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From a health perspective it may be good that green tea does not contain all-too much caffeine. From a fat loss perspective, it clearly lacks caffeine.

When it comes to dietary supplements, people like to pay tons of money for unproven ingredients with funky names and dubious or non-existent safety profiles; agents that have been scientifically proven to work, are save and cheap, on the other hand, are non-sellers or at least considered to be non-effective.

Obviously, I cannot really explain why that is the case (I suspect it is because people effect drug-like effects without drug-like side-effects from supps and are thus always on the lookout for the "next big"... hoax), I can tell you, though, that a recent study that is going to be published in one of the upcoming issues of the Journal of Strength and Conditioning Research (Jo. 2015) shows that caffeine and green tea, two supplements that belong to the previously described category, are everything but useless.
In said study, Edward Jo and colleagues investigated the effects of a caffeine + green tea polyphenol mix (250mg caffeine + 400mg of a green tea extract with 50% EGCG and 5mg of caffeine per serving) on (a) metabolic rate and fat oxidation at rest, as well as following a bout of sprint interval exercise (SIE) and (b) the performance during a standardized sprint-interval test.

The study was a double-blind, randomized, placebo-controlled, crossover study that involvd 12 subjects (male: n=11; female:1 n=1) whose antroprometric data, i.e "body mass=76.1±2.2 kg; height= 169.8±1.6 cm; BMI= 22.7±3.0 kg/m2; body fat %= 21.6±2.0% [DXA data]" (Jo. 2015), already tell you that they were healthy recreationally active, but not necessarily athletic (it may be worth mentioning that they were relatively stim-naive with an intake of < 201mg of caffeine per day).

Figure 1: Energy expenditure (kcal/h) and fat oxidation (g/day) measure before (at rest) and during (during SIE) the sprint interval exercise 10 and 55 minutes after the ingestion of caffeine + GTE or placebo (Jo. 2015).

During the two testing sessions at the Human Performance Research Laboratory of the California State Polytechnic University, the subjects' resting energy expenditure (REE) was measured for 45 minutes starting 10 minutes after the ingestion of the aforementioned caffeine + polyphenol mix - a mix that was consumed on an empty stomach after an 8-h overnight fast (don't be fooled by the way the scientists report their data in "g/day" and kcal/day - I changed the latter, already but the values for fat in g would have become to small - we are talking about 45 + 30 min and a 24h measurement here).
After the initial 45-minutes, during which the subjects' resting energy expenditure had been measured, the subjects were placed on a computer-integrated cycle ergometer on which they performed a standardized 30 minute sprint interval exercise (SIE) protocol., the scientists describe as follows:

"Sprint-Interval Exercise Protocol. The SIE protocol was performed on the Velotron DynaFit Pro cycle ergometer and comprised of four 30-second maximal effort intervals each separated by 5 minutes of low-intensity, constant workload cycling (Figure 2). First, the ergometer was properly adjusted for the subject. Adjustment specifications for each subject was recorded during their familiarization visit and repeated for all experimental trials. Subjects initiated the SIE protocol with a 5-minute interval of low-intensity cycling at a constant workload of 75W. Immediately after, subjects cycled with maximal effort for 30 seconds against an added resistance that is 7.5% of BW for males and 7.2% for females. These two intervals were repeated three additional times. After the last 30-second sprint interval, the subjects performed an additional low-intensity 75W interval plus an extra 3 minutes of cool-down at a constant workload of 30W. The total duration of the SIE protocol was 30 minutes" (Jo. 2015).

A workout that had little effect on the effect of the caffeine + green tea combo which did, as you can see, when you compare the "at rest" and "during SIE" values in Figure 1, increase the energy at rest and during sprint interval training to a similar extent. More specifically, the increase in energy expenditure and fat oxidation was - within the margin of inter-individual variability - in the range of +10% during both conditions.

Figure 2: Illustration of the sprint interval exercise test performed 55 min after ingesting the supplement (Jo. 2015).

In view of the fact that we may safely assume that this effect should last for at least 2-3h this can be a practically relevant effect if it is complemented by a reduced energy intake and a caloric deficit. If the latter is not present, even the extremest increases in fat oxidation and energy expenditure will fizzle out and be as irrelevant as the effect of the caffeine + green tea combination on exercise average and peak power (W) during the sprint training, Jo et al. observed in their study... and "Yes!", that is disappointing, but in view of the low dose of caffeine and the non-existent effects of green tea on performance during a test like this not really surprising.
References:

• Jo et al. "Dietary Caffeine And Polyphenol Supplementation Enhances Overall Metabolic Rate And Lipid Oxidation At Rest And After A Bout Of Sprint Interval Exercise." Journal of Strength & Conditioning Research: Post Acceptance: November 23, 2015. doi: 10.1519/JSC.0000000000001277

Bicycling to work is one of these "small things" that may make the difference between lean and obese.

