Compounds in green tea can reprogram muscle metabolism to boost insulin sensitivity and glucose tolerance, according to new findings that further our understanding of why this ancient drink has long been linked to protection against obesity.
Green tea has long been linked with improved blood sugar control and reduced risk of metabolic diseases, but much of that evidence stems from animal studies undertaken in cool laboratory environments. Those conditions can artificially trigger calorie burn and mask the true physiological effects being tested. Researchers from Cruzeiro do Sul University wanted to remove that bias to see if green tea still delivered metabolic benefits in obesity models.
In this study, supported by Brazilian science institution Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), scientists fed mice a high-fat “cafeteria diet,” designed to mimic a high-fat Western diet, for four weeks, before supplementing the eating plan with green tea extract. The high-fat diet (HFD) had a caloric density of 531 kcal/100 g, almost double that of a standard diet of 288 kcal/100 g. The obese mice were kept in temperatures of around 28 °C (82.4 °F) for the duration of the study.
“We give them chocolate, filled cookies, dulce de leche, condensed milk,” said lead researcher Rosemari Otton from Cruzeiro do Sul University, who has been studying green tea for 15 years. “In other words, the same type of food that many people consume on a daily basis.”
The mice were split into three groups: Control (standard chow and water), HFD plus water and HFD plus green tea extract (500 mg/kg/day, Monday-Friday). All mice were on these diets for 16 weeks, however, the HFD and green tea group began daily supplementation after four weeks.
What they found was that the mice receiving green tea had significantly improved glycemic control compared to the mice on just the HFD, with better glucose tolerance and insulin sensitivity, and lower fasting glucose. This suggests that green tea still boosted metabolic function even though temperature ruled out additional calorie burn.
In addition to this, these animals’ skeletal muscle had structural and molecular shifts consistent with better glucose handling. Green tea increased muscle fiber cross-sectional area without affecting intramuscular triglyceride or cholesterol. At the signaling level, green tea increased the expression of Insr, Irs1, Glut4, Hk1, and Pi3k – genes that are important for glucose uptake and use in muscles. Lactate dehydrogenase (LDH) – an enzyme that helps recycle energy during muscle exertion and essential for glucose metabolism – was restored to pre-HFD levels.
“One way to assess muscle function is to look at fiber diameter,” Otton explained. “If it increases, we have more active muscle components. Green tea managed to maintain this diameter, showing that it protects muscle against the harmful effects of obesity.”
While body weight wasn’t the focus of the study, the researchers demonstrated just how green tea is able to improve insulin sensitivity while boosting energy processing in skeletal muscle – functions that drop off through obesity.
“This study suggested that green tea treatment attenuates the negative effects of HFD by improving muscle fiber cross-sectional area in the gastrocnemius muscle and increasing the expression of genes involved in lipid metabolism,” the researchers noted. “Although no effect was observed on fatty acid oxidation, green tea improved insulin and glucose sensitivity, as evidenced by glucose and insulin tolerance tests. It also increased the expression of genes associated with glucose uptake and lactate dehydrogenase activity in skeletal muscle.”
The researchers also found that the benefits depended on the protein hormone adiponectin, which is secreted by fat cells and regulates glucose and fat (lipid) metabolism. In mice with adiponectin production muted, green tea failed to deliver the benefits, indicating that the hormone may play a key role in how the extract impacted metabolic function.
“We conducted a study with adiponectin-knockout mice, meaning they don’t produce it,” Otton said. “And in these animals, green tea had no effect. This suggests that adiponectin is a key player in the mechanism of action of the tea.”
What’s more, the researchers couldn’t point to any particular compound in the green tea that was responsible for its benefits – instead, they believe, it’s the sum of its parts that appear to be key.
“Green tea is a complex matrix with dozens of bioactive compounds,” said Otton. “We’ve tried to separate these compounds and study their effects individually, but the whole extract is always more effective. There’s a synergy between the compounds that we can’t reproduce when they’re isolated.”
While the mice dose of green tea equated to around three cups a day, the scientists warn that there’s more nuance involved. Not all commercial green tea products meet the same quality standards that were used in the laboratory-grade extract, and dosage, bioavailability and polyphenol content can vary widely between preparations.
“The ideal is chronic consumption, as we see in Asian countries,” Otton said. “In Japan, for example, people consume green tea every day, throughout their lives, and obesity rates are low. But this is different from drinking tea for five months and expecting a miraculous weight loss effect.
“What we see in animals doesn’t always reproduce in humans,” she added. “But if we want to make this translation to real life, we need to think about all the details, such as ambient temperature. It’s these precautions that increase the validity of our data. We’re far from having all the answers, but we’re getting closer and closer.”
The study was published in the journal Cell Biochemistry & Function.
Source: Fundação de Amparo à Pesquisa do Estado de São Paulo
