Quercetin in onion appeared mainly in the free-form as the aglycone. In garlic cloves, 72% of the total flavonoids is myricetin, 23% apigenin, and 5% quercetin. In chive, garlic chive, and leek the predominant flavonoid is kaempferol. Onion and garlic are an excellent source of calcium, potassium and manganese providing up to 10% of the human daily requirements of these elements. Onion and garlic can also accumulate selenium if grown in selenium-rich soils in the form of selenocystein and selenoproteins. Based on this information Ip and Lisk showed that garlic plants fertilized with high selenium and low sulfur fertilizer accumulated between 110 and 150 ppm dry weight selenium, while onion plants accumulated up to 28 ppm. Ip and Lisk proposed that the selenium enriched garlic and onion provide an ideal system to deliver selenium efficiently and safely into the human body for cancer prevention since inhibition of cancer by selenium in animal models requires between 1 to 3 ppm. Most of the onions and garlics contain very low concentrations of selenium. Ip and Lisk reported that “Valencia” topset garlic cloves contained 0.06 ppm selenium, while “Stuttgart” onion bulbs contained 0.02 ppm. Interestingly, onions also contain chromium and are considered a good source of this element. Chromium has been recently linked to diabetes prevention and appears to act as such by potentiating insulin receptor kinases and to be a part of the glucose tolerance factor, involved in cellular responses to insulin. Clinical studies on diabetic patients have shown that chromium can decrease fasting glucose levels, improve glucose tolerance, lower insuline levels and decrease total cholesterol and triglycerid levels.
Onions are a rich source of dietary fibers and especially of inulin, a polyfructosan. The health benefits of inulin-type fructans to human health have now been studied for more than one decade. It has prebiotic properties as it is preferably fermented by beneficial bowel bacteria like Lactobacilli and Bifidobacteria, thereby altering the bacterial mycoflora of the intestine in such a way that pathogenic, ebb flow table or harmful bacteria become less abundant. Neokestose, another fructan found in onion, has recently been shown to be an excellent promoter of the growth of beneficial bacteria. Frutans also promote the absorption of calcium and could thus be useful in the prevention of osteoporosis. High fructan diets have also been shown to lower concentration of colesterol, tryacylglycerol, phospholipids, glucose and insulin in the blood of middle-aged men and women. Owing to the presence of prebiotic polysaccharides , which are poorly degraded by the gut enzymes, and the presence of flavonoids, onions have been shown to possesss antidiabetic potential. Sharma et al. showed that onions had antihyperglycemic effects. Such effects were confirmed by Tjokroprawiro et al. who conducted a crossover comparative study with twenty diabetic patients to assess the effect of a diet comprising onions and green beans on serum glucose levels. They showed that the consumption of 20 g fresh onion three times daily significantly reduced blood sugar levels. The therapeutic value of onions, garlic and other Allium vegetables is confirmed by multiple epidemiological and experimental studies. Consumption of Allium vegetables has been found to retard growth of several types of cancers. For instance, there appears to be a strong link between the consumption of onions and the reduced incidence of stomach and intestine cancers. A number of epidemiological studies show inverse correlations between the consumption of alliums like onions and garlics, and the reduced incidence of cancers. A synthesis of case control studies carried in Italy and Switzerland reveals that consumption of one to seven portions of onions per week reduces the risks of colon, ovary, larynx, and mouth cancers. Other epidemiological studies clearly show the correlation between moderate garlic intake and a low esophageal and stomach tract cancers incidence. These studies have been performed on diferent geographical areas/continents and countries: China, Japan, Netherlands, Italy, Hawaii, Venezuela, Uruguay. Overall, a protective effect was reported, despite the obvious genetic variance existing among the populations examined in the different studies.
A regular consumption of garlic has been associated with the reduction in the incidence of preneoplastic lesions occurring in the gastric mucosa of individuals infected by Helicobacter pylori. A reduced cancer risk has been widely documented also for colorectal and prostate cancers. Other studies analyzing the preventive effect of garlic extracts on colorectal cancer have evidenced their suppressive potential on the development and progresssion of colorectal adenomas. A population based study analyzing the impact of a diet rich in Allium vegetables on the incidence of prostate cancer showed that the anti-cancer effects were more pronounced in men presenting localized rather than advanced forms. Mortality due to prostate cancer also appears to be reduced by a diet making a large place for onions. A limited number of studies explored the impact of a regular intake of Allium vegetables on the incidence of cancers affecting breast, endometrium and lungs. The risk of breast cancer was shown to decrease as consumption of alliums was increased in a French case-control study. It was found that garlic and some of its constituents prevent tumor initiation by inhibiting the activation of pro-carcinogens and by stimulating their elimination. Some studies say that onion extracts can inhibit the mutation process and reduce the proliferation of cancer cells. This effect is being attributed to quercetin in particular. Prevention of cardiovascular diseases has been attributed to regular garlic consumption. Onions also contain a number of bioactive molecules that can presumably reduce the risks for cardiovascular diseases. A preliminary study conducted in humans showed that the consumption of the equivalent of three onions in a soup was sufficient to significantly reduce the blood platelet aggregation. Tomato is the second most consumed and widely grown vegetable in the world after potato. Tomato is popular fresh and in many processed forms . In addition to their culinary role in the diet, tomatoes represent a low energy dense food with unique constituents that may positively affect health. Compositionally, the tomato has a unique nutritional and phytochemical profile. The major phytochemicals in tomato are the carotenoids consisting of 60% to 64% lycopene, 10% to 12% phytoene, 7% to 9% neurosporene, and 10% to 15% carotenes. Based on a fresh weight basis, tomato contains about 35 mg/kg of lycopene, with red cultivars containing in average 90 mg/kg of lycopene and yellow ones only 5 mg/kg. Processed tomatoes contain 2- to 40-fold higher lycopene than fresh tomatoes. Tomatoes and tomato-based foods are the world richest sources of lycopene.The average daily intake of lycopene in the human diet is about 25 mg/day; nearly 85 percent is obtained from fresh and processed tomato products. Tomato contains significant amounts of α-, β-, γ-, δ-carotene ranging in concentrations form 0.6 to 2.0 mg/kg, which ranks tomato as the fourth leading-contributor of provitamin A and vitamin A in the American diet. In addition to lycopene, tomatoes are one of the top contributors of potassium in developed countries and in the American diet.