I know this is not what your friends will want to hear, but for 99.9% of them it's not their genes which are to blame for their inability to lose weight. It's much simpler than that: It's their inability or unwillingness to induce a large enough caloric deficit to force their bodies to tap into the fat stores. Don't get me wrong: It's obvious that genes, with their direct and indirect influence on one's basal energy requirements (think of being tall vs. being small, if nothing else), amount of muscle, and even ability to handle glucose and fats, will factor in here. In the end, however, it's everyone's knowledge about the dos and don'ts of dieting, effort and dietary adherence that will make the difference.

That's no news for you? I think the results of a recent study from the University of Kansas Medical Center and the Texas Tech University will still come as a surprise. Usually, we talk about cheating on one's diet, skipping scheduled workouts and eating foods people are not supposed to eat, when we discuss the reasons why people fail. The results of said study, however, suggest that something else may have a much larger impact: Our regular non-exercise physical activity.
If you've read my March 2015 article "It Doesn't Have to be an Exhaustive Workout - Increasing Physical Activity Just as Effective as Strength, Endurance or Combined Exercise to Lose Fat and Build Muscle" (read it now), you will know about the importance of "taking the stairs", standing instead of seating, bicycling to work and other often overlooked low-intensity non-exercise physical activity in our everyday lives. Interestingly enough, it is just this type of low intensity 'non-exercise' of which the so-called "Midwest Exercise Trial 2" indicates that it is what distinguishes the weight loss responders (>5% weight loss) from the non-responders (<5% weight loss) in a relatively tightly controlled "work out five times per week for 10 months to lose weight"-intervention by Herrmann et al. (2015).

Before we are dealing with this surprising result, though, let's first take a look at what exactly the N=141 18-30 year-old men overweight/obesity (BMI 25-40 kg/m²) subjects had to do in this 'exercise for weight loss study' (details can be found in the description the scientists published when they registered their clinical trial | Donelly. 2012):

• Table 1: It is important to note that there were no sign. baseline differences in weight, age, etc. between responders and non-responders among the study participants (Herrmann. 2015).

  the subjects exercised on 5 days of the week - one of the session was "choose the activity you want", the other sessions were performed under supervision walking/jogging on treadmill
• the duration / intensity of all workouts was matched to initially burn 150 kcal; from months for on, 400 and 600 kcal/session (this is in line with the "2008 Physical Activity Guidelines for Americans" | USDA. 2008) 

I see you're looking for the dietary advise? Well, there was none. That may look awkward, but in view of the fact that the scientists wanted to see whether the simple adherence to the USDA "Physical Activity Guidelines for Americans" would make a difference, the subjects were told to stay on their regular (junk food?) diets.
As the name of the study or rather the index "2" in the name implies, the "Midwest Exercise Trial 2" is a follow up study. It's a follow up that was supposed to elucidate (a) what distinguishes responders from non-responders and whether (b) the gender differences in weight response Donelly et al. observed in their 2003 predecessor study were coincidental or something we have to keep in mind, whenever we are designing exercise-based weight loss routines for men and women..

Figure 1: Total daily energy expenditure (TDEE), nonexercise energy expenditure (NEEx), and resting metabolic rate (RMR) at baseline and 10 months in responders (RS) and nonresponders (NR) to an aerobic exercise intervention.

What should I say? If you look at the data it would almost seem as if (2) was the case: Men and women appear to respond very differently to this kind of exercise-based weight loss efforts. Yes, the data in Figure 1 shows that there is a trend for non-responders from both sexes to be less active in their free time and thus having a lower non-exercise induced energy expenditure (NEEEx). On the other hand, though, ...

1. Figure 2: Nonexercise physical activity (NEPA) and sedentary time across 10 months in responders (RS) and nonresponders (NR) to an aerobic exercise intervention (Herrmann. 2015).

   only the male non-responders are truly characterized by their tendency to take the five weekly workouts as an excuse to use the elevator and drive the 200m to the next fast-food outlet by car, while 
2. there is no such compensatory effect on NEEEx in the women and even an increase in resting metabolic rate (so no metabolic damage or shut-down) in the female non-responders

This trend for a "compensatory effect" on non-exercise physical activity (NEPA) in male and its absence is in female non-responders becomes even more obvious in Figure 2.

Figure 3: Rel. energy balance in male and female weight loss responders and non-responders in month 10 - data calculated as (total intake / total expenditure - 1)*100 based on data from Herrmann et al. (2015).

