Not true, unlike many other mammals humans can survive a long time without food largely because the liver regulates metabolism in response to calorie intake. The thyroid hormone T3 is the main regulator aka “gas pedal” of cellular metabolism, and conversion of inactive T4 into T3 is done mostly by the liver. Conversion is halted when fasting or severely restricting calories… much to the frustration of dieters. This is why mainstream advice is to not lose weight too fast.
Indeed, and there are a few other mammals for which there are mechanisms to reduce ATP generation, reduce VO2, etc., and some which do it much better than humans do. Emperor Penguins, for example.
Did you read the study?
The study is about prolonged fasting, up to 58 hours.
They observed that metabolism
Increased during fasting.
> During human fasting, metabolic markers, including butyrates, carnitines, and branched-chain amino acids, are upregulated for energy substitution through gluconeogenesis and use of stored lipids.
You are misunderstanding what that means… it is an up regulation of catabolism- breaking down other molecules and tissues for survival during fasting. Overall metabolic rate is drastically reduced during prolonged fasting, which is good, it preserves nutrients for survival.
58 hours is not a long fast in terms of total adaptability to fasting- humans can fast much longer, sometimes for months, and the biggest down regulation of metabolism usually happens later than this.
Catabolism is an integral component of metabolism. Both anabolic (growth) and catabolic (breakdown) processes constitute the entirety of metabolic activities.
The study that I linked examines a fasting period of 58 hours. Fasting for 58 hours is considered a form of prolonged fasting. Prolonged fasting refers to an extended period without consuming food, typically lasting more than 24 hours.
Yes, every chemical reaction in a living system is part of metabolism, but in common speech when we talk about increasing metabolism we are specifically talking about increasing the metabolic rate- e.g. calories burned to generate ATP per an amount of biomass. What is happening in that paper is the opposite of a "metabolic increase" in that context. You are trying to correct people by citing a paper that supports what they were saying, and not what you are saying.
I am an academic PI whose lab studies metabolism... however I use common speech when talking online so people know what I mean, I would only use more specific terminology when talking with other metabolism researchers. In many cases the common terms and academic jargon are strictly at odds, but even a researcher wouldn't say "increases metabolism" when talking about fasting adaptations, they would talk about increased metabolic flux through specific pathways and/or changes in metabolite concentrations.
The 58 hours is another example of the same- it isn't long enough to observe many of the adaptations to fasting, especially the pronounced fall in T3 and resulting decrease in metabolic rate. The fact that 58 hours would be called "prolonged" in a journal article is not relevant to the point I was making.
Whether "burning calories" is used for nutrition or for "burning fat," both terms are common in everyday speech. The rate of "burning" increases during fasting. I have linked one study published in Nature.
Let's go over some excerpts that will answer your questions.
* Metabolism increases during fasting.
> Thus diverse, pronounced metabolite increases result from greatly activated catabolism and anabolism stimulated by fasting.
* The hormone t3 produced by the thyroid is used to control metabolism. In this study they instead looked at markers for actual metabolism.
> We performed non-targeted, accurate semiquantitative metabolomic analysis of human whole blood, plasma, and red blood cells during 34–58 hr fasting of four volunteers. During this period, 44 of ~130 metabolites increased 1.5~60-fold. Consistently fourteen were previously reported. However, we identified another 30 elevated metabolites, implicating hitherto unrecognized metabolic mechanisms induced by fasting.
* The study also mentioned prolonged fasting. Which is earlier stated longer then 24 hours.
> Quantification of blood metabolites from 4 volunteers during prolonged fasting. (a) Experimental procedures employed to study metabolomic changes during human fasting for 58 hr.
Jesus people, please stop with the unequivocal statements about equivocal/nuanced/complex things.