Green tea is made from the unfermented dried leaves of the plant Camellia Sinensis. The important components of green tea for health and fat loss benefits are ‘catechins’ and the world’s favourite ‘drug’, caffeine. A large mug (250ml) of brewed green tea contains 50–100 mg of catechins and 30–40 mg of caffeine, but these can vary depending on brewing time or water temperature. Therefore, green tea extract (GTE) has been developed, for research purposes, as well as a nutritional/sports supplement, providing a constant level of catechins and caffeine to elicit their proposed benefits. Green tea has many health benefits with catechins having been reported to have anti-diabetic, anti-bacterial, anti-inflammatory and antiobesity properties.
GTE ingestion, rich in catechins and caffeine, has been reported successful for gradual weight loss, in combination with moderate food energy restriction and increased exercise. It has also been shown that GTE supplements can help weight maintenance after weight loss. However, further investigations are needed to confirm the dose and content of tea catechins and caffeine required to achieve weight loss or maintenance goals.
It is important to note that routine caffeine consumption can override these weight loss effects of GTE, and so a higher dose of GTE may be required for those who consume caffeine regularly. GTE ingestion may be a better alternative to consumption of caffeine alone, as no change in heart rate or blood pressure has been reported, even after consumption of high doses. This is in contrast to ingestion of high doses of caffeine which has been shown to increase blood pressure, produce anxiety, palpitations and headaches among other potential side effects.
It has been reported that using mixtures of green tea catechins and caffeine improves body-weight loss and body-weight maintenance over the long-term, mainly through their effects on energy expenditure, appetite suppression and fat oxidation. Other studies have shown body-weight maintenance occurs after body-weight loss induced by an energy-restricted diet while ingesting green tea catechins and caffeine mixtures. However, ethnicity and habitual caffeine consumption were moderating factors in these studies.
Green tea catechins with caffeine have also been shown to decrease body-weight, body mass index (BMI) and waist circumference when compared with caffeine supplementation alone. It has been found that not only high habitual caffeine consumption, but also foods or meals in general, influences green tea’s actions on energy metabolism. It is important to note that green tea catechin caffeine intake does not add synergistically to the beneficial effect of a high-protein diet during weight maintenance after weight loss. However, protein intake alone as well as consumption of the green tea mixture showed beneficial effects in the conservation of fat free body mass and sustained energy expenditure.
It summary, GTE has the potential to increase fat metabolism and weight loss, both at rest and during exercise, however the mechanism is not completely understood and there are doubts as to which dose would be most effective, particularly in highly trained athletes, and if these effects are from purely metabolism boosting actions or other, potentially more important factors, are at play.
Paul Rimmer (BSc, MSc, PhD).
