Asparagus Folate, Vitamin K, and Pregnancy

Table of Contents

1. What Folate Does, and How It Differs From Folic Acid
2. How Much Folate Is Actually in Asparagus
3. The Neural Tube Trials
4. Food Folate Versus the Supplement
5. Homocysteine, Stroke and the Heart
6. Cooking: Keeping the Folate in the Spear
7. Vitamin K: Clotting, Bone, and Green Versus White
8. Vitamin K and Warfarin: Consistency, Not Avoidance
9. Who Gets the Most From This, and Who Should Be Careful
10. Key Research Papers
11. Connections
12. Featured Videos

What Folate Does, and How It Differs From Folic Acid

Folate is vitamin B9 as it occurs in food: a family of related molecules, mostly in the form called 5-methyltetrahydrofolate, attached to chains of glutamate. Folic acid is the manufactured form used in supplements and in fortified flour and breakfast cereal. It is more stable and more completely absorbed than food folate, which is exactly why it was chosen for the trials and for fortification programmes, but once inside the body both end up doing the same job.

That job is carrying single carbon atoms from one molecule to another. It sounds trivial and is anything but: those one-carbon transfers are how the body builds the bases of DNA, how it makes new red blood cells, and how it recycles the amino acid homocysteine back into methionine. Any tissue that divides quickly — bone marrow, the gut lining, a developing embryo — runs short of folate first. The classic sign of adult deficiency is megaloblastic anaemia: red cells that are too large and too few, because the marrow cannot copy DNA fast enough to divide on schedule.

Two things follow for a page about a vegetable. First, folate is water-soluble and heat-sensitive, so how you cook the spears matters as much as which spears you buy. Second, because the body does not store much folate, it is a nutrient you need to keep eating rather than one you can bank — which is where a vegetable that supplies more than a third of a day's need in a single side dish earns its place.

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How Much Folate Is Actually in Asparagus

The figures below are from USDA FoodData Central for asparagus, cooked, boiled, drained (SR Legacy). USDA gives two household portions for this food: four spears weigh 60 g and half a cup weighs 90 g.

• Per 100 g: 149 micrograms of folate (as dietary folate equivalents), 22 calories, 2.0 g fibre, 50.6 micrograms of vitamin K, 224 mg potassium.
• Four spears (60 g): about 89 micrograms of folate — roughly 22% of the 400 microgram daily value — for 13 calories.
• Half a cup (90 g): about 134 micrograms, or a third of the daily value.
• A full cup (180 g, ten to twelve spears): about 268 micrograms, two-thirds of the daily value, for 40 calories.

For pregnancy the target is higher — 600 micrograms a day is the usual recommendation — and a cup of asparagus supplies just under half of it. Few vegetables do better per calorie. The comparison people usually reach for is spinach, which is similar per 100 g raw but shrinks so much in cooking that a cooked cup ends up higher; lentils and other pulses are higher again per cup but at ten times the calories. Asparagus's real advantage is that a generous portion is easy to eat, cooks in minutes, and costs almost nothing in energy.

Two cautions on the numbers. These are averages for boiled spears; the raw figure is a little lower per 100 g because water is lost in cooking, and a spear boiled for a long time in a lot of water will have lost folate into the pot (see the cooking section). And “dietary folate equivalents” already account for the fact that food folate is absorbed less completely than folic acid, so the USDA number is the one to compare with the 400 microgram target.

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The Neural Tube Trials

The neural tube is the strip of tissue that folds into a tube and becomes the brain and spinal cord. It closes between the third and fourth week after conception — usually before a pregnancy is confirmed. If it fails to close at the bottom the result is spina bifida; if it fails at the top, anencephaly. Through the 1960s and 1970s researchers noticed that these defects were commoner in poor families, in winter conceptions and among women with low blood folate, and two trials settled the question.

The Medical Research Council Vitamin Study (1991) was a randomised, double-blind trial in 33 centres across seven countries. It recruited women who had already had a pregnancy affected by a neural tube defect, and so were at high risk of another, and randomised them to 4 mg of folic acid a day, other vitamins, both, or neither, starting before conception. Folic acid produced a 72% protective effect (relative risk 0.28, 95% confidence interval 0.12 to 0.71); the other vitamins did nothing measurable. The trial was stopped early because the result was so clear that it would have been unethical to keep giving placebo.

The Hungarian trial of Czeizel and Dudás (1992) asked the other question: does it work for a first occurrence, in women with no history? 4,753 women planning a pregnancy were randomised to a multivitamin containing 0.8 mg of folic acid or to a trace-element supplement without it. Among the pregnancies with a known outcome there were six neural tube defects in the trace-element group and none in the vitamin group (P = 0.029), and congenital malformations overall were significantly fewer (13.3 versus 22.9 per 1,000).

