Citrus

TL;DR. Almost every citrus fruit you can buy descends from just three or four wild ancestors, hybridised and back-crossed for two thousand years: the orange is a mandarin-pomelo cross, the lemon is a citron-sour orange cross, the grapefruit is a pomelo-orange cross. They are the classic vitamin C source and their peel is where the aromatic oils and most of the polyphenols live. Grapefruit is the one fruit in this book with a genuine, documented, occasionally fatal drug interaction: it inhibits the enzyme that metabolises dozens of common medicines, and the effect lasts a day or more.

1. What it is

Genus Citrus, family Rutaceae. The fruit is a hesperidium: a modified berry with a leathery, oil-glanded rind and interior segments filled with juice vesicles, which are individual hair-like sacs of juice. That structure is unique to citrus.

The astonishing genetics: genomic work published in Nature in 2018 by Wu and colleagues established that essentially all cultivated citrus derives from a small number of wild ancestors:

From which:

• Sweet orange = a complex mandarin × pomelo hybrid, roughly 3 parts mandarin to 1 pomelo
• Sour (Seville) orange = a different mandarin × pomelo hybrid
• Grapefruit = pomelo × sweet orange, arisen in Barbados in the eighteenth century
• Lemon = citron × sour orange
• Key lime = citron × papeda
• Persian lime = a triploid involving key lime and lemon
• Clementine = mandarin × sweet orange
• Tangelo = mandarin × grapefruit or pomelo

Citrus hybridises readily and many types are apomictic (producing seeds that are clones of the mother), which is why chance seedlings occasionally produce stable new types.

2. Where it comes from

Origin: the eastern Himalayan foothills, spanning northeast India, northern Myanmar, and southwest China, from where the genus radiated about 8 million years ago. Domestication began in China and Southeast Asia; the citron reached the Mediterranean first (by around 300 BCE), followed by sour oranges and lemons via Arab trade routes in the tenth century, and sweet oranges via Portuguese traders in the fifteenth.

Citrus shaped naval history. Scurvy killed more sailors than combat until James Lind's 1747 trial and the eventual Royal Navy adoption of lemon, then lime, juice rations, which is the origin of "limey" as a term for a British sailor (Chapter 15).

Who grows them now (approximate):

Brazil and Florida built the frozen concentrated orange juice industry; Florida's has been devastated by citrus greening disease (huanglongbing), covered below. Spain and Morocco supply most of Europe's fresh fruit.

3. How it is grown

Climate. Subtropical evergreen trees with no chilling requirement. They want warm summers, mild winters, and no frost. Cold tolerance varies sharply:

A single hard freeze can kill an orchard. The Florida freezes of the 1980s pushed the state's citrus industry substantially south.

Cool nights are needed for colour. Peel colour comes from chlorophyll breaking down to reveal carotenoids underneath, and that breakdown requires cool nights. Oranges grown in consistently warm tropical climates stay green while being perfectly ripe inside, which is why commercial packers degreen them with ethylene, and in some countries dye them (Chapter 8).

Soil. Well-drained, pH 6.0 to 7.5. Extremely intolerant of waterlogging, which invites Phytophthora root and collar rot. Sensitive to salinity, which is a growing problem in irrigated arid regions.

Rootstocks are everything. Almost all commercial citrus is budded onto a rootstock chosen for disease resistance, cold hardiness, salinity tolerance, and soil adaptation: trifoliate orange and its hybrids (Carrizo, Swingle citrumelo) are common. The historical lesson is tristeza virus, which killed millions of trees on sour orange rootstock worldwide in the twentieth century and forced a global rootstock change.

Harvest. Citrus is non-climacteric: it does not ripen after picking, so it must be harvested ready to eat. Sugar and acid are measured as a Brix-to-acid ratio, and legal minimum maturity standards exist in most producing regions. Fruit can be "stored on the tree" for weeks or months in many varieties, which is a useful commercial flexibility.

The disease that is destroying the industry. Huanglongbing (citrus greening), caused by a bacterium spread by the Asian citrus psyllid, has no cure. Infected trees produce small, lopsided, bitter, green fruit and decline over a few years. It has devastated Florida, where production has fallen by more than 90 percent from its peak, and it is present in Brazil, China, and much of Asia. Responses include psyllid control, antibiotic trunk injection, and transgenic resistance research. It is the citrus equivalent of the banana's Race 4 and substantially further advanced.

4. What is inside it

Per 100 g of edible portion:

A medium orange (about 130 g) supplies roughly 70 mg of vitamin C, comfortably a full day's requirement.

Beyond vitamin C:

• Flavanones: hesperidin (oranges and mandarins), naringin (grapefruit, which is where the bitterness comes from), eriocitrin (lemons). Concentrated in the pith and membranes, which is why juice contains far less than whole fruit.
• Limonoids: limonin and nomilin, bitter triterpenes. Limonin is why juice from some varieties turns bitter hours after squeezing, as a precursor converts in acid.
• Essential oils in the rind: predominantly d-limonene, plus linalool, citral (the lemon note), and dozens more. The rind is a completely different ingredient from the flesh and delivers most of the aroma.
• Carotenoids: beta-cryptoxanthin in oranges and mandarins (a provitamin A carotenoid, and mandarins are one of the best dietary sources), lycopene in pink and red grapefruit, and beta-carotene.
• Pectin, heavily concentrated in the peel and pith: commercial pectin for jam-making is made overwhelmingly from citrus peel and apple pomace.

