being absorbed. High-sensitivity C-reactive protein did not change.

The honest summary of both: raisins in a sensible portion, substituted for other snacks, do not harm glycaemic control even in people with diabetes, and may modestly help blood pressure. They are not a treatment for anything.

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If You Have Diabetes

The advice people with type 2 diabetes most often hear about dried fruit is "avoid it". The evidence does not support a blanket ban, and the Greek trial above was run specifically in this population.

• Portion is the whole game. The trial dose was 36 grams a day — about two tablespoons, or a small handful. That is a fruit serving, not a snack bag.
• Substitute, do not add. Both trials replaced other snacks with raisins. Raisins added on top of an existing diet are simply extra carbohydrate.
• Never eat them alone. Pair with nuts, seeds, cheese, plain yogurt or a meal. The almond finding is directly actionable.
• Test if you can. Individual glucose responses to identical foods vary widely between people. If you use a glucose meter or continuous monitor, a small handful of raisins with and without nuts will tell you more about your own physiology than any published glycaemic index.
• Fresh grapes are the lower-risk option if you find raisins hard to stop eating. Same fruit, same polyphenols, a quarter of the sugar per mouthful and far more satiety.

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Teeth: The Conventional Warning

The standard dental position on dried fruit has a clear rationale and it should be stated fairly before the counter-evidence.

Tooth decay is driven by acid. Oral bacteria — Streptococcus mutans foremost among them — ferment dietary sugar and excrete acid, and when plaque pH falls below roughly 5.5, tooth enamel begins to demineralise. Saliva buffers the acid and restores pH over about twenty to forty minutes, and minerals return to the enamel. Decay happens when the acid attacks come too often, or last too long, for that repair to keep up.

On this model dried fruit looks bad for two reasons. It is sugary, giving the bacteria plenty of substrate. And it is sticky, lodging in the fissures of molars and between teeth, so the sugar is not cleared by saliva in minutes but sits there feeding bacteria for far longer. Duration of exposure matters more than total quantity in caries risk, which is why sipping a sweet drink all afternoon is worse than drinking it in one go. A raisin wedged in a molar is, on this reasoning, a slow-release sugar supply.

That logic is sound as far as it goes, and it is why paediatric dentists have long put dried fruit alongside sweets. But it was reasoning from first principles, not from measurement.

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Teeth: What Was Actually Measured

The most direct evidence comes from Christine Wu's work at the University of Illinois at Chicago, reviewed in the Journal of Nutrition in 2009.

Her group started from a chemical question: do raisins contain compounds that act against oral bacteria? Through antimicrobial assay-guided fractionation they identified a list of active constituents — oleanolic acid, oleanolic aldehyde, betulin, betulinic acid, linoleic and linolenic acids, rutin, 5-(hydroxymethyl)-2-furfural, beta-sitosterol and beta-sitosterol glucoside — that inhibited growth of oral pathogens. Oleanolic acid suppressed the adherence of cariogenic Streptococcus mutans biofilm in vitro, which is mechanistically interesting because sticking to the tooth is the first thing a decay-causing bacterium has to do.

Then they measured what happens in real mouths. In children aged 7 to 11, plaque acidogenicity was tested after eating raisins and after eating raisin-containing bran cereal. The finding was that raisins did not drive plaque pH below 6 over the 30-minute test period — that is, they did not push the mouth into the demineralising range at all. And a mixture of raisins with plain bran flakes, with no added sugar, produced a smaller plaque pH drop than commercial bran flakes or commercial raisin bran cereal did.

That last comparison is the most useful thing in the paper, and it points somewhere unexpected: the problem with raisin bran is the added sugar in the cereal, not the raisins. Wu's overall conclusion was that raisins represent a healthy alternative to commonly consumed sugary snack foods.

Three honest caveats. First, this is a small body of work from one research group, published in a journal supplement, in a field with clear industry interest — it is not a large independent replication. Second, plaque pH over 30 minutes is a mechanistic marker, not a count of cavities; no trial has randomised children to raisins and counted fillings years later. Third, none of this makes raisins protective; the reasonable reading is that they are considerably less cariogenic than their reputation, not that they are good for teeth.