The 2015 Cochrane review pooled five trials with 7,391 women and confirmed the protective effect of periconceptional folate supplementation against neural tube defects, with no evidence of harm to mother or baby. This is one of the best-established findings in preventive medicine, and it is why more than 80 countries now fortify flour with folic acid and why every major health authority advises a daily 400 microgram folic acid supplement for anyone who could become pregnant.

Notice what those trials tested: folic acid tablets, at doses of 0.8 to 4 mg, started before conception. They did not test asparagus, or any food. That matters for the next section.

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Food Folate Versus the Supplement

The obvious question — if asparagus is this rich in folate, why do I still need the tablet? — has three honest answers.

Timing. The neural tube closes before most women know they are pregnant. Protection depends on folate status being adequate at conception, which means the habit has to be in place beforehand, every day, regardless of what is in season. A supplement is the reliable way to guarantee that; a vegetable you eat three times a week in spring is not.

Absorption. Food folate is absorbed less completely than folic acid. The best measurement is a four-week Dutch feeding study by Winkels and colleagues (2007): 72 healthy adults ate either a high-folate diet (369 micrograms a day from fruit, vegetables and liver) or a low-folate diet topped up with measured folic acid capsules, all while taking a small dose of isotope-labelled folic acid as a tracer. Food folate turned out to be about 80% as available as folic acid (78% by the tracer method, 85% by the change in blood folate). That is better than the 50% figure older textbooks assumed, and it is the basis of the “dietary folate equivalent” conversion — but it still means a cup of asparagus does a little less than the label number suggests.

Dose. The trials used 0.4 to 4 mg of folic acid a day. Reaching even the lowest of those from food alone would take a very deliberate diet, every day, without fail.

So the practical picture is the one this site gives on the main asparagus page: the tablet is the proven insurance policy for the first month of pregnancy, and asparagus is one of the best ways to keep everyday folate status high for the rest of life — for red blood cells, for homocysteine, and for the population-wide protection that fortification was designed to give. Winkels' own conclusion was that a diet rich in food folate “can improve the folate status of a population”, and a vegetable that provides a third of the daily value per 100 g is a large part of how.

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Homocysteine, Stroke and the Heart

Beyond pregnancy, folate's other well-measured job is keeping homocysteine down. Homocysteine is an amino acid produced when the body uses methionine; folate (with vitamins B12 and B6) recycles it. High homocysteine is associated with heart attack and stroke in observational studies, and lowering it with folic acid is easy and cheap — so the question of whether that lowers cardiovascular risk has been tested in dozens of large trials.

How much folate lowers homocysteine is settled. The Homocysteine Lowering Trialists' Collaboration (2005) pooled individual data from 25 randomised trials and 2,596 people: 0.2 mg of folic acid a day cut homocysteine by 13%, 0.4 mg by 20%, 0.8 mg by 23%, and higher doses added little. The effect was largest in people who started with low folate and high homocysteine — that is, in people whose diets were short of foods like asparagus.

Whether that translates into fewer heart attacks and strokes is where the evidence splits, and this page reports the split. The 2016 meta-analysis by Li and colleagues in the Journal of the American Heart Association pooled 30 randomised trials with 82,334 participants. Folic acid supplementation reduced stroke by 10% (relative risk 0.90, 95% CI 0.84 to 0.96) and overall cardiovascular disease by 4%, but had no effect on coronary heart disease (relative risk 1.04). The stroke benefit was concentrated in people with low folate at baseline and in populations without flour fortification. In other words: raising folate helps most where it was low to begin with, it seems to protect the brain's blood vessels more than the heart's, and it is not a treatment for existing coronary disease.

For a home cook the translation is straightforward. Those trials used tablets, but the 0.2 to 0.4 mg a day that produced most of the homocysteine effect is the same order of magnitude as a daily serving of asparagus, lentils or leafy greens. Nobody has run a randomised asparagus trial for stroke, and this page does not pretend one exists.

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Cooking: Keeping the Folate in the Spear

Folate dissolves in water and breaks down with heat, so the cooking method decides how much of the USDA figure actually reaches your plate. The clearest measurement of ordinary home cooking is McKillop and colleagues' 2002 study in the British Journal of Nutrition, which surveyed how UK shoppers really cook their main folate sources and then measured folate before and after.

