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In my experience of memory retention, it seems that, at the time this occurs, the brain – consciously or unconsciously – places a memory into a category with a rating of importance varying from "retention essential" to "almost discardable". Memories in the top category can be retained almost indefinitely, while those in the lowest, although …

If thinking hard doesn't cause the brain to consume more energy, it doesn't make evolutionary sense that stress hormone levels rise when we need to concentrate. It is well known that elite chess players lose weight during tournaments . Even though the brain is only responsible for a small percentage of the extra calories burned, …

Jeffrey Lapides has discovered microbes in the brain seemingly implicated in Alzheimer's disease, an unexpected finding. Will this, and similar discoveries, signal a rethink about neurological and mental illnesses? Suspicions must arise that, overall, this may not be a benign microbiome, especially if pathogens are entering the brain. We have medicines that act on some …

Meat is the most "ultra-processed food". It is ultimately plants that have been chewed, digested, broken down, chemically altered, restructured as animal flesh, slaughtered, skinned, butchered and processed even further. Any factory processing of plant-based foods pales in comparison ( 24 February, p 21 ).

Dismissing concerns over ultra-processed food isn't so simple. We can't rely on cravings and appetite for health if we eat stuff that tells our body, by flavour and texture, that it has one nutritional profile while delivering an utterly different one. And what about the removal of essential and beneficial micronutrients when, for example, maize …

James Dinneen's review of A City On Mars was interesting. As its authors say, there has been, to our knowledge, no documented human sexual activity in space. However, given our species' predilections, I would be surprised if, with the person-hours racked up on past and present space stations, not to mention lunar and other orbital …

To find evidence to back string theory, Joseph Conlon suggests looking for certain primordial gravitational waves from very early in the universe. Presumably, these would be stretched as the universe expands, just as light waves are. Radiation we see as the cosmic microwave background was stretched many, many times from its original form . Gravitational …

Michael Paine wants to refer to extreme weather-related events as "climate disasters", rather than natural ones. Such disasters have never been natural, whether related to climate or geology ( Letters, 24 February ). That idea has been widely accepted in disasters research for at least 50 years. Disasters that emerge from a (natural) hazard event …

You discuss manipulating animal genes to bolster meat production to allow people to keep eating what they are used to, noting that meat is a big factor in two of the greatest problems we face: global warming and biodiversity loss. We face a third major problem: our wilful failure to acknowledge the biological fact that …

The only use of quantum computers is to absorb vast amounts of research money and effort for no practical benefit whatsoever. Can I have my $5 million now please( 9 March, p 10 )?