Health Benefits Of Lettuce

What is lettuce?

Lettuce, also known as Lactuca sativa L.,  is a green leafy vegetable that is consumed worldwide, mainly as a base in salads. It is often argued not to be a nutritional food as it contains mostly water, approximately 95%, and very little to no nutrients or calories.¹ 

China is the largest producer of lettuce, and the US ranks lettuce as the 3^rd most consumed vegetable.¹ There are six main types of lettuce, all with different colours, shapes and sizes, including iceberg lettuce or crisphead lettuce, head lettuce, romaine or cos lettuce, leaf or cutting lettuce, stem or stalk lettuce, and Latin lettuce.¹

Health benefits of lettuce

Every type of lettuce has a unique nutrient profile and therefore, offers different health benefits.¹  However, generally speaking, the main health benefits that lettuces provide include: 

Protection and improvement of eyesight

Lettuce is rich in beta carotene, an antioxidant that the body converts to vitamin A.¹ Antioxidants are compounds that protect cells from damage caused by the accumulation of free radicals (highly reactive and unstable molecules).²

Recent studies have shown that regular consumption of beta-carotene-rich foods such as lettuce may help in the promotion of eye health and significantly decrease the risk of developing eye diseases associated with vision loss such as age-related macular degeneration (AMD).³

Strengthening of bones

In addition to the beta carotene (precursor of vitamin A), lettuce is packed with the antioxidant vitamin C as well as vitamin K,¹ all of which facilitate bone strength by achieving the following:

• Besides promoting eye health and enhancing immune function, vitamin A helps build new bone cells (osteoblasts)⁴
• Vitamin C helps with the formation and maintenance of bones by stimulating collagen production.⁵ Collagen is a protein that provides strength, structure and elasticity to the skin, bones, teeth, cartilage and tendons
• Vitamin K helps enhance bone mineral density and decrease the risk of developing osteopenia (low bone density) and fractures (osteoporosis)⁶ 

Helping with weight loss

 This health benefit is not at all surprising or shocking because lettuce is: 

• Extremely low in calories: According to the USDA national nutrient database,⁷ 1 cup (47g) of shredded raw romaine lettuce contains only 7.99 kilo calories (kcals)
• A low energy density food: Lettuce is very high in both water (95%) and fibre,¹ meaning that it will induce feelings of satiety and fullness (due to large density) on fewer calories (due to low energy content),⁸ making it an ideal food for weight loss

Improving heart health

 According to research studies, consuming lettuce in regular quantities may protect against cardiovascular diseases by:

• Decreasing liver cholesterol levels⁹ 
• Decreasing LDL (low-density lipoprotein) levels: the “bad cholesterol’’ that ‘clogs’ arteries⁹ 
• Increasing HDL (high-density lipoprotein) levels: the “good cholesterol’’ that ‘unclogs’ arteries⁹
• Increasing homocysteine metabolism: Lettuce is rich in folate,¹ a B vitamin¹⁰ (vitamin B9) that converts homocysteine to methionine. Elevated homocysteine levels due to insufficient folate intake¹⁰ have been shown to damage the heart by inducing vascular inflammation (inflammation to blood vessels) and plaque (cholesterol build-up in arteries) formation 

Improving sleep

Interestingly, lettuce extract from lettuce seeds has been recently shown to improve sleep duration as it contains a sedating substance known as lactucin (the active compound of lactucarium).¹¹ Extracts from romaine lettuce seeds have the highest lactucin content compared to other types of lettuce seeds, including red leaf lettuce and green leaf lettuce, thus promoting the longest sleep.  

Nutritional facts

As stated above, the nutritional value as well as the bioactive compounds of lettuce vary depending on its type.¹ Generally speaking, lettuce is low in calories, fat, cholesterol, and sodium whilst being rich in dietary fibre, folate, beta carotene (precursor of vitamin A) and vitamin C.¹ 

In fact, the folate content of lettuce exceeds that of spinach, with fresh romaine lettuce containing 2.2 μg g−1 of folate compared to 1.9 μg g−1 in fresh spinach! Butterhead lettuce and red leaf lettuce are other good sources of folate. Additionally, despite spinach containing more calcium than lettuce, the calcium bioavailability of lettuce is argued to be better than that of spinach because lettuce, unlike spinach, does not contain oxalates, which decrease calcium absorption.

Leaf lettuce and romaine lettuce are considered to be the most nutritious types of lettuce.¹ On the other hand, iceberg lettuce or crisphead lettuce is the least nutritious type of lettuce.

Compared to other types of lettuce, romaine lettuce contains the highest amount of dietary fibre, potassium, folate and beta carotene whilst green leaf lettuce contains the most vitamin C and vitamin E.¹ On the other hand, red leaf lettuce contains the most phenolic acids which are compounds produced by plants that are produced to defend and protect themselves from insect and predatory attack. Red leaf lettuces also contain flavonoids, particularly anthocyanins which are responsible for the red colour (pigment) of their leaves. 

When consuming lettuce, it is worth adding a healthy fat source like olive oil to maximise the absorption of its fat-soluble nutrients, including the vitamins A, E, and K. 

Side effects and other concerns 

Despite being a food generally safe for everyone to consume in large amounts, lettuce may give rise to some serious side effects, including: 

Food poisoning or foodborne illnesses

Lettuces are susceptible to contamination by harmful germs^12, such as E. coli O157, norovirus, Salmonella, Listeria, and Cyclospora. If accidentally consumed, these harmful germs may lead to foodborne illnesses or food poisoning. To prevent this, it is recommended to store lettuce¹² in a 40^oF (4^oC) fridge, wash it thoroughly before eating, and keep it separated from raw meat, poultry and seafood

Nutrient deficiencies and malnutrition

As lettuce is very low in calories, fat, protein and important minerals, including iron, zinc and calcium, if overconsumed, especially alongside a very restrictive diet, including vegan or vegetarian, lettuce may cause nutrient deficiencies and later on, malnutrition.¹

Excess vitamin K blood levels

 Lettuce is rich in vitamin K, which, apart from its role in bone health, plays an important role in the induction of blood clot formation (coagulation).¹ Although this is mostly a good thing, it can be a bad thing too, especially for people who are taking blood thinners like warfarin,¹³ which works to prevent blood clot formation (anticoagulation). Therefore, having too much lettuce may decrease the efficiency of warfarin, resulting in high vitamin K blood levels and serious heart issues

Summary

• Lettuce (Lactuca sativa L.) is a leafy vegetable that is consumed heavily worldwide in salads. Although often argued to be a nutrition-lacking vegetable, lettuce generally contains many nutrients and bioactive compounds as well as antioxidants
• The nutritional value of lettuce varies depending on the lettuce type and leaf colour. Compared to all lettuce types, romaine lettuce is the most nutrient-dense, containing the highest amount of folate, potassium, beta-carotene and dietary fibre. Iceberg lettuce, on the other hand, provides the least amount of nutrients, vitamins and minerals
• When consumed as part of a healthy balanced diet alongside an active lifestyle, lettuce can provide amazing health benefits, including the possibility of improving eye health, bone health, heart health, sleep, and helping with weight loss
• That being said, just like any food, if consumed in excess and/or relied on solely for health purposes, lettuce can have a harmful effect mainly because it contains very little to no calories, proteins, fat or minerals. In addition, they can also be contaminated by harmful germs and can decrease the efficiency of blood thinners