• Boiling for typical times left only 49% of the folate in spinach (192 to 94 micrograms per 100 g) and 44% in broccoli (177 to 77 micrograms). The rest was in the cooking water.
• Steaming the same vegetables caused no significant loss, even at the longest steaming times tested.

Asparagus was not one of the foods in that study, but its folate is the same water-soluble molecule in the same kind of green tissue, and there is no reason to expect it to behave differently. Note too that the USDA figure this page uses is already for boiled, drained spears — so a steamed, roasted or grilled spear will if anything carry a little more than 149 micrograms per 100 g, and a spear boiled hard for ten minutes in a big pot will carry less.

Practical rules that follow: steam, roast, grill or sauté rather than boil; if you do boil, use little water, keep it short (two to four minutes for medium spears), and use the water in a soup or sauce; buy spears that are firm with tightly closed tips, since folate declines in storage; and keep them cold and eat them within a few days. The bonus is that every one of those rules also gives you better-tasting asparagus — the grey, limp spear of institutional cooking is the folate-poor one.

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Vitamin K: Clotting, Bone, and Green Versus White

Asparagus's second headline vitamin is K1, phylloquinone: 50.6 micrograms per 100 g cooked according to USDA, which is 42% of the 120 microgram daily value, and about 91 micrograms in a cup. Vitamin K is the cofactor for the enzyme that activates the clotting factors made in the liver — without it, prothrombin and several other factors are made but cannot bind calcium and do not work. The same enzyme activates osteocalcin in bone and matrix Gla protein in blood vessel walls, which is why vitamin K is studied for bone density and arterial calcification as well as for clotting.

Phylloquinone sits in the chloroplast membranes of green plants, alongside chlorophyll. That has two practical consequences. Green asparagus carries more vitamin K than white, because white spears are grown under soil where no chlorophyll forms; the USDA entry does not separate the colours, but the biology is unambiguous. And because it is fat-soluble, vitamin K is absorbed several times better when the spears are eaten with some fat — olive oil, butter, a poached egg, hollandaise. The traditional pairings turn out to be the nutritionally right ones.

For most people the vitamin K in asparagus is simply good: it is a nutrient many diets fall short of, and there is no upper limit for K1 because the body has no trouble with it. The one group for whom it needs thought is people taking a vitamin K antagonist, and that is the next section.

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Vitamin K and Warfarin: Consistency, Not Avoidance

Warfarin and the older anticoagulants like it work by blocking the recycling of vitamin K, so a sudden change in how much vitamin K you eat changes how well the drug works. For decades patients were told to avoid green vegetables altogether. That advice was wrong in an important way, and the evidence now supports a different rule: eat a steady amount, every week, and do not swing.

Holbrook and colleagues' 2005 systematic overview in Archives of Internal Medicine rated the quality of every published warfarin–food and warfarin–drug interaction; high vitamin K foods were among the few dietary interactions supported by decent evidence. Franco and colleagues (2004) measured it directly: in 12 stably anticoagulated patients put through four-day hospital diets, an 80% cut in vitamin K intake pushed the INR from 2.6 to 3.3 within a week (that is, made the blood thinner and raised the bleeding risk), and a five-fold increase lowered it from 3.1 to 2.8. Both directions moved the numbers; the depleted diet moved them more. In their observational arm, vitamin K intake predicted both over- and under-anticoagulation.

The 2016 systematic review by Violi and colleagues, which pooled two dietary trials and nine observational studies, found the evidence conflicting on whether ordinary day-to-day variation in vegetable intake affects INR control — a fair reading is that big, abrupt changes matter and small habitual ones mostly do not. The most striking trial points the other way from the old advice entirely: Sconce and colleagues (2007, Blood) took 70 warfarin patients whose control was unstable, noted that unstable patients typically ate less vitamin K than stable ones, and randomised them to 150 micrograms of vitamin K a day or placebo for six months. The vitamin K group's INR became significantly more stable (time in range rose by 28 points versus 15 for placebo), and control improved in 33 of 35 people taking it. A steady background supply of vitamin K buffers the drug against fluctuations.

What that means for asparagus: if you take warfarin you do not need to give it up. A cup a week, every week, is a small and steady input that your dose will already have been set against. What you should not do is eat none all winter and then a bunch a day for the three weeks of the local season — that is the swing that moves an INR. If you are changing your habit, tell whoever manages your dose so they can check the INR a week or two later. The newer anticoagulants (apixaban, rivaroxaban, dabigatran) do not work through vitamin K and have no such interaction.

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Who Gets the Most From This, and Who Should Be Careful

Anyone who could become pregnant benefits most, in the sense that folate status at conception is the single nutritional factor with the largest proven effect on birth outcomes. Asparagus is one of the best foods for keeping that status high — alongside, not instead of, the 400 microgram supplement.

