Caffeine is known to cause a transient increase in the plasma levels of free fatty acids. This likely results from a concomitant increase in plasma levels of adrenaline, which increases the lipolysis rate of adipocytes and thus increases circulating levels of free fatty acids. Free fatty acids are known to be a causative factor in insulin resistance, and this explains the 15 % decrease in insulin sensitivity seen with caffeine infusions (here). Green tea is a source of caffeine, and green tea is known to cause increases in the release of adrenaline, which leads to increased lipolysis and an increase in plasma free fatty acids. However, green tea has consistently been shown to be a weight loss food and consumption of green tea is associated with improved body composition. It is therefore likely that green tea has an insulin sensitising effect, and it is by this mechanism that it is able to induce weight loss. In fact studies investigating the effects of green tea show a beneficial effect on insulin sensitivity and its associated conditions.
For example, researchers1 administered a capsule containing 379 mg of green tea extract to obese individuals with high blood pressure. These subjects showed many of the characteristic symptoms of the metabolic syndrome and were at high risk of cardiovascular disease. Another group of subjects received a placebo tablet. After 3 months of treatment the subjects consuming the green tea extract had improvements in a number of the symptoms of the metabolic syndrome including a reduction in insulin resistance, an improvement in antioxidant status, a reduction in inflammation, a reduction in systolic blood pressure, a reduction in diastolic blood pressure, a reduction in fasting triglycerides, a reduction in total cholesterol levels, and a reduction in fasting insulin levels, when compared to the placebo group. The body mass indices of the green tea group did not differ to the controls, suggesting that the benefits of the treatment did not occur because of any weight loss experienced by the subjects.
The reasons that adrenaline and free fatty acid rises following caffeine are not detrimental is because the free fatty acids are oxidised as a source of fuel and so increases in levels are only transient. The obesity pathology is associated with permanently raised levels of free fatty acids, and these are a symptom of a metabolic disfunction. The transient rise in free fatty acids following caffeine ingestion is associated with an increased oxidation of the fatty acids in a similar way to exercise. Green tea is also a potent and biologically active antioxidant, with the D ring gallate group in epigallocatechin gallate (EGCG) and epicatechin gallate (ECG) conferring addition antioxidant capacity to the dihydroxyl and trihydroxyl groups on the B ring and the meta-5, 7-dihydroxyl groups on the A rings of flavan-3-ols. This makes them effective at reducing systemic inflammation, a possible contributory factor to the oxidative stress that may decrease insulin sensitivity.
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