5. What it does in your body

Vitamin C is the headline and it does more than prevent scurvy: it is the essential cofactor for collagen synthesis, and it converts dietary non-haem iron to the absorbable ferrous form. That second point is the most actionable: a glass of orange juice or a squeeze of lemon over a lentil or spinach dish can multiply the iron you absorb from it several-fold (Chapter 16).

Citric acid binds calcium in urine and raises urinary citrate, which inhibits crystal formation. This is a genuine, clinically used effect: lemonade therapy (typically 120 mL of concentrated lemon juice diluted daily) is a recognised adjunct for recurrent calcium oxalate kidney stones, and potassium citrate is the pharmaceutical version.

Flavanones (hesperidin, naringenin) have shown modest effects on endothelial function and blood pressure in trials, generally small.

Limonene from the peel has been studied for a range of effects with mostly preliminary evidence.

Acidity and teeth. Citric acid at pH around 2.2 to 2.4 (lemon and lime juice) dissolves tooth enamel directly. This is the most under-appreciated harm in this chapter: habitual lemon water, sucking on lemon slices, and sipping citrus drinks through the day produce documented, irreversible dental erosion. Dentists see it constantly.

6. What the evidence actually shows

Established: vitamin C prevents and cures scurvy. Citrate raises urinary citrate and reduces calcium oxalate stone recurrence. Vitamin C substantially enhances non-haem iron absorption.

Strong: citrus intake is part of the general fruit-and-vegetable association with lower cardiovascular disease and stroke. Beta-cryptoxanthin intake is associated with lower inflammatory arthritis risk in cohort studies.

Mixed: flavanone supplementation effects on blood pressure and endothelial function, which are small and inconsistent. Grapefruit for weight loss: one small 2006 trial found a modest weight difference, and it has not been convincingly replicated.

Thin to absent: vitamin C preventing colds (it does not reduce incidence in the general population; it shortens duration by roughly 8 percent, about half a day Chapter 15). Lemon water for detox, alkalising the body, or boosting metabolism: no mechanism and no evidence. Your blood pH is held between 7.35 and 7.45 by mechanisms nothing you eat can override, and the "alkaline diet" rests on a misunderstanding of that.

7. Who should eat more, who should be careful

Good for: almost everyone. Particularly useful for people relying on plant iron sources, for recurrent calcium oxalate stone formers, and as a low-FODMAP fruit option for people with IBS (oranges, mandarins, lemons, and limes are all low-FODMAP, which makes citrus one of the better fruit choices in that context).

Be careful if:

• You take medication. See the grapefruit section immediately below. This is the single most important safety item in the fruit half of this book.
• You have reflux (GORD). Citrus is a common trigger through direct mucosal irritation.
• You have dental erosion or sensitivity. Drink citrus with meals rather than sipping through the day, use a straw, rinse with water afterwards, and do not brush for 30 minutes after, because brushing softened enamel removes it.
• You take a proton pump inhibitor or antacids. No interaction, but reflux patients often need to know the trigger.
• Phototoxicity. Citrus peel oils, especially bergamot, lime, and to a lesser extent lemon, contain furocoumarins that cause phytophotodermatitis: handling limes and then going into the sun produces a burn and long-lasting brown pigmentation in the pattern of the splashes. "Margarita burn" is a real dermatological presentation, and it is often mistaken for a chemical burn or abuse.
• Citrus allergy exists and is uncommon; oral allergy syndrome to citrus is more common.

Ages. Fine from around 6 months, though acidity commonly causes a harmless perioral rash in babies. Whole segments are a choking risk for toddlers if the membranes are tough; cut them small. Excellent through life.

8. Grapefruit and the drug interaction

In short: Grapefruit inhibits an intestinal enzyme that normally destroys much of an oral drug dose before it reaches the blood, so blood levels of dozens of medicines can rise sharply, and one glass can do it for over 24 hours.

This deserves its own section because it is the clearest example in the whole book of a food behaving as a drug.

The mechanism. Many oral drugs are partly destroyed on their first pass through the intestinal wall and liver by the enzyme CYP3A4 (Chapter 62). Grapefruit contains furanocoumarins, principally bergamottin and 6',7'-dihydroxybergamottin, which irreversibly inactivate intestinal CYP3A4. Because the inactivation is irreversible, the effect lasts until the cells make new enzyme, which takes 24 to 72 hours. Spacing the grapefruit and the tablet by a few hours does not avoid it.

Grapefruit also inhibits the OATP transporters that carry some drugs in, which produces the opposite effect: reduced absorption and reduced efficacy for a different set of medicines.

