The lemon ( Citrus limon ) is a small evergreen tree native to Asia and the tree's ellipsoidal yellow fruit.
The fruit is used for culinary and non-culinary purposes primarily for its juice, which contains about 5-6% of citric acid, and secondary for the pulp and the rind, which are also used in cooking and baking.
The lemon is widely used in medicine, all over the world and since ever: historically the sailors consumed it to prevent scurvy during their long trips, later it has been used against diarrhoea.
In Turkey and South Africa, the traditional medicine actually uses lemon in place of more expensive drugs while the Indian Siddha Medicine exploit the antibacterial properties of the lemon juice.
Nowadays, our mothers and grandmothers still use to prepare boiled rice with lemon juice and hot lemonade in order to treat diarrhoea, as well as marinating meat and fish to inhibit the bacteria growth.
Eating a healthy diet has become an important item of the everyday life. In this sense, fruits and vegetables are necessary compounds of a balanced diet, particularly for their role in prevention of diseases (i.e., obesity and diabetes) and certain types of cancer.
Lemon is a rich source of nutrients, including citric acid, vitamin C, flavonoids and minerals.
Lemon, raw, without peel
|Nutritional value per 100 g (3.5 oz)|
|Energy||121 kJ (29 kcal)|
|Dietary fiber||2.8 g|
|Thiamine (vit. B1)||0.040 mg (3%)|
|Riboflavin (vit. B2)||0.020 mg (2%)|
|Niacin (vit. B3)||0.100 mg (1%)|
|Pantothenic acid (B5)||0.190 mg (4%)|
|Vitamin B6||0.080 mg (6%)|
|Folate (vit. B9)||11 μg (3%)|
|Vitamin C||53.0 mg (64%)|
|Calcium||26 mg (3%)|
|Magnesium||8 mg (2%)|
|Phosphorus||16 mg (2%)|
|Potassium||138 mg (3%)|
|Zinc||0.06 mg (1%)|
|Percentages are relative to US recommendations for adults. Source: USDA Nutrient Database|
These values vary depending on the cultivar and the circumstances in which the fruit was grown.
The average lemon contains approximately 3 tablespoons (50 mL) of juice, characterized by the following parameters:
- pH, about 2,37;
- content of vitamin C, as Ascorbic Acid;
- antioxidant capacity;
with decreasing contents as follows:
eriocitrin > (12 mg/100 mL) > hesperidin(6 mg/100 mL) > diosmin.
Traditionally, the healthpromoting properties of lemon juice have been associated to its vitamin C contents, but recently it has been shown that flavonoids may also play a role in relation to antioxidant, antiinflammatory, cardioprotective, anticarcinogenic, antimicrobical and blood-lipid lowering properties:
Update on Uses and Properties of Citrus Flavonoids: New Findings in Anticancer, Cardiovascular, and Anti-inflammatory Activity, Journal of Agriculture and Food Chemistry 2008.
Vitamin C or L-ascorbic acid is a cofactor in at least eight enzymatic reactions including several collagen synthesis reactions that, when dysfunctional, cause the most severe symptoms of scurvy: malaise and lethargy, followed by formation of spots on the skin, spongy gums and bleeding from the mucous membranes.
Spots are most abundant on the thighs and legs and a sick person looks pale, feels depressed and is partially immobilized.
As scurvy advances, there can be open and suppurating wounds, loss of teeth, jaundice, fever, neuropathy and death.
Scurvy can be prevented by having 10 milligrams of vitamin C a day, so lemons were historically used for their high content of ascorbic acid: British sailors were given a ration of citrus fruits on long voyages to prevent the onset of scurvy, hence the British nickname of Limey.
Flavonoids (from the Latin word flavus meaning yellow, their colour in nature) are a class of plant secondary metabolites, characterized by a common benzo-γ-pyrone structure.
They were referred to as Vitamin P (probably because of the effect they had on the permeability of vascular capillaries) from the mid-1930s to early 50s, but the term has since fallen out of use.
More than 8000 compounds with a flavonoid structure have been identified.
This large number arises from the various combination of multiple hydroxyle, methoxyl and O-glycoside group substituents on the basic benzo-γ-pyrone.
Several types of flavonoids occur in Citrus:
- Flavones: Apigenin, Diosmin and Tangeritin;
- Flavanones: Eriocitrin, Hesperidin;
- Flavonons: Quercitin.
The concentration of these compounds depends upon the age of the plant and the highest levels are detected in tissues showing pronunced cell divisions.