If we go back to my initial comment on caloric deficits and do the math that's required to calculate the relative difference between the total reported energy intake and the estimated energy expenditure, we get an excellent explanation for the fact that the male "non-responders" don't lose weight: They simply weren't in caloric deficit (see Figure 3); and that - and this is actually the most interesting finding - not because they ate more (the reported energy intake didn't change much), but because they moved less in their everyday lives!
References:

• Donnelly, Joseph E., et al. "Effects of a 16-month randomized controlled exercise trial on body weight and composition in young, overweight men and women: the Midwest Exercise Trial." Archives of Internal Medicine 163.11 (2003): 1343-1350.
• Donnelly, Joseph E., et al. "A randomized, controlled, supervised, exercise trial in young overweight men and women: the Midwest Exercise Trial II (MET2)." Contemporary clinical trials 33.4 (2012): 804-810.
• Herrmann, Stephen D., et al. "Energy intake, nonexercise physical activity, and weight loss in responders and nonresponders: The Midwest Exercise Trial 2." Obesity 23.8 (2015): 1539-1549.
• Klesges, Robert C., Linda H. Eck, and JoAnne W. Ray. "Who underreports dietary intake in a dietary recall? Evidence from the Second National Health and Nutrition Examination Survey." Journal of consulting and clinical psychology 63.3 (1995): 438.
• USDA, Physical Activity Guidelines Advisory Committee. "Physical activity guidelines for Americans." Washington, DC: US Department of Health and Human Services (2008): 15-34.
• Poppitt, S. D., et al. "Assessment of selective under-reporting of food intake by both obese and non-obese women in a metabolic facility." International journal of obesity and related metabolic disorders: journal of the International Association for the Study of Obesity 22.4 (1998): 303-311.
• Smith, Wayne T., Karen L. Webb, and Peter F. Heywood. "The implications of underreporting in dietary studies." Australian journal of public health 18.3 (1994): 311-314.

"Yes", GTE will increase your total energy expenditure (TEE) and decrease the ratio of glucose to fat you're burning at rest and during your workouts, but "NO", it won't make your abs appear w/out dietin'.

You will remember that I am not exactly a fan of green tea extracts. In spite of the fact that there are studies that suggest reductions of both thyroid hormone and testosterone production, when you consume high(er) amounts, their anti-inflammatory effects appear to help (obese) people lose weight. In view of the fact that a recent study suggests that this does not interfere with the adaptational response to exercise, like vitamin C + E, for example (learn more), they are thus still among the most promising over-the-counter anti-obesity agents.

With that being said, the increase in fatty oxidation is often mentioned when people try to explain why green tea helps (obese) people lose fat.
As a waredownloadsoft reader, however, you know that a mere increase in the use of fats over carbohydrates will not translate into practical weight loss. Against that background it is important to investigate the effects of green tea extracts on both, the respiratory exchange ratio (RER), which is the quotient of glucose / fat that's used to fuel your basal and exercise-induced metabolic demands, as well as their effects on your total energy expenditure.

Luckily, a recent study from the Charles Darwin University and the University of New South Wales in Australia includes both measures of changes in RER, resting energy expenditure (REE) and the total energy expenditure (REE + activity induced energy expenditure).

Figure 1: Diagrammatic representation of the study design. * indicates blood collection. Lactate assessed at *1, *2, *4, *6, *9; catecholamines at *1, *4, *5, and glycerol at *1–*9 (Gahreman. 2015).

But that's not all. The scientists also took a look at the potential mediators of increased energy expenditure and fatty acid use like catecholamines and they did so in normal-weight young women (age: 21.5 +/- 0.5 years; body mass: 65.7 +/- 1.8 kg; BMI: 24.3 +/- 0.4 kg/m²; maximal oxygen consumption; VO2max): 32.1 +/- 1.7 mL/kg/min) and not the usual subjects: Overweight or obese post-menopausal women or male and female patients with type II diabetes.
I have plotted the most relevant results of experiment the design of which is represented graphically in in Figure 1 for you in Figure 2. Please note that only the changes in VO2 which are indicative of an increased oxidation of fatty acids and the corresponding RER, the quotient of carbohdydrate oxidation and fat oxidation changed significantly.

Figure 2: Response at rest and during and after intermittent sprinting exercise in the green tea and placebo conditions; data expressed as relative differences between GTE and PLA (Gahreman. 2015).

It is thus all the more important to put the effects the consumption of three GTE (250 mg of camellia sinesis extract w/ 187.5 mg polyphenols, 125 mg EGCG) or placebo (cellulose) capsules the day before and one capsule 90 min before a 20-min intermittent sprinting exercise (ISE) cycling protocol consisting of

• a 5-min warm-up at 30 W, and 20 min of ISE on a Monark Ergomedic 839E ergometer at 110 RPM during the sprint phase and 40 RPM during the recovery phase (pedal resistance for the sprint phase was calculated as 60% of each participant’s maximal power output) 
• during which the subjects performed a total of sixty 8-s/12-s bouts totaling 8 min of sprinting and 12 min of easy pedaling recovery 

had on the metabolism of the fourteen untrained non-habitual coffee or green tea drinkers (< 2 cups per day) during the exercise, as well as before and after the ISE protocol:

Table 1: Mean power output, rating of perceived exertion, and lactate response to the sprinting and recovery components of the intermittent sprinting exercise for the placebo and green tea conditions (mean and SEM | Gahrmen. 2015).