The size of the effect can be extreme. For some drugs, a single glass of grapefruit juice increases blood levels several-fold. Terfenadine, a former antihistamine, was withdrawn partly because interactions of this kind caused fatal cardiac arrhythmias.

Drug classes affected (this is illustrative, not exhaustive; always check the leaflet):

Which fruits are involved. Grapefruit and pomelo are the main ones. Seville (bitter) oranges and their marmalade contain furanocoumarins too, as do tangelos and limes to a lesser degree. Sweet oranges, mandarins, clementines, satsumas, and lemons do not significantly inhibit CYP3A4 and are safe alternatives.

The practical rule: if you take a prescription medicine, check the patient information leaflet for grapefruit, ask a pharmacist, and if the answer is to avoid it, avoid it entirely rather than trying to time it. The list of affected drugs is long and grows.

9. Choosing, storing, and ripening

Choosing. Heavy for size is the single best test, because weight means juice. Thin, fine-textured skin generally indicates more juice than thick, coarse skin. Firm with slight give; avoid soft spots, which indicate rot beginning. Colour is unreliable: a green orange can be fully ripe, and colour may be the result of degreening or dye. Loose, puffy skin on mandarins is normal for some types (satsumas) and indicates over-maturity in others.

Storing. Citrus is non-climacteric and only degrades.

• Room temperature: about a week. Fridge: two to four weeks, in the crisper.
• Refrigeration is better for storage life. Bring to room temperature before eating or juicing, because cold fruit yields less juice and tastes less aromatic.
• Grapefruit is the most chilling-sensitive citrus and can develop pitting below about 10 °C over long periods.
• Cut citrus goes in the fridge, covered, and lasts two to three days.
• Zest and juice freeze well. Zest citrus before juicing, always; the zest is the more valuable part and it is impossible to recover afterwards.

10. The varieties worth knowing

11. Myths and confusions

Don't be confused: the pith is not waste. The white spongy layer holds most of a citrus fruit's flavanones and a large share of its fibre. It is bitter, and the standard advice to remove it entirely throws away much of what is beneficial. Eating an orange with some pith attached is better than juicing it.

• "Lemon water alkalises your body." Your blood pH is defended within 0.1 of a unit by respiration and kidney function. Nothing you drink changes it, and if it did you would be critically ill. Lemon juice does make your urine more alkaline, which is the source of the confusion and is not the same thing.
• "Vitamin C prevents colds." It does not reduce incidence in the general population.
• "Orange juice is as good as an orange." It has the fibre removed, most of the flavanones lost with the pith, and a much faster sugar delivery. Whole fruit is associated with lower diabetes risk; juice with higher.
• "Fresh juice is always better than from concentrate." Nutritionally they are similar once reconstituted. Both are still juice.
• "Citrus is bad for you if you have a cold sore / arthritis / an ulcer." No good evidence for any of these. Acidity may irritate an existing mouth ulcer directly.
• "You should avoid citrus at night." No, unless you get reflux.

12. The bottom line

• Nearly all citrus is a hybrid of mandarin, pomelo, citron, and papeda, cross-bred over two thousand years.
• One orange covers the day's vitamin C. The bigger practical value is that vitamin C multiplies the iron you absorb from plant foods eaten with it.
• Citrus does not ripen after picking, so buy it heavy for its size and eat it. Store in the fridge, zest before juicing.
• Grapefruit, pomelo, and Seville oranges inhibit CYP3A4 for a day or more and interact seriously with dozens of common drugs. Sweet oranges, mandarins, and lemons do not.
• Citric acid erodes tooth enamel. Habitual lemon water is a real dental problem, and the detox and alkalising claims behind it are false.

Sources and notes

Composition figures are from USDA FoodData Central; production figures are approximate FAOSTAT values for the early 2020s. Citrus phylogeny and hybrid origins follow Wu et al., Nature, 2018. Cultivation, rootstock history, tristeza, and huanglongbing follow University of Florida and University of California citrus extension publications and USDA reporting on Florida production decline. Scurvy history follows Lind's A Treatise of the Scurvy, 1753, and Carpenter, The History of Scurvy and Vitamin C, 1986. Citrate and kidney stone prevention follows the American Urological Association guideline and Kang et al.'s lemonade therapy work. Flavanone content and its concentration in pith follow Phenol-Explorer. Dental erosion from citric acid follows Lussi's erosion literature. The grapefruit interaction follows Bailey, Dresser, and Arnold, Canadian Medical Association Journal, 2013, which lists the affected drug classes and the irreversible CYP3A4 mechanism; furanocoumarin identification follows Guo and Yamazoe's work. Vitamin C and colds follows the Cochrane review by Hemila and Chalker. Phytophotodermatitis follows dermatology case literature.

Open questions. The full list of drugs affected by grapefruit continues to grow and is not comprehensively characterised; the practical rule is therefore to check per drug rather than rely on any list. Flavanone health effects in humans remain small and inconsistent across trials.

👉 Next: berries, the most nutrient-dense fruit per calorie and the most fragile in the supply chain.