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Practical Dental Rules

The evidence changes the ranking of the risk but not the sensible habits.

• With a meal, not between meals. This is the single most effective dental rule for any sweet food. Saliva flow is high during a meal, the buffering is strong, and one acid episode is far better than five.
• Rinse with water afterwards, and swallow. It clears far more residue than doing nothing and takes five seconds.
• Do not brush immediately. Enamel is temporarily softened after an acid exposure; brushing at once can abrade it. Wait about thirty minutes, or rinse now and brush later.
• Watch the fissures. Sticky food lodges in the biting surfaces of molars and between teeth, where a toothbrush reaches poorly. Floss or interdental brushes matter more than brushing harder.
• The company matters more than the raisin. A raisin in a sugar-coated cereal, a chewy bar or a chocolate-covered snack is a different food from a plain raisin. Read the ingredient list; if sugar or syrup appears, the dental question has been answered by the manufacturer.
• For young children, the same rules apply, plus one from a different article entirely: whole grapes and other round, firm foods are a choking hazard for the under-fives. That is covered in Pesticides and Choking Safety, and it is a more urgent risk than a cavity.

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Fresh Grapes, Raisins and Juice Compared

Three forms of the same fruit, and they are not interchangeable.

Fresh grapes are the best default. High water content, roughly a gram of fibre per cup, intact cell walls, all the skin polyphenols, and enough physical bulk that a normal person stops after a reasonable amount. Frozen grapes count as fresh for these purposes and make an excellent substitute for ice cream.

Raisins are the concentrated, portable, shelf-stable version. They keep the fibre, the potassium, the iron and much of the polyphenol content, and lose the water and most of the vitamin C. They are genuinely useful in a lunchbox, on a hike, or anywhere fresh fruit will not survive — and their measured glycaemic index is moderate. Their weakness is entirely behavioural: they are trivially easy to overeat.

Grape juice is the form we would recommend least, and this is not a close call. Juicing destroys the cell walls and removes essentially all the fibre, so the sugar arrives in free solution and is absorbed fast. A cup of juice is drunk in seconds and does not fill you up. It retains some polyphenols — more if it is pressed with skins, as purple Concord juice is, which is why the vascular trials used it — but as a daily habit it is a sugar delivery vehicle. If you want the polyphenols, eat the fruit.

A note on sulfites: golden raisins and sultanas are usually treated with sulfur dioxide to preserve their pale colour. Most people are entirely unaffected. A minority — particularly some people with asthma — react to sulfites, and dried fruit is one of the highest-sulfite foods in an ordinary diet. Dark, sun-dried raisins are typically unsulfured; if sulfites are a problem for you, the colour is the tell, and the ingredient list will say.

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Practical: Portions, Pairing and Buying

• A serving is a small handful — the Greek trial used 36 grams a day, about two tablespoons, and treated that as two fruit servings. That is a good working portion.
• Decant. Never eat from the bag. Put a portion in a bowl and put the bag away. This is the single highest-yield change for anyone who finds raisins hard to stop eating.
• Always pair with fat or protein. Almonds, walnuts, pumpkin seeds, cheese, or plain whole-milk yogurt. The measured effect of adding almonds to a dried-fruit meal was a lower glycaemic index in nearly every combination tested.
• Use them where they replace refined sugar. Raisins in oatmeal, in brown rice pilaf, in a spiced stew, in a salad with walnuts and bitter leaves — in each case they are doing the work that sugar or a sweet dressing would otherwise do, and bringing fibre and polyphenols with them.
• Buy plain. Check the ingredient list: it should say raisins, and possibly a little oil to stop them clumping. Yogurt-coated, chocolate-coated and sweetened varieties are confectionery.
• Dark over golden if you want polyphenols, since the sulfuring that keeps sultanas pale also preserves lighter-coloured fruit with less anthocyanin to begin with.
• Store them airtight, cool and dark. Refrigeration keeps them soft for months and prevents the sugar crystallisation that makes old raisins gritty.
• Keep them away from dogs. Grapes and raisins cause acute kidney injury in dogs, and raisins are more dangerous than grapes because they are concentrated. See Pesticides and Choking Safety.

