Asian businesswoman online working at the night. Busy and exhausted of work overtime at home.

Mental strain can lead to changes in brain physiology that cause feelings of tiredness.Credit: Getty

Why thinking hard makes us feel tired

It’s not just in your head: a desire to curl up on the couch after a day spent toiling at the computer could be a physiological response to mentally taxing work, according to a study that links fatigue to changes in brain metabolism (A. Wiehler et al. Curr. Biol.; 2022).

The study found that participants who spent more than six hours working on a tedious and mentally demanding assignment had higher levels of glutamate — an important signalling molecule in the brain — than did those who had performed easier tasks. Too much glutamate can disrupt brain function, and a rest period could allow the brain to restore proper regulation of the molecule, the authors note.

The work is important in its effort to link cognitive fatigue with neurometabolism, but more research — potentially in non-human animals — will be needed to establish a causal link, says behavioural neuroscientist Carmen Sandi at the Swiss Federal Institute of Technology in Lausanne. “It’s very good to start looking into this aspect,” she says. “But for now this is an observation, which is a correlation.”

Healthier foods are better for the planet, huge study finds

Healthier foods tend to be more environmentally friendly than those with low nutritional value, finds an analysis of more than 57,000 food items sold in the United Kingdom and Ireland (M. Clark et al. Proc. Natl Acad. Sci. USA 119, e2120584119; 2022).

The mammoth study is among the first to estimate the environmental impact of products made using multiple ingredients, rather than looking at individual commodities. Researchers used an algorithm to estimate how much of each ingredient was in thousands of products sold in major supermarket chains. They then gave items an environmental-impact score out of 100 — with 100 being the worst — by combining the impacts of the ingredients in 100 grams of each product. They considered several factors, including greenhouse-gas emissions and land use.

Food for thought: Scatter plot comparing the environmental-impact and nutrition impact for a range of foods.

Source: M. Clark et al. Proc. Natl Acad. Sci. USA 119, e2120584119 (2022).

Comparing environmental-impact score with nutritional information revealed that healthier foods tended to have low environmental impacts. There were some notable exceptions: both nuts and seafood have a good nutrition score but relatively high environmental impacts, for example.

The information could help consumers to understand how items compare in terms of both nutrition and sustainability, the researchers say.

Health workers perform a Covid-19 swab test on a resident in the Xuhui district of Shanghai, China

A health-care worker swabs a person in Shanghai, China, for SARS-CoV-2.Credit: Qilai Shen/Bloomberg via Getty

COVID rebound is common without antiviral therapy

In some people taking the antiviral drug Paxlovid for COVID-19, symptoms and detectable virus levels resurge days after vanishing. Now, studies suggest it is common for SARS-CoV-2 to return in untreated cases, and hint that the virus makes a fiercer comeback in people who take Paxlovid.

Jonathan Li, a physician-scientist at Brigham and Women’s Hospital in Boston, Massachusetts, and his team analysed data from hundreds of people who received a placebo in a trial of COVID-19 drugs (R. Deo et al. Preprint at medRxiv; 2022). More than one-quarter of participants infected with SARS-CoV-2 reported that symptoms rebounded; 1 in 8 saw the virus return to high levels.

In a second study (M. Charness et al. Preprint at Research Square; 2022), infectious-disease physician and vaccine scientist Kathryn Stephenson at Beth Israel Deaconess Medical Center in Boston and her team followed 11 people who took Paxlovid for COVID-19 and 25 who did not. More than one-quarter of the Paxlovid recipients rebounded, in terms of SARS‑CoV-2 levels, compared with just one of the untreated people. Moreover, people with Paxlovid rebound had high levels of virus for several days. Neither study has been peer reviewed.