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Water is the main constituent of all cells and it is required for various reactions inside the cell and body tissues. The body cannot survive without enough water. The lack of water in the body not only contributes cellular dysfunction but will also lead to dysfunction of major organs such as kidney, heart and brain along with increases blood viscosity leading to artery obstruction caused by blood clot or dehydration shock. Therefore, water is vital for all known forms of life. Along with water tea and coffee are the most common drinks across the world.

It is astonishing that the medical community have never studied about the quantity of drinking water for body requirements. The recommendation for common water intake is drinking at least 8 glasses (1 glass ~ 240 cc) of water every day called 8×8 recommendation. When investigators sought to find the origin of this advice, there was one-man anticipation with the absence of scientific literature supporting the 8×8 recommendation[224].

The research considered as the best study about the amount of water intake per day, is the Adventist Health Study, which prospectively assessed the associations between intake of water and other fluids with the risk of fatal coronary heart disease. Among the 8,280 male and 12,017 female participants aged 38–100 years who were without heart disease, stroke or diabetes at baseline and most of them drinking water and non-alcoholic fruit juices, were followed up for 6 years. A total of 246 fatal coronary heart disease events occurred during the 6-year follow-up. When compared with subjects drinking two or fewer glasses of water daily (low), those drinking from three to four glasses (medium) had a 35% lower risk of fatal coronary heart disease and those drinking five or more glasses (high) had a 54% lower risk of fatal coronary heart disease. These results indicated that high water intake reduces mortality risk. The associations remained virtually unchanged when adjusting for confounding factors such as fluids other than water, exercise, and when weight replaced body mass index.

In contrast, this study found that intake of fluids other than water (most of them is fruit juice without fiber or residues) was associated with increased risk of fatal coronary heart disease.

This study supported the recommendation that normal people should drink at least five or more glasses of water and should not drink high amount of fluids other than water such as fruit juice without fiber or residues.

In addition to the quantity, consumers should consider the quality of drinking water. The scientific evidences considered tap water which is always flowing is the lowest contaminated with bacteria. Nonetheless, most of consumers would rather drink bottled water than tap water because they presume that bottled water is clean and healthier than tap water. In fact, it is not always correct. A study about the quality of bottled water, 35 different brands of bottled water i.e., spring water fortified tap water, purified tap water, carbonated water and distilled waters were randomly collected from local grocery stores in the greater Houston area, USA. The results found that four of the 35 brands of the bottled water samples analyzed were contaminated with bacteria which were higher than tap water.[225].

While water is the best drink. Tea and coffee are other common drinks that are beneficial for health rather than disadvantages and considered as good drinks which are secondary to drinking water.

Health effects of these two other popular drinks are discussed below

Tea is plant leaves containing a variety of antioxidants. Both green tea and white tea are from the same type of plant and have equally amount of antioxidants. However, white tea provide more antioxidants if you add lemon in it. An In vitro study found that tea had anti-proliferative property in several different tumors including cancers. An evidence about anti-tumor growth property of tea was from a systematic review and meta-analysis which proved that Polyphenol E ointment extracted from green tea treated the external ano-genital warts with a very low recurrence rate compared with placebo[226].

A prospective study about the association between green tea consumption and mortality due to all causes was conducted in 90,914 Japanese (aged between 40 and 69 years). After 18.7 years of follow-up, 12,874 deaths were reported. When the association was analyzed, the investigators found that participants who drank many cups of tea per day had a lower risk of all-cause mortality, particularly heart disease and cerebrovascular disease, compared with those who did not drink less than 1 cup of tea per day[227].

A meta-analysis of 13 cohort studies assessing the association between consumption of tea and risk of developing type 2 diabetes found that drinking tea more than 3 cups per day was associated with a decreased risk of developing type 2 diabetes[228].

A meta-analysis of randomized clinical trials in 1,536 participants revealed that green tea intake resulted in significant small reduction in systolic blood pressure (1.94 mmHg) and moderate reduction in total cholesterol and LDL cholesterol (8%)[229].

A cross-sectional study examining the association between green tea consumption and teeth loss in 25,078 Japanese aged between 40 and 64 years found that consumption of more than 1 cup/day of green tea was significantly associated with decreased risk of teeth loss [230].

A meta-analysis of 2,215 patients with Parkinson's disease and 145,578 controls found that higher tea consumption was associated with a lower risk of Parkinson's disease[231].

There was no significant association between tea or coffee consumption and bone fracture incidence. A meta-analysis of studies performed before 2013 in 195,992 individuals with 9,958 cases of hip fractures to examine the association between tea or coffee consumption and the risk of hip fracture (due to osteoporosis) found that there was no significant association between coffee consumption and the risk of hip fracture[232].

A study in cross-over design, 50 coffee drinkers (habitually consuming 3–6 cups per day) participated in two trials, each lasting three consecutive days. The coffee trial involved participants consuming four mugs (200 ml) of black coffee per day equating to a caffeine intake of 4 mg/kg body mass. The water control trial involved participants consuming four mugs of water (200 ml) per day. In addition, the participants had controlled physical activity, food, and fluid intake. Total body water (TBW), body mass (BM), urine osmolality, hematocrit, serum osmolality at 24 hr. urine volume and hydration markers were assessed. The results showed that there were no significant differences between coffee drinkers or non-coffee drinkers on hydration status if water ingestion from drinking water and foods in both groups were equal.[233].