The main health-related properties of citrus flavonoids have been found to include:
Flavonoids may act in the different development stages of malignant tumors by inhibiting the proliferation of cancerous cells (tangeretin: inhibition of several kinases involved in signal transduction and arrest cell-cycle progression), the invasion of surrounding tissue (diosmin: increased capillary resistance) and the angiogenesis (quercitin: inhibition of HIF-1α and VEGF expression).
Their activities are dependent upon particular structural motifs: oxidant stage, substituent and presence of glycolsilation;
- CARDIOVASCULAR PROTECTION:
Regular flavonoids intake is associated with a reduced risk of coronary heart disease by improving coronary vasodilatation, decreasing the ability of platelets in the blood to clot and preventing LDLs from oxidizing (apigenin).
Moreover, the flavonoids can be used to treat chronic venous insufficiency because they reduce the permeability and fragility of capillary walls, inhibiting the action of hyaluronidase enzyme (hesperidin; diosmin);
- HYPOCHOLESTEROLEMIC PROPERTIES:
Flavonoids decrease the availability of lipids for assembly of apoB-containing lipoproteins, an effect mediated by reduced activity of ACAT (Acyl CoA:cholesterol acyltransferase) and MTP (Metallo Proteinase);
- ANTI-INFLAMMATORY ACTIVITY:
Flavonoids inhibit the synthesis and biological activities of different pro-inflammatory mediators, mainly the arachidonic acid derivatives (hesperidin);
- INHIBITION OF ADIPOGENESIS:
The anti-adipogenic activity of citrus flavonoids is mediated by the inhibition of Akt activation, which induced the down-regulation of lipid accumulation and lipid metabolizing genes:
Citrus aurantium flavonoids inhibit adipogenesis through the Akt signaling pathway in 3T3-L1 cells, BMC Complementary and Alternative Medicine 2012;
The flavonoids are potent radical scavengers, because they inhibit the generation of lipid hydroperoxides and protect α-tocopherol;
The lemon juice can be used for the treatment of oral candidiasis in an HIV population, because some flavonoids (hesperidin) have an inhibitory effect on Candida Albicans growth:
Treatment of oral thrush in HIV/AIDS patients with lemon juice and lemongrass (Cymbopogon citratus) and gentian violet, Phytomedicine 2009.
Citrate, the conjugate base of citric acid, is a central component of the tricarboxylic acid cycle (TCA):
it is synthesized in mitochondria from oxaloacetate and acetyl-CoA (coenzyme A) by the enzyme citrate synthase;
in blood, the concentration of citrate (complexed to divalent ions, such as calcium and magnesium) is relatively constant, ranging from 0.05 mM to 0.3 mM;
at the glomerulus, it is filtered freely and its reabsorption takes place predominantly in the proximal tubule, from 65% to 90%;
the restant citrate is excreted in urine.
Low urinary citrate excretion, hypocitraturia, (Hypocitraturia: Pathophysiology and Medical Management Reviews in Urology 2009) is a known risk factor for the development of kidney stones.
The modulation of citrate excretion in the kidney is most influenced by the pH: acidosis decreases renal citrate excretion, whereas alkalosis increases it.
Since lemons contain the highest concentration of citrate of any fruit, the consumption of 120 ml of lemon juice per day, when mixed with two liters of water, has been shown to reduce the rate of kidney stone formation in people susceptible.
Transport of citrate across the apical membrane in the proximal tubule has has been extensively studied.
A portion of the citrate absorbed in the kidney will pass into the mitochondria for utilization in the TCA cycle.
This enzyme catalyzes the conversion of citrate and CoA to oxaloacetate and acetyl-CoA. Acetyl-CoA produced through this pathway may be used in the synthesis of fatty acids and cholesterol and is also involved in other types of metabolism.
Oxaloacetate is a substrate for gluconeogenesis, which is catalyzed by the enzyme phosphoenolpyruvate carboxykinase.
Citrate works through a variety of mechanisms to prevent the formation of kidney stones.
In the renal tubule, citrate complexes with calcium, increasing its solubility and reducing the concentration of free calcium in the urine.
This calcium-citrate complex limits calcium supersaturation and prevents nucleation of both calcium oxalate and calcium phosphate.
Additionally, citrate prevents crystal agglomeration and growth through its ability to bind to the crystal’s surface and may also prevent adhesion of calcium oxalate to renal epithelial cells.