Writing in the Royal Society of Chemistry journal Food & Function, Taiwanese academics Hsiu-Chen Huang and Jen-Kun Lin said that (water aside) tea was the most widely drunk beverage worldwide.
The four general forms of tea included unfermented green, partially fermented oolong, fully fermented black and post-fermented pu-erh tea.
Tea had been used as a ‘crude medicine’ in China for more than 4,000 years, the scientists said, with effects such as blood vessel protection, reduction of serum cholesterol levels and prevention of arteriosclerosis reported.
Beneficial compounds within tea – especially polyphenols: either catechins or flavanols – reduced the risk of a variety of diseases, the authors wrote.
They noted that many widely studied biological functions of tea polyphenols included its anti-fungal, anti-inflammatory, anti-oxidative, anti-tumour and anti-carcinogenic effects.
According to Huang and Lin: “Notably, recent studies in humans and animals have shown that tea consumption has hypolipidemic and anti-obesity effects. However, the exact mechanism remains elusive.”
Rising obesity rates
The scientists linked common sweetener high fructose corn syrup to rising obesity and metabolic syndrome rates; they said refined sweeteners in processed foods had markedly increased consumer fructose intake, with 30%+ of carbohydrates consumed coming from added sugars in the US.
Hyperlipidemia (a high blood cholesterol level) is caused by lipid metabolic changes, and is a major cause of CVDs such as atherosclerosis and coronary heart disease (Wang, Lin-Shiau, Chen, Len 2000).
The present authors cited a 1998 study, which found that green tea had significant hypolipidemic and growth suppressive effects in rats after 63 weeks of feeding.
And in 2003 they said that Yeh et al. had demonstrated the significant suppression of fatty acid synthase (FAS) – responsible for fatty acid creation – by tea and tea polyphenols in human breast carcimona and liver cancer cells.
“These results strongly suggested that green tea leaves exerted a hypolipidemic effect, and therefore might have a protective effect against the atherosclerotic process,” Huang and Lin wrote.
Studies also reported an apparent protective effect from coronary heart disease (CHD) or stroke from high intakes of black tea or polyphenols, according to the current authors, although they said no protective effects were evident within one large cohort study (Muramatsu, Fukoya and Hara 1986).
Given these findings, the authors set out to investigate the hypolipidemic and growth suppressive effects of oolong, black, pu-erh and green teas in fructose-fed rats.
With results measured against a control group on a basal diet of Purina rat chow, other groups were fed diets comprising 36% rat chow, 60% high fructose syrup and 4% of each tea, while one group was fed 40% chow and 60% corn syrup.
Previous studies have shown that high fructose (60%) diets induced metabolic syndromes such as glucose intolerance, high serum triacyglycerols and insulin resistance in rats (Moura et al. 2009).
Investigating the effects of various tea leaves on fructose-induced hyperlipidemia, the team divided rats into six groups and calculated their weight up to 12 weeks.
Results showed that at 8 weeks the group fed fructose/oolong tea showed no body weight reduction, while the groups fed fructose/green tea, fructose/black tea and fructose/pu-erh tea showed “remarkable decreases in body weight compared with the fructose-fed group.
“After 12 weeks of feeding, all the groups with tea leaves included in the diet had significant decreases in their body weight as compared with the fructose-fed group.”
Human studies needed
Relative epididymal apipose tissue and kidney tissue weight was lower in all rats whose diets were supplemented with tea leaves as opposed to fructose alone, although the kidney difference was not statistically significant when the oolong/fructose and fructose-only diets were compared.
Positive effects of tea leaves on lipid metabolism, hepatic fatty acid synthase and p-AMPK protein levels were also noted, and Huang and Lin summed-up their findings thus:
“We provide evidence that tea leaves improve lipid and leptin metabolism in the fructose-fed rat model. Our results show that oral administration of tea leaves of different degrees of fermentation to fructose-fed rats results in the reduction of triacylglycerol, total cholesterol, leptin, insulin concentrations and enhancement of insulin resistance.
With pu-erh tea showing the most promise in terms of body and relative hypolipidemix effects in rats, the team noted that these “striking effects” deserved further study.
“Our results add new understanding to how natural products such as pu-erh tea affect lipid metabolism in vivo,” they wrote.
If the results were as striking in humans, the findings might contribute to new treatment strategies for fatty liver and obesity-related disorders in future, Huang and Lin added.
Title: ‘Pu-erh tea, green tea and black tea suppresses hyperlipidemia, hyperleptinemia and fatty acid synthase through activating AMPK in rats fed a high-fructose diet’.
Authors: Hsiu-Chen Huang and Jen-Kun Lin
Source: Food & Function, accepted November 16, 2011, doi: 10.1039/c1fo10157