Based on a 1999-2000 USA National Health and Nutrition Examination Survey food intake data, tomatoes rank seventh after milk, potatoes, beef, coffee, poultry, and orange/grapefruit juice as a potassium source. Besides in USA potassium is a nutrient of concern, as most Americans consume amounts well below the Dietary Reference Intake . In 2004, the new adult DRI for potassium was substantially higher than the amount previously reported in the 1989 Recommended Dietary Allowance . The increased recommendation was based on evidence indicating that 4700 mg potassium should help lower blood pressure, reduce the adverse effect of excess sodium intake on blood pressure, reduce the risk of kidney stones, hydroponic grow table and possibly reduce age-related bone loss. Tomatoes provide at least twice the potassium per 100 kcal compared with other common sources, except coffee, a nonsignificant calorie source of potassium. Consuming potassium from fruits and vegetables is ideal because it occurs with a biologically advantageous ratio of bicarbonate or citrate, important for bone health. Increasing potassium intake through increased tomato intake is a healthful, calorically sensible strategy for world developed countries. Tomato fruits are also an excellent source of ascorbic acid, about 200 mg/kg and are the major source of vitamin C next to citrus. Tomato contains small but significant amounts of lutein, α-, β-, and γ– tocopherols, and conjugated flavonoids. In a study of 20 tomato cultivars, total flavonoids content ranged from 1.3 to 22.2 mg/kg with about 98% present in the skin. Flavonoids in fresh tomato are present only in the conjugated form as quercetin and kaempferol, but processed tomato products contain significant amounts of free flavonoids. Flavonoids content is affected by cultivar and culture. For example, cherry tomatoes have a markedy d higher flavonoids content than standard or beef tomato cultivars and field-grown fruits have higher flavonoids content than greenhousegrown. Tomato cultivars are available with double the normal vitamin C , forty times normal vitamin A , high levels of anthocyanin , and two to four times the normal amount of lycopene . Tomato lycopene is found in appreciable levels in human serum and tissues when tomatoes and tomato products are consumed frequently. Several research investigations have shown an inverse relationship between plasma/serum lycopene concentrations and risk of some cancers. Similar associations have been reported for markers of cardiovascular disease, osteoporosis, cognitive function, and body weight. Moreover, relationships between dietary intakes of tomato products or tomato extract supplements have been observed in epidemiological studies and clinical trials examining markers of some cancers, cardiovascular disease, and ultraviolet light-induced skin erythema. The majority of research conducted in the area of tomato and lycopene intake and cancer risk has been observational. In a recent review of the literature, 178 original research articles were compiled reporting findings in humans on the relationship between lycopene, tomatoes and tomato-based products, and cancer risk.
Among these publications, nearly 90% were observational, highlighting the paucity of cause and effect investigations in this area. Reports on 13 cancer types were identified, of which breast, colorectal, gastric/upper gastrointestinal, and prostate cancers have the most original research published in humans, ranging from 17 to 60 publications. For breast, colorectal, and gastric cancers, the data support a neutral, although potentially protective, relationship between tomato/lycopene intake and cancer risk. Although the data are limited for gastric and lung cancers; the protective association is strongest with tomato intake verses dietary lycopene intake. Among the cancers investigated relative to lycopene and tomato intake, prostate cancer is the most widely researched. Although randomized controlled trial data are less available than observation data, a small number of dietary intervention trials using processed tomato products have been conducted. The results have been relatively successful as measured by improvements in prostate-specific antigen concentrations or increased apoptotic cell death in carcinomas. A prostate cancer risk reduction of nearly 35% was observed when the test subjects consumed ten or more servings of tomato products per week and the effect was much stronger for subjects with more aggressive and advanced stages of cancer. People consuming diets rich in tomato and tomato based products, which are rich in the carotenoid lycopene were found to be less likely to develop stomach and rectal cancers than those who consume lesser amounts of lycopene rich vegetables. Tomatoes, with their distinctive nutritional attributes may play also an important role in reducing the risk of cardiovascular and associated diseases through their bioactivity in modulating disease process pathways. In 2004, Sesso et al. reported an inverse association for women consuming greater intakes of tomato-based products and cardiovascular disease; an association not observed with lycopene intake alone. Several hypotheses are being tested related to the antioxidant properties of lycopene and a combination of carotenoids with coexisting water-soluble constituents delivered by tomatoes, such as vitamin C. The antioxidant capacity of plasma decreases when tomatoes and tomato products are removed from the diet and increased when they are added back. Consuming tomato products daily for 2 to 4 weeks increases antioxidant enzyme defenses and has been shown to reduce plasma lipid peroxides and the susceptibility of low density lipoprotein to oxidation. Oxidative modification of low density lipoprotein is a key step in the development of atherosclerotic lesions.