Hot News About Cold Tea: Cold Maté Tea Increases Energy Expenditure 2x More than Hot Maté | No Strings Attached?

I don't want to spoil your beloved maté tea for you, but if you expect it to shed slabs of body fat off your frame, this belief may well spoil your fat loss efforts for you ;-)

I've previously addressed the potential benefits of cold-brewing your coffee and the effect of water temperature on the extraction of bioactive substances from coffee in my "Coffee 101" (Moussa 2017). For (caffeinated) tea, I haven't done this yet... what I haven't done for either of the two is to address the effect of the beverages' temperature at consumption.

In their latest RCT, Maufrais et al. (2018) did just that... more specifically, they "compare[d] the cardiovascular, metabolic and cutaneous responses to the ingestion of [Yerba Mate] at cold or hot temperature in healthy young subjects."
Just to ensure you see the difference, here: We're talking about the serving temperature, not the brewing (and for coffee, roasting) temperature, which I previously analyzed for coffee and of which the contemporarily available evidence says that it will - just like an increased water/leaf ratio, by the way - promote the rate of extraction of selected soluble constituents (rule of thumb: higher brewing temperature = highest anti-oxidant content + caffeine), i.e. the stuff that could cause the previously cited effects on the heart, metabolism and skin Maufrais et al. investigated in their latest study (Astil 2001).

Figure 1: You don't need the review by Astil et al. to know that the brewing time will significantly effect the ratio of caffeine to bioactive catechines... you can actually feel the difference, but that's something for another study ;-)

Speaking of Maufrais' study, ... the researchers included twenty-three healthy subjects (twelve men and eleven women) with a mean age of 24 ± 1 years and body mass index (BMI) 22.5 ± 0.5 kg/m². Overweight individuals, as well as those with a daily exercise workload exceeding 60 min per day were excluded.
The same goes for subjects who had any diseases or were taking any medication affecting cardiovascular or autonomic regulation. With respect to the actual study protocol, the scientists highlight the following aspects in the methodology section of their paper:

• Physical activity was limited, diet was "standardized" (subjects reported to the lab in the AM fasting after having had their habitual dinner, no caffeine or alcohol 24h prior was allowed) and the bladder was emptied (don't laugh, with body impedance analysis (BIA) to 'measure' body composition that's relevant).
• Every subject attended two separate experimental sessions (each session separated at least by 2 days) according to a randomized crossover study. Randomization was performed using a random sequence generator where the session order was determined for 23 subjects before the study started. Women were only tested during the follicular phase of their menstrual cycle. 

After the subjects had been hooked up to the measurement devices and had reached cardiovascular stability (usually ~10–15 min), a stable baseline recording was made for 30 min.

"Then, the subjects ingested over 5 min, either 500 mL of cold tea (~3°C) or hot tea (~55°C) with a dose of 3.4 g of instant unsweetened [to avoid interference due to sugar] Yerba Mate (Wisdom Natural Brands®, 1203 West San Pedro Street Gilbert, Arizona 85233) according to recommendations, containing 99 mg of caffeine (2,900 mg/100 g) [...] Monitoring continued for another 90 min post-drink ingestion." (Maufrais 2018)

In that, the scientists chose the instant tea over tea bags to ensure that the exact same dose of tea was dissolved in water. Thus, Maufrais et al. could be relatively certain that any inter-group differences they measured over the 90 min post-drink ingestion would be solely due to the temperature of the drinks - and there were differences:

• Figure 2: The reduction in heart rate w/ cold vs. hot mate achieved sign. for the whole study period (Maufrais 2018)

  compared to hot tea, cold tea induced a decrease in heart rate (cold tea: −5 ± 1 beats/min; hot tea: −1 ± 1 beats/min, p < 0.05), double product, skin blood flow, and hand temperature 
• compared to hot tea, cold tea also increased the baroreflex sensitivity, fat oxidation and energy expenditure (cold tea: +8.3%; hot tea: +3.7%, p < 0.05)
• in spite of the metabolic effects, the researchers did not observe consistent differences of tea temperature on cardiac output work and mean blood pressure

What should be mentioned, though, is that the blood pressure and total peripheral resistance increased much earlier with the cold vs. hot drink.