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Key Research Papers

1. Viguiliouk E, Jenkins AL, Blanco Mejia S, Sievenpiper JL, Kendall CWC. Effect of dried fruit on postprandial glycemia: a randomized acute-feeding trial. Nutrition & Diabetes. 2018;8(1):59. — doi:10.1038/s41387-018-0066-5
2. Zhu R, Fan Z, Dong Y, Liu M, Wang L, Pan H. Postprandial glycaemic responses of dried fruit-containing meals in healthy adults: results from a randomised trial. Nutrients. 2018;10(6):694. — doi:10.3390/nu10060694
3. Anderson JW, Weiter KM, Christian AL, Ritchey MB, Bays HE. Raisins compared with other snack effects on glycemia and blood pressure: a randomized, controlled trial. Postgraduate Medicine. 2014;126(1):37–43. — doi:10.3810/pgm.2014.01.2723
4. Kanellos PT, Kaliora AC, Tentolouris NK, et al. A pilot, randomized controlled trial to examine the health outcomes of raisin consumption in patients with diabetes. Nutrition. 2014;30(3):358–364. — doi:10.1016/j.nut.2013.07.020
5. Wu CD. Grape products and oral health. The Journal of Nutrition. 2009;139(9):1818S–1823S. — doi:10.3945/jn.109.107854
6. Costabile G, Vitale M, Luongo D, et al. Grape pomace polyphenols improve insulin response to a standard meal in healthy individuals: a pilot study. Clinical Nutrition. 2019;38(6):2727–2734. — doi:10.1016/j.clnu.2018.11.028
7. Kay CD. Aspects of anthocyanin absorption, metabolism and pharmacokinetics in humans. Nutrition Research Reviews. 2006;19(1):137–146. — doi:10.1079/NRR2005116
8. Crowe-White KM, Evans LW, Kuhnle GGC, et al. Flavan-3-ols and cardiometabolic health: first ever dietary bioactive guideline. Advances in Nutrition. 2022;13(6):2070–2083. — doi:10.1093/advances/nmac105
9. Cassidy A, Mukamal KJ, Liu L, Franz M, Eliassen AH, Rimm EB. High anthocyanin intake is associated with a reduced risk of myocardial infarction in young and middle-aged women. Circulation. 2013;127(2):188–196. — doi:10.1161/CIRCULATIONAHA.112.122408
10. Dohadwala MM, Hamburg NM, Holbrook M, et al. Effects of Concord grape juice on ambulatory blood pressure in prehypertension and stage 1 hypertension. The American Journal of Clinical Nutrition. 2010;92(5):1052–1059. — doi:10.3945/ajcn.2010.29905
11. Raman G, Avendano EE, Chen S, Wang J, et al. Dietary intakes of flavan-3-ols and cardiometabolic health: systematic review and meta-analysis of randomized trials and prospective cohort studies. The American Journal of Clinical Nutrition. 2019;110(5):1067–1078. — doi:10.1093/ajcn/nqz178
12. Live literature search — raisins and glycaemic control: PubMed: raisins and glycaemic control
13. Live literature search — dried fruit and dental caries risk: PubMed: dried fruit and dental caries

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Connections

• Grapes — the main topic page.
• Grapes — Benefits Deep Dive — the hub for this set of articles.
• Grapes: History and Origins — including how raisin grapes diverged from wine and table grapes.
• Polyphenols and Vascular Health — what survives the drying process and what it does.
• Pesticides and Choking Safety — residue, children and dogs.
• Type 2 Diabetes — where portion and pairing matter most.
• Insulin Resistance — the upstream process behind post-meal glucose spikes.
• Hypertension — the blood-pressure findings in both raisin trials.
• Potassium — abundant in raisins and part of the blood-pressure story.
• Anthocyanins — the pigments that survive drying in dark raisins.
• Apples and Cherries — other fruits with a fresh-versus-dried question.
• Gut Health — where the unabsorbed polyphenols end up and what happens to them.

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