It is true that coffee intake acutely increases blood pressure, but over time, attenuation of this effect does occur. Nevertheless, it is not true that habitual consumption of coffee increases the risk of developing hypertension. Nowadays, some physicians recommend the patients with hypertension that they should stop drinking coffee because of the physicians’ misperception that coffee is associated with hypertension. That perception had ever been in worldwide physicians until Harvard Medical School published a large prospective cohort study in the Journal of the American Medical Association[234]. Over 150,000 women were followed up for over 12 years to examine the association between coffee intake (defined as the number of cups of coffee per day) and hypertension incidence. There was clear conclusion that there is no association between coffee consumption and the incident of hypertension.

A cross-sectional study of 8,013 subjects in Japan to evaluate the effect of coffee consumption on four major acid-related diseases:

• Non-erosive reflux disease (NERD),

• Gastric ulcer (GU),

• Duodenal ulcer (DU), and

• Reflux esophagitis (RE) Based on the large-scale multivariate analysis in 5,451 coffee drinkers and 2,562 non-coffee drinkers found that there is no significant relationship between coffee consumption and the four major acid-related upper gastrointestinal disorders[235].

A study found that coffee consumption improved short-term cognitive performance (simple reaction time, choice reaction time, incidental verbal memory and visio-spatial reasoning). Result of previous studies have shown significant, positive trend between coffee intake and all cognitive performance. Additionally, the greater improvement in cognitive performance occurred among the elderly. A study in Oman aged 80 or older found that those with higher lifetime coffee consumption had a better cognitive performance compared with non-coffee drinkers or less coffee drinkers[235].

There are evidence from two studies supporting prevention effect of coffee on dementia which are just moderate level of evidences, 1.Match case-control study, 54 patients with Alzheimer’s disease and 54 accompanying persons with normal cognition function, matched for age and sex, were participated. Levels of caffeine consumption were detected in previous 20 years. The result shows that patients with Alzheimer’s disease had an average caffeine intake of 73.9 mg/day (equal to a half cup of coffee, approximately), whereas the controls had an average caffeine intake of 198.7 mg/day (equal to a half cup of coffee, approximately). These finding indicated that subjects with higher caffeinated coffee intake had a significantly lower risk of Alzheimer’s disease compared with non-drinkers [6].
2.In a prospective analysis in over 6,000 Canadian subjects with normal cognitive function aged 65 years or older , a risk factor questionnaire to examine the risk factors or preventive factors for Alzheimer’s disease and all factors were recorded. Five years later, there were 194 Alzheimer’s disease cases and 3,894 cognitively normal controls. When risk factors were analyzed, the factors were significantly associated with increased risk of Alzheimer’s disease were increasing age, fewer years of education, and the apolipoprotein E ε4 allele (ApoE4). Three factors associated with a reduced risk of Alzheimer’s disease were use of nonsteroidal anti-inflammatory drugs (NSAIDs), coffee consumption, and regular physical activity[363].

The study evidence from the prospective study of over 40,000 women in the Nurses’ Health Study of Harvard Medical School for 12 year follow-up found that women with high coffee consumption had a lower risk of type 2 diabetes compared with non-coffee drinkers or less coffee drinkers[363].

A study found that participants who consumed higher coffee (up to 3 and a half cups/day) had a lower risk of Parkinson’s disease compared with non-coffee drinkers[238].

In a large prospective study in the Cancer Prevention Study II Nutrition Cohort (CPS II–Nutrition), incident cases of Parkinson’s disease (197 men and 120 women) were detected and it was found that subjects drinking higher coffee had a lower risk of Parkinson’s disease.[239].

In a randomized controlled trial, patients with Parkinson’s disease were given either caffeine 100 mg twice daily 3 weeks, then 200 mg twice daily 3 weeks, or matching placebo for 6 weeks. The results showed that caffeine had no significant benefit on excessive daytime sleepiness but improved objective motor measures in patients with Parkinson’s disease.[239].

In a prospective study National Institutes of Health-AARP Diet and Health Study, nearly 400,000 participants who were followed up for 14 year to examine the association of coffee consumption and mortality. The study found that coffee drinkers with higher coffee consumption had a lower total mortality compared with non-coffee drinkers or less coffee drinkers. The association was ranging from 0 to 6 cups of coffee per day Health benefits of coffee [241].
1. Reduce risk of symptomatic gallstone disease in both men and women [242].
2. Coffee drinker had a lower risk of gout compared with non-coffee drinker.
3. Coffee stimulates bowel movement causing defecate easily.
4. Coffee drinker had a lower risk of various cancers of mouth, esophagus, throat, breast, endometrium, liver and prostate gland compared with non-coffee drinker.

Disadvantages of coffee
1. Coffee addiction. 2. Coffee can cause sleeplessness in some cases but it can cause over sleep in some cases.
3. Coffee may cause anxiety and anger easily in some cases.
4. Coffee interferes iron absorption and increase risk of anemia caused by iron deficiency if drinking coffee immediately after meal.
5. Coffee contains substances causing LDL cholesterol increment. However, these substances are filtered out if you make coffee with filter paper.
6. Coffee increases risk of infant death during labor in pregnancy.
7. Drinking coffee with painkiller such as paracetamol may increase liver damage.