The winter bug season has truly arrived, with cases of flu in England reported to be higher than usual last month – earlier than the previous four years, according to the UK Health Security Agency.
But can you rely on that adage, you need to ‘feed a cold, starve a fever’ to tackle flu or a cough?
In fact, there may be some truth in it, but not as you might think.
As John Tregoning, a professor in vaccine immunology at Imperial College London, explains: ‘Fevers and colds are the body’s response to an infection, so getting a high temperature or a runny nose are immune reactions to fight off the viruses or bacteria. Without these responses we’d be much sicker.’
It’s thought increased body temperature helps to clear viruses and bacteria by slowing their growth, so the immune system can kill them.
So while colds generally have milder symptoms than other viruses, such as flu (which does cause fever), it’s possible to get a temperature with a cold too.
It’s thought increased body temperature helps to clear viruses and bacteria by slowing their growth, so the immune system can kill them
‘A mild respiratory infection might just mean someone has a snotty nose, while if it’s moderately severe it can get into your lungs and cause a cough – but if it’s very severe you might get a fever because your body is working really hard to fight it off,’ says Professor Tregoning.
Symptoms vary depending on factors such as genetics and having a pre-existing respiratory condition such as asthma – ‘as your lungs are working that much harder to fight off viral or bacterial infections’, he explains.
As for whether you should deliberately ‘starve’ a fever, this may reflect what’s happening in the body anyway, as you may be less likely to want to eat.
Research by Imperial College London in 2024 found that mice infected with the cold virus lost weight. This is thought to be due to the immune system triggering the release of interleukin-1 alpha, a protein that sends messages to the brain that we don’t need as much food – in turn, levels of the hormone leptin are increased, which suppresses appetite.
‘It’s probably a similar pathway in humans,’ says Professor Tregoning, who worked on the study.
As to the evolutionary logic driving this, a study by the University of Bonn in Germany suggested reduced food intake means the body is forced to use up fat stores for energy – this produces energy-rich molecules, called ketone bodies, which fuel immune cells.
The research, published in Nature in 2022, found that people with flu produced high levels of ketone bodies.
However, other experts say this appetite-reducing mechanism may not be helpful, and that you need to ensure you eat well.
‘Fever increases the body’s metabolism and energy requirements, so maintaining adequate nutrition is important,’ says Dr Praveen Molanguri, a consultant respiratory physician at Epsom Hospital and Spire St Anthony’s Hospital in Surrey.
‘Not eating enough makes the recovery longer. If you have a reduced appetite, then it’s best to eat small meals more frequently to maintain your food intake.’
Generally, whenever the immune system works harder, then more fuel actually helps, says Nathan Davies, a professor of biochemistry at University College London.
As a rough estimate, energy expenditure can increase by around 10 to 13 per cent for every one degree rise in body temperature, says Dr Molanguri.
So someone who normally burns around 1,500 calories a day at rest might burn an additional 150-200 calories if they have a temperature around 1C above normal throughout the day.
‘A mild cold without a significant fever is unlikely to cause a major increase in calorie requirements,’ says Dr Molanguri. ‘Also, people tend to be less physically active when unwell, so their overall daily calorie expenditure may not necessarily increase.’
John Tregoning is a professor in vaccine immunology at Imperial College London
The choice of what you eat may also have an impact, but not necessarily the traditional options.
For while research has shown that chicken soup and fresh ginger reduce viral infections, these have been lab studies, so the findings can’t be extrapolated to humans, says Professor Tregoning. ‘Other research found giving mice compounds found in broccoli meant that they recovered better from flu – but again, you can’t say people should eat more broccoli when they’re sick.’
Part of the 2022 study by the University of Bonn showed that a ketogenic diet (low in carbohydrates but high in fat) or adding ketone bodies to water boosted the function of immune cells in mice with Covid.
But carbs provide energy, which is important for the immune system, says Dr Molanguri.
You also need plenty of protein to help the body make antibodies which fight infection, he adds.
And drinking enough fluid is vital. When we have a fever, we sweat more – we also lose fluid by producing more mucus to flush out germs, says Professor Davies.
‘If your lungs have more mucus, there’s less surface area for oxygen to cross over, so you have to breathe faster to make sure you’re getting the same amount of oxygen – and when we breathe, fluid comes out of our lungs,’ explains Professor Tregoning.
