Asparagus benefits: 60% of people cannot smell it in their urine

Key benefits

1. 01Nutrition data

   Folate and vitamin K inside 20 kcal

   Per 100 g of raw asparagus, against adult daily requirements:

   • Vitamin K 41.6 µg — 35%
   • Folate 52 µg — 13%
   • Vitamin E 1.13 mg — 8%
   • Thiamine 0.143 mg — 12%, riboflavin 0.141 mg — 11%
   • Fibre 2.1 g, protein 2.2 g, potassium 202 mg
   • Beta-carotene 449 µg, copper 0.189 mg

   That it is 20 kcal is the premise of this table. A full 100 g is a fifth of half an apple.

   1.13 mg of vitamin E is high for a vegetable. Vitamin E usually comes from oils and nuts, not from vegetables. Spinach is higher at 2.03 mg; broccoli lower at 0.78 mg.

   Iron at 2.14 mg is also high for a vegetable — but it is non-haem, absorbed at 2–10%. Vitamin C in the same meal raises that.

   Green and white are the same plant. White asparagus is grown banked with soil to keep the light off. No chlorophyll forms, and nor do some of the compounds that form alongside it. The table above is for green.

   The compounds concentrate towards the tip. The lower stem is fibrous and the vitamins sit higher up, near the bud. Trimming the base is about texture, not about discarding nutrition.

2. 02Moderate evidence

   60% cannot smell it — the genes decided

   A 2016 genome-wide association study in the BMJ, drawing on the Nurses' Health Study and Health Professionals Follow-up Study cohorts: 6,909 people of European-American descent with genome-wide data.

   First the proportions.

   • 58.0% of men (1,449 of 2,500) were anosmic
   • 61.5% of women (2,712 of 4,409) were anosmic

   Then the genetics. Around nine million single nucleotide polymorphisms were tested, and 871 reached genome-wide significance (P < 5×10⁻⁸).

   All 871 sat in one place — a region on chromosome 1, 1q44 (248,139,851–248,595,299), containing multiple genes of the olfactory receptor 2 (OR2) family.

   Conditional analysis narrowed it to three independent markers: rs13373863, rs71538191 and rs6689553.

   What this result says has to be read precisely.

   What the study measured is the ability to smell. Participants were sorted by one self-reported statement. It did not measure whether the odorous metabolites appear in the urine.

   So "anosmic" is a fact about the nose — most people do produce the compounds (sulphur compounds from the breakdown of asparagusic acid), and more than half of us lack the receptor to detect them.

   This is also one population. Only European-American participants were analysed. Whether the proportions hold in other ancestry groups is not something this study answers.

   The authors' conclusion is plain: a large proportion of people have asparagus anosmia, and genetic variation near multiple olfactory receptor genes is associated with the ability to smell asparagus metabolites in urine.

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3. 03Not established

   The hangover and diuretic claims are not established

   Two claims attach to asparagus regularly.

   "Good for a hangover." A widely cited cell study reported that asparagus extract affects alcohol-metabolising enzymes in liver cells. There is no randomised trial measuring hangover symptoms in people. What happens in a dish of cells and what happens the morning after are different questions.

   "It is a diuretic." It has been used traditionally as one, and the amino acid asparagine is offered as the explanation. Controlled trials measuring urine output in people are hard to find.

   The purine content is genuinely moderate. Asparagus turns up regularly in gout advice. But as data accumulate that plant purines do not raise urate the way animal purines do, recent guidance treats vegetable purines less strictly than it used to.

   So: the reasons to eat asparagus are the taste, and the vitamin K and folate packed into 20 kcal. Hangovers and diuresis are unestablished territory.

   And in Europe it is a spring vegetable. In season, thick and firm, it is far better. That claim needs no evidence.

How to eat it well

What to watch out for