Romaine Lettuce

Food · Vegetable · Food, Typed Supps

Romaine is mostly water and crunch, but it packs a real dose of vitamin K and vitamin A into very few calories.

Stage by stage

Chewing & Release (Mouth): Chewing releases romaine's water, fiber, and fat-soluble vitamins A and K from its cell walls, though they absorb best when the leaves are eaten with a source of dietary fat.
Stomach Breakdown (Stomach): In the stomach, acid and mechanical churning break down romaine's plant fibres and begin freeing its carotenoids and vitamin K1 from the leaf tissue.
Intestinal Absorption (Small intestine): In the small intestine, bile emulsifies the fat-soluble vitamin K1 and beta-carotene so they can be packaged into micelles and absorbed into intestinal cells alongside dietary fat.
Liver Activation (Liver): Absorbed vitamin K travels to the liver, where it activates clotting factors II, VII, IX, and X, while absorbed beta-carotene is converted to vitamin A to support vision and immune function.

Timing

Pair romaine with a source of fat, such as olive oil dressing, nuts, or avocado, since its vitamin K and vitamin A are fat-soluble and absorbed better with a lipid present.

Time to effect

Days to weeks: Romaine digests within hours, but its vitamin K only meaningfully affects blood-clotting-factor levels after days to weeks of consistent intake, which is why clinicians managing warfarin worry about sudden large swings in leafy-green consumption, not a single salad.

Allergens

None of the fourteen major allergens. Derived from what this food is, not from a product label.

Eating patterns

Fits: Vegan, Vegetarian, Pescatarian, Halal, Kosher, Keto, Low-FODMAP, Gluten-free.

In season

May to September in the northern hemisphere, November to March in the southern.

Satiety

The satiety index is measured on 240 kcal servings. At 17 kcal per 100 g that is 1.4 kg of this, which is not a serving anyone eats — so the index has nothing to say about it.

Environmental footprint

0.5 kg CO2e and 108 litres of freshwater per kg, matched to "Other vegetables" in Poore & Nemecek (2018), Science 360.

Cost band

Cheap — roughly £2.5 per kg, as a band midpoint rather than a price.

Key nutrients

Nutrition per 100g

Calories: 17
Protein: 1.2g
Carbohydrate: 3.2g
of which sugars: 1.2g
Fat: 0.3g
of which saturated: 0g
Fibre: 1.8g
Sodium: 8mg

Food vs. supplement

Because romaine's vitamin K and vitamin A are fat-soluble and absorption depends on eating some fat alongside the leaves, whole romaine gives a lower, more variable dose per serving than a supplement — which is exactly why steady, food-based intake, rather than sporadic large doses, is recommended for people on vitamin K-sensitive medications like warfarin.

Kitchen tip

Dress romaine with an oil-based dressing or other fat source to boost absorption of its fat-soluble vitamins K and A, and wash it thoroughly — or re-rinse pre-washed bags — given romaine's repeated history of E. coli contamination and recalls.

Common myth

"Lettuce is basically just water with no real nutrition." In reality: Romaine is about 95% water and low in calories (roughly 17 kcal per 100 g raw), but per calorie it is unusually rich in vitamin K, provitamin-A carotenoids, and folate — and its main real-world safety issue is recurring E. coli O157:H7 contamination linked to multiple U.S. multistate outbreaks and recalls since 2018, not a lack of nutrients.