The cited study is real and is correctly named. The authors examined the effects of temperature acclimation on brown adipose tissue, energy balance, and substrate metabolism in a prospective crossover study of four months duration, consisting of four consecutive one-month blocks of overnight temperature acclimation (24°C, then 19°C, then 24°C, then 27°C) in five healthy men housed in a temperature-controlled research facility. Sequential monthly acclimation modulated BAT reversibly, boosting and suppressing its abundance and activity in mild cold and warm conditions respectively, independent of seasonal fluctuations.

BAT acclimation did not alter cold-induced thermogenesis but was accompanied by diet-induced thermogenesis and postprandial insulin sensitivity enhancement, evident only after cold acclimation.

The reversibility is central and is omitted from the post. All BAT parameters increased after one month of mild cold acclimation at 19°C, decreased to nearly baseline level after the thermoneutral month at 24°C, and BAT was nearly completely muted at the end of the one-month mild warm exposure in the final month.

The specific "42% / 10%" figures trace to press communication around the study rather than to a headline sentence in the abstract. After one month of exposure to mild cold temperatures, the five participants had a 42 percent increase in brown fat volume and a 10 percent increase in fat metabolic activity, and during the final warm month the results were completely reversed. The same figures are repeated in consumer coverage: a 2014 study published in Diabetes found men who slept in rooms cooled to 66 degrees for a month increased their levels of calorie-burning brown fat by 42 percent and their ability to metabolize fat by 10 percent.

On whether this translates into fat loss, the broader literature is far more cautious. A systematic review of 47 clinical trials of agents and cold exposure found significant heterogeneity in intervention duration and metrics, and observed that changes in energy expenditure do not correlate with major weight changes across interventions commonly known to stimulate thermogenesis; even though cold exposure appears to consistently activate BAT and induce thermogenesis, studies are small, and it appears to be an unlikely sustainable therapy to combat obesity. A meta-analysis of acute cold exposure found measurable but modest effects: compared with 24°C, energy expenditure increased after acute cold exposure at 16 to 19°C by a mean difference of about 188 kcal/day, with increases also in BAT volume and BAT activity.