• There was a significant increase in fat oxidation post-exercise compared to at rest in the placebo condition (p < 0.01). 
• After GTE ingestion, however, at rest and post-exercise, fat oxidation was significantly greater (p < 0.05) than that after placebo. 
• Plasma glycerol levels at rest and 15 min during post-exercise were significantly higher (p < 0.05) after GTE consumption compared to placebo. 
• There was no significant increase in total energy expenditure during or after exercise, though - that's in line with results Gregersen et al. (2009) generated in a study in normal-weight men, but different from some studies in normal-weight-to-moderately overweight men like Dullo et al. (1999) that report increases in TEE of ~2,8% over both, placebo and caffeine.
• Compared to placebo, plasma catecholamines increased significantly after GTE consumption and 20 min after ISE (p < 0.05 | not shown in Figure 2). 
• The effects are almost certainly not triggered by caffeine, because the capsules contained only 20mg of caffeine and previous studies have shown that only oral dose of more than 100 mg caffeine will elicit a significant increase in thermogenic response (Bracco. 1995; Dulloo. 1998 - suggest that 600-1,000mg/day is necessary for sign. increases in thermogenesis).
• It's also worth noting that there were no significant differences in mean power output, RPE, lactate levels, RPM (Table 1), and HR levels between the GTE and placebo trials. This leaves little doubt that the effects were not mediated by direct ergogenic effects (= higher exercise performance / effort) in response to the GTE supplementation.
• Lastly, it should be said that even though this was not tested in the study at hand, previous clinical trials like Bérubé-Parent et al. (2005) report identical thermogenic effects for low and high dosages of GTE. Thus, simply taking more GTE probably wouldn't have changed the results considerably.

Overall, the results of the study do thus support the ubiquitous claim that the ingestion of green tea extracts can significantly increase the fat oxidation at rest, during and after exercise when compared to placebo. In spite of the fact that the conclusion of the abstract to the study at hand makes it appear as if that was a practically highly relevant finding, there's one question neither the abstract nor the full-text of the paper actually address: Do the increase in fatty oxidation and the (albeit non-significant) increases in total and resting energy expenditure mean that consuming green tea extracts is going to help young, healthy, non-overweight women like the subjects in the study at hand lose body fat?
References:

• Bérubé-Parent, Sonia, et al. "Effects of encapsulated green tea and Guarana extracts containing a mixture of epigallocatechin-3-gallate and caffeine on 24 h energy expenditure and fat oxidation in men." British Journal of Nutrition 94.03 (2005): 432-436.
• Bracco, David, et al. "Effects of caffeine on energy metabolism, heart rate, and methylxanthine metabolism in lean and obese women." American Journal of Physiology-Endocrinology and Metabolism 269.4 (1995): E671-E678.
• Dulloo, A. G., et al. "Normal caffeine consumption: influence on thermogenesis and daily energy expenditure in lean and postobese human volunteers." The American journal of clinical nutrition 49.1 (1989): 44-50.
• Dulloo, Abdul G., et al. "Efficacy of a green tea extract rich in catechin polyphenols and caffeine in increasing 24-h energy expenditure and fat oxidation in humans." The American journal of clinical nutrition 70.6 (1999): 1040-1045.
• Gahreman, Daniel, et al. "Green Tea, Intermittent Sprinting Exercise, and Fat Oxidation." Nutrients 7.7 (2015): 5646-5663.
• Gregersen, Nikolaj T., et al. "Effect of moderate intakes of different tea catechins and caffeine on acute measures of energy metabolism under sedentary conditions." British journal of nutrition 102.08 (2009): 1187-1194.
• Hill, Alison M., et al. "Can EGCG reduce abdominal fat in obese subjects?." Journal of the American College of Nutrition 26.4 (2007): 396S-402S.
• Jówko, Ewa, et al. "The effect of green tea extract supplementation on exercise-induced oxidative stress parameters in male sprinters." European journal of nutrition (2014): 1-9.
• Phung, Olivia J., et al. "Effect of green tea catechins with or without caffeine on anthropometric measures: a systematic review and meta-analysis." The American journal of clinical nutrition 91.1 (2010): 73-81.
• Rains, Tia M., Sanjiv Agarwal, and Kevin C. Maki. "Antiobesity effects of green tea catechins: a mechanistic review." The Journal of nutritional biochemistry 22.1 (2011): 1-7.