Cum: what it is made of, how it looks and how it tastes

Mostly gland fluid with a few per cent of sperm, a gel that melts in half an hour, and a taste that no diet has ever been tested on

Everyone has seen it, most gay men have tasted it, and very few know what is actually in it. Semen is a cocktail mixed in a couple of seconds from several glands, and the part everybody talks about, the sperm, is the smallest part of the drink.

Here is what it is made of, why it turns to jelly and back, what the colours mean, what really changes the smell and the taste, and the honest answer on swallowing. For how much comes out and how far it goes, see Ejaculation.

What it is made of

Most of the volume comes from the seminal vesicles, two glands behind the bladder, roughly two thirds to three quarters of it. The prostate adds most of the rest. The testicles, where the sperm come from, supply only a few per cent.

A few per cent is still a crowd. The WHO reference values for fertile men give a median of 66 million sperm per ml and 210 million per ejaculation. Only 14% of them, in the median man, pass the lab's strict test for a perfect shape, and the lower limit of normal is 4%. Wonky sperm are the rule, not a problem.

The fluid around them has its own recipe. The seminal vesicles add fructose, the sugar that fuels sperm, and the prostate adds zinc and citric acid: labs measure exactly these to check how each gland is working. Seminal plasma carries about 35 to 55 g of protein per litre, and the most abundant are the semenogelins. It is slightly alkaline, with a pH of 7.2 or more in a normal sample.

Gel, lumps and liquid

Fresh semen is a semi-solid, gel-like clump. Within a few minutes it starts to thin into a lumpy mix, and at room temperature it is usually fully liquid within 15 to 30 minutes. If it is still not liquid after an hour, a lab writes it down.

The gel is the semenogelins at work, trapping sperm right after ejaculation. The thinning is done by an enzyme from the prostate that cuts them up. Its name is prostate-specific antigen, PSA, the same PSA your doctor may one day measure in your blood.

What the colour tells you

Blood is the one that scares people. It is most common in men under 40, and in them it is usually benign and goes away on its own. The commonest cause is a medical procedure, above all a prostate biopsy, followed by infections. A urology review recommends a basic check even for a single episode: questions, an examination, blood pressure and a urine test. Repeated episodes, blood after 40, or pain call for further tests.

Smell, taste and what changes them

The smell is distinctive enough to have its own name in smell science. In 1975 researchers found that a small molecule, 1-pyrroline, smells very much like semen, called its odour "spermous", and found that about one person in five simply cannot smell it. The WHO notes the same: people vary a lot in how strongly they perceive it. A strong smell of urine or of rot is not normal and worth reporting.

Most men describe it as salty and slightly bitter. The fructose adds at most a hint of sweetness, and as the next section shows, there is not much of it. Then come the diet rules: pineapple and cinnamon sweeten it, asparagus, coffee and garlic ruin it. We could not find a single study that tested any of them on semen.

The closest real science is about urine: asparagus gives urine its famous smell in some people, and only some people can smell it. Nobody has run the same experiment on semen. What is documented to change it is duller: a longer break turns it yellowish, some medicines and vitamins change the colour, and traces of urine can push its pH down. On water intake there is nothing either way.

Calories, protein and zinc

The numbers make the protein-shake joke fall apart. A median 3 ml load at 35 to 55 g of protein per litre holds 0.1 to 0.17 g of protein, less than one calorie's worth. The WHO lower limit for fructose is 13 µmol per ejaculation, about 2 mg of sugar. The lower limit for zinc is 2.4 µmol, about 0.16 mg, against the 11 mg a day adult men are advised to eat. Swallowing will not feed you, fatten you or fix your zinc.

Swallowing: safe to eat, worth testing for

As food, semen is proteins, a sugar and some minerals. The real question is infections. For HIV the per-act risk of oral sex is rated low, against 138 per 10,000 for receptive anal sex. PrEP cut new HIV infections by 86% in gay men in the PROUD trial, and an HIV-positive partner with an undetectable viral load produced no linked transmissions in 782 gay couples. That is U=U.

Gonorrhoea is another story: it lives happily in the throat, usually without symptoms. US guidelines advise men who suck cock to have a throat test at least once a year, every three to six months with many partners, plus rectal and urine tests for the other roles you play and a syphilis blood test. Doxycycline taken after sex, doxyPEP, cut bacterial infections from 31.9% to 10.7% of quarterly check-ups among gay and bisexual men and trans women on PrEP in one trial.

Swallow or spit, it is your call. Just get the throat swab.