Figure 3: The cold mate had surprisingly more pronounced vasoconstrictive effects (Maufrais 2018).

If you check out the data in Figure 3 you will see that this was partly due to a significant, treatment (=cold) specific effect on peripheral resistance and skin blood flow. An observation of which those of you with a basic understanding of the interaction between catecholamine release, vasoconstriction (learn more, here), and metabolism (Webber 1993) won't be surprised to see:

Figure 4: The subjects who drank the cold maté saw greater increases in energy expenditure and a higher contribution of fat (vs. glucose) to the subjects' energy supply (Maufrais 2018) - don't jump to conclusions, though!

A significant increase in energy expenditure was observed for both cold and hot maté, much in contrast to what I personally would have expected, though, this increase was significantly less pronounced in the (red) hot maté trial.
The respiratory quotient (RER, a measure of the ratio of te contribution of glucose and fat to the energetic demands of an individuum) even showed opposing reactions in the first 30 minutes after ingestion ...

• with the cold maté producing a non-significant decrease in RER, i.e. the ratio of glucose/fat that was used as fuel, and ...
• the hot maté triggering a short and transient increase in the RER and thus the ratio of the contribution of glucose vs. fat to the energetic demands of the test subjects.

I know what (some of) you may be thinking, now: "Wait, increased energy expenditure + increased contribution of fatty acids to fuel the subjects' energetic demands... that must mean fat loss, right?".

Doubled increase in energy expenditure, greater contribution of fat... great!? Not really...

Even though statistically significant increases in fat oxidation and energy expenditure sound great, their real-world relevance often is small and non-significant. Let's try to estimate if what we see here is relevant to your efforts to get beach-ready:

Table 1: Compounds identified in Yerba Mate leaves and some of their biological activities (from Heck 2007)

• the actual increases in energy expenditure were ~0.35 kj/min in the cold maté group and ~1.8 kg/min in the hot maté group 
• in kcal that's an extra energy expenditure of 0.08 kcal/min for the cold and 0.04 kcal/min for the hot maté group
• according to cocker, the subjects who drank the cold tea did thus burned an extra 7.2 kcals over the 90 minute period, the ones who received the cold drink burned the tremendous (irony alert) amount of 3.6 extra kcals over 90 minutes
• even if 100% of those extra-calories ended up as a deficit, it would take roughly 486 days and 972 days, or 1.33 and 2.6 years, respectively to drop a single pound of body fat if you consumed cold or hot maté on a daily basis (warning: rule of thumb)

I know the pervious calculation is just an estimation and I didn't include the RER at all, but in view of the absolute effect size, it would be a waste of time and efford to (a) go beyond the -3500kcal ~ 1lbs of body fat rule or thumb and/or to (b) consider the already totally overrated importance of the respiratory exchange ratio (RER) in this context... on a related note: in the long(er) run, i.e. over weeks and years, the RER makes absolutely no difference in terms of the amount of fat you lose.

References:

• Astill, Conrad, et al. "Factors affecting the caffeine and polyphenol contents of black and green tea infusions." Journal of agricultural and food chemistry 49.11 (2001): 5340-5347.
• Heck, Caleb I., and E. Gonzalez De Mejia. "Yerba Mate Tea (Ilex paraguariensis): a comprehensive review on chemistry, health implications, and technological considerations." Journal of food science 72.9 (2007).
• Maufrais, Claire et al. "Cardiovascular and Metabolic Responses to the Ingestion of Caffeinated Herbal Tea: Drink It Hot or Cold?" Front. Physiol. (2018), published online on April 06, 2018.
• Webber, J., and I. A. Macdonald. "Metabolic actions of catecholamines in man." Baillière's clinical endocrinology and metabolism 7.2 (1993): 393-413.