People with low folate for other reasons: heavy alcohol use, coeliac or inflammatory bowel disease, some anti-seizure and rheumatology drugs (methotrexate works by blocking folate), and older adults on a limited diet. A serving a day of a folate-rich vegetable is an easy correction, and a blood folate test (see the site's Folate lab test page) will tell you where you stand.

People concerned about stroke risk, particularly where flour is not fortified: the trial data say raising a low folate intake helps, and food is the safest way to do it.

Who should think twice? Anyone on warfarin should keep intake steady rather than seasonal, as above. People with untreated B12 deficiency should know that high folic acid intakes can correct the anaemia of B12 deficiency while the nerve damage progresses; that is a concern with high-dose supplements and fortification rather than with vegetables, but it is the reason a B12 level is usually checked alongside folate. Otherwise there is no such thing as too much asparagus folate: the upper limit of 1,000 micrograms applies to folic acid from supplements and fortified foods, not to food folate, for which no toxicity has ever been shown.

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Key Research Papers

Author names, titles and journals are plain text; only the PMID is a link. Every PMID below was checked against PubMed before publication, and the abstract read to confirm it supports the sentence it is attached to.

1. MRC Vitamin Study Research Group (1991). Prevention of neural tube defects: results of the Medical Research Council Vitamin Study. Lancet. — PubMed PMID: 1677062
2. Czeizel AE, Dudás I (1992). Prevention of the first occurrence of neural-tube defects by periconceptional vitamin supplementation. The New England Journal of Medicine. — PubMed PMID: 1307234
3. De-Regil LM, Peña-Rosas JP, Fernández-Gaxiola AC, Rayco-Solon P (2015). Effects and safety of periconceptional oral folate supplementation for preventing birth defects. The Cochrane Database of Systematic Reviews. — PubMed PMID: 26662928
4. Winkels RM, Brouwer IA, Siebelink E, Katan MB, Verhoef P (2007). Bioavailability of food folates is 80% of that of folic acid. The American Journal of Clinical Nutrition. — PubMed PMID: 17284745
5. Homocysteine Lowering Trialists' Collaboration (2005). Dose-dependent effects of folic acid on blood concentrations of homocysteine: a meta-analysis of the randomized trials. The American Journal of Clinical Nutrition. — PubMed PMID: 16210710
6. Li Y, Huang T, Zheng Y, Muka T, Troup J, Hu FB (2016). Folic Acid Supplementation and the Risk of Cardiovascular Diseases: A Meta-Analysis of Randomized Controlled Trials. Journal of the American Heart Association. — PubMed PMID: 27528407
7. McKillop DJ, Pentieva K, Daly D, et al. (2002). The effect of different cooking methods on folate retention in various foods that are amongst the major contributors to folate intake in the UK diet. The British Journal of Nutrition. — PubMed PMID: 12493090
8. Holbrook AM, Pereira JA, Labiris R, et al. (2005). Systematic overview of warfarin and its drug and food interactions. Archives of Internal Medicine. — PubMed PMID: 15911722
9. Franco V, Polanczyk CA, Clausell N, Rohde LE (2004). Role of dietary vitamin K intake in chronic oral anticoagulation: prospective evidence from observational and randomized protocols. The American Journal of Medicine. — PubMed PMID: 15121490
10. Violi F, Lip GY, Pignatelli P, Pastori D (2016). Interaction Between Dietary Vitamin K Intake and Anticoagulation by Vitamin K Antagonists: Is It Really True?: A Systematic Review. Medicine. — PubMed PMID: 26962786
11. Sconce E, Avery P, Wynne H, Kamali F (2007). Vitamin K supplementation can improve stability of anticoagulation for patients with unexplained variability in response to warfarin. Blood. — PubMed PMID: 17110451

PubMed Topic Searches

1. PubMed: folate and neural tube defect prevention
2. PubMed: bioavailability of food folate from vegetables
3. PubMed: dietary vitamin K and warfarin INR stability

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Connections

• All Food
• Asparagus
• Asparagus Benefits: What the Evidence Actually Shows
• Asparagus: Nutrient Profile
• Asparagus Inulin, Prebiotic Fibre, and the Gut
• Asparagus, the Diuretic Tradition, Kidneys, and Purines
• Asparagus Glutathione, Saponins, and Antioxidants
• Vitamin B9 (Folate)
• Vitamin K
• Folate Blood Test
• Spina Bifida
• Anemia
• Spinach
• Broccoli

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