Crispy green/crimson-red coloured lettuce is the most sought after leafy vegetable; be it your crunchy green salad or healthy vegetable sandwiches!
Botanically this marvelous, nutrition rich leafy green belongs to the daisy family of Asteraceae. Scientific name: Lactuca sativa.  
The leaves exude milk like fluid when cut. There are about six varieties of cultivars exist based upon head formation and leaf structure. The leaf varieties with more bitter taste are rather rich in anti-oxidants.

Here are some popular varieties grown around the globe:-

Butter-head, with loose heads; it has a buttery texture. Butter head cultivars are most popular and widely grown in Europe.

Chinese variety or celtuce, generally have long, tapering, non-head forming, strong flavoured leaves unlike its Western counterparts. They are, therefore, used preferred in stir fried dishes and stews.

Crisp-head variety forms tight, dense heads that resemble cabbage. They are generally the mildest form, valued more for their crunchy texture than flavour. Cultivars of crisp head are the most familiar type used in the USA.

Loose-leaf- with tender, delicate and fully flavoured leaves with loose bunch. This group includes green oak leaf, red oak leaf, valeria and lolla-rosa-types.

Romaine-lettuce grows in a long head of sturdy leaves with a firm rib almost reaching to the tip of the leaf. Cultivars of Romaine..

Summer Crisp variety forms moderately dense heads with a crunchy texture; this type is intermediate between crisp-head and loose-leaf types.

Health benefits

Lettuce leaves are the store house of many phyto-nutrients that have health promotional and disease prevention properties.

Vitamins in lettuce are plentiful. Fresh leaves are an excellent source of several Vitamin A and beta carotenes. Just 100 g of fresh, raw-lettuce provides 247% of daily vitamin A, and 4443 mcg of beta-carotene (Carotenes convert to vitamin A in the body; 2 mcg of carotene is considered equivalent to 1 IU of vitamin A). These compounds have antioxidant properties. Vitamin A is required for maintaining healthy mucus membranes and skin, and is also essential for vision. Consumption of natural fruits and vegetables rich in flavonoids helps to protect body from lung and oral cavity cancers.

Zeaxanthin (1730 mcg per100) an important dietary carotenoid in lettuce is selectively absorbed into the retinal macula lutea where it is thought to provide antioxidant and protective light-filtering functions, thus it offers some protection against age related macular disease (ARMD) in the elderly.

It is a rich source of vitamin K, Vitamin K has potential role in the increase of bone mass by promoting osteotrophic activity in the bone. It also has established role in Alzheimer's disease patients by limiting neuronal damage in the brain.

Fresh leaves contain good amounts folates and vitamin C.  Folates require for DNA synthesis and therefore, vital in prevention of neural tube defects in-utero  fetus during pregnancy. Vitamin C is a powerful natural antioxidant; regular consumption of foods rich in vitamin C helps body develop resistance against infectious agents and scavenge harmful, pro-inflammatory free radicals.

It also contain good amounts of minerals like iron, calcium, magnesium, and potassium which are very essential for body metabolism. Potassium in an important component of cell and body fluids that helps controlling heart rate and blood pressure. Manganese is used by the body as a co-factor for the antioxidant enzyme superoxide dismutase. Copper is required in the production of red blood cells. Iron is essential for red blood cell formation.

It is rich in B-complex group of vitamins like thiamin, vitamin B-6 (pyridoxine), riboflavins.

Regular inclusion of lettuce in salads is known to prevent osteoporosis, iron deficiency anaemia and believed to protect from cardiovascular diseases, ARMD, Alzheimer's disease and cancers.

Anti-Inflammatory Properties: Lettuce possesses anti-inflammatory properties that help in controlling inflammation. In experimental models, lettuce extracts have shown significant controlling power over inflammation induced by biocatalysts like lipoxygenase and carrageenan.

Protects Neuronal Cells: Neurons are brain cells that form physical connections to make up memory. The death of neurons in particular connections or circuits can result in the loss of memory. In some extreme cases, significant neuronal death can result in the onset of diseases like Alzheimer’s. The extracts from lettuce showed considerable control of neuron cell death due to its role in glucose/serum deprivation (GSD). The research has also mentioned that lettuce has the potential to be used in neuro-protection as a common remedy for neurodegenerative diseases.

Lowers Cholesterol Levels: Lettuce can be beneficial in lowering cholesterol levels that often lead to cardiovascular disease and other dangerous conditions. High cholesterol levels, particularly LDL or bad cholesterol levels, are harmful and can cause heart attacks and strokes. A study was conducted on mice to test the impact of lettuce consumption on fat and cholesterol. The results indicated a significant reduction of cholesterol levels compared to mice that weren’t fed lettuce. Lipid peroxidation was observed in most cases, which was concluded to be responsible for important this form of cholesterol control.

Induces Sleep: One of the major traditional uses of lettuce in Unani medicine was its use as a sleep inducer. Research into the extracts of lettuce resulted in the isolation of a depressant chemical. This chemical, when administered in experimental animals, showed significant sedative effects. Decreased heart rate and ventricular contractions were also observed. This particular chemical acts by blocking the excitatory signal processes of muscular and neural tissues.

Antioxidant Properties: Studies have shown that lettuce possesses antioxidants with significant free radical-scavenging capabilities. Antioxidants are a wide range of bio-chemicals that are mostly found in our diet; they are also very necessary for human health. Antioxidants act as barriers to free radicals, which are produced during cellular metabolism. These free radicals attack healthy tissues, cells and the DNA inside them. They can often cause healthy cells to mutate into cancer cells. The result is the development of various diseases. Antioxidants on the other hand, counteract these free radicals and neutralize them before the free radical attacks take place.

Antimicrobial Properties: The latex of lettuce possesses antimicrobial properties. Candida albicans and a number of other yeasts were completely deformed upon coming into contact with the latex from lettuce. Bio chemicals that are considered to possess these antimicrobial properties are the terpenes and cardenolids, as well as enzymes like glucanases.

Controls Anxiety: The neurological properties of lettuce have long been suggested and exploited during ancient times and the middle Ages in medical treaties such as the Unani system. Detailed research in recent times has led to the conclusion that lettuce possesses anxiolytic properties. When lab animals were given lettuce extracts, their locomotive activity was reduced, suggesting considerable anxiolytics.

Anti-Cancer Properties: Lettuce leaf extracts can control certain type of cancer. Research on human cancer cells, particularly leukaemia cells and breast cancer cells, were controlled to a significant extent after being treated with lettuce extracts. The experiments also suggested that the weight ratio of human lettuce consumption required to kill 50% of leukaemia cells would be 3 kg.