Hello Nature readers,
Today we learn how a junk-food splurge can change your brain activity, hear from US postdocs and PhD students hit hard by Trump’s crackdown on science and explore what molecular ‘clocks’ can tell you about your health.

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Junk food binges can alter brain activity

Stints on a high-calorie diet can lead to changes in the brain’s response to appetite-suppressing insulin, even without weight gain. In a trial, 18 healthy male volunteers went on a high-sugar, high-fat diet for five days. After that, scans taken after participants used an insulin nasal spray revealed that activity in three brain regions involved in responses to dietary changes and rewards was higher in these men than in those who ate their normal diet. This brain-activity pattern is similar to that seen in people with obesity or insulin resistance.

US PhDs and postdocs hit by funding freeze

Repeated threats to US science funding have left early-career scientists, who are most reliant on federal grants for their income, fearing for their futures. In response, some are looking for other jobs — even if that means leaving science. “I love my research, but I cannot afford to lose a pay cheque,” says neuroscience postdoc Haroon Popal, whose grant from the National Institutes of Health is threatened by a clampdown on funding that falls under diversity, equity and inclusion. Others who had applied to research positions in the United States are now looking elsewhere.

Ancient walkers might have pulled carts

Gently curving tracks preserved in New Mexico could be evidence of one of the earliest-known uses of transport technology: handcarts without wheels, called travois. The tracks were found alongside footprints that the same team earlier revealed could be around 22,000 years old — if so, they are the oldest evidence of human settlement in the Americas, setting the date thousands of years earlier than other timelines. Researchers built their own travois and dragged them through sand to reproduce a pattern that they say might indicate “adults pulled the simple, probably improvised travois, while a group of children tagged along to the side and behind”.

The trouble with biological age

In the world of gerontology — the study of ageing — money is pouring into the development of tests that measure our ‘biological age’. Many such tests focus on biomarkers such as chemical marks on DNA, known as methylation. Biomarkers could prove useful as part of efforts to develop therapies that could stall the negative effects of ageing. But often, results that use them are presented without mention of the uncertainties that plague the field. Even among gerontologists, there’s little consensus as to what ‘biological age’ actually means.

The racial injustice of COVID-19 laid bare

The COVID-19 pandemic and the US government’s promises to tackle racial injustice after the murder of George Floyd in 2020 seem long forgotten now in Chicago, sociologist Claire Laurier Decoteau argues in her new book. In Emergency, she methodically dissects COVID-19’s impact on marginalized groups in the city and exposes the failure of short-term interventions to address systemic problems. “Emergency is a stark reminder that the deep divisions made visible by COVID-19 are still present in our society,” writes sexual-health physician Annabel Sowemimo in her review.

How to make concrete more sustainable

The production of concrete — particularly cement —accounts for 7% of all anthropogenic CO₂ emissions. The construction industry can become more sustainable by using recycled materials, renewable energy sources and greener cement-production methods, says a group of civil engineers and materials scientists. The journey to net-zero concrete can be made step-by-step, starting with more-efficient production methods and eventually with the use of concrete engineered to sequester CO₂. But, the industry must lay the groundwork now if we’re ever going to get there, the group argues.