New Year’s resolutions work better if you know what to measure

As 2025 comes to a close, it's traditional to look back on the past year and look forward to what might come next. Many of us will be contemplating New Year's resolutions like getting fit, eating better and strengthening our immune systems, but how do you know if your new habits are actually working?

To begin with, “boosting” your immune system is a misnomer, as having an overactive immune defense would be a bad thing, but as new research suggests (see page 26), we can assess our ability to resist infection by measuring the levels of certain immune cells. From this, your “immunity level” can indicate whether you are in good shape.

However, tests themselves are useless if you don't know what you're measuring. We are increasingly aware of the need to have a diverse gut microbiome, and there are numerous DIY fecal tests available now. The problem is that no one can agree on which microbes give you a high score. That's about to change, thanks to research using the Zoe Health app, which can now score the overall health of your microbiome from 0 to 1000 (see page 11).

It is important not to be blinded by statistics. Body mass index (BMI), for example, is one of the most commonly used health indicators, but it has serious shortcomings. It remains popular because it is a simple calculation of a person's weight relative to their height, but it does not take into account whether a person's weight is high because of excess fat, which can reflect poor health, or because of greater bone and muscle mass. This is why researchers have proposed completely new definition obesity earlier this year.

From all this we can take two things away. First, if you're trying to change your life, make sure you have the right numbers to measure those changes. It's no use promising to get up early and exercise every day if you measure success by what time your alarm goes off. Second, the science of what works is constantly changing, and you should strive to keep up with the best evidence available. Of course, if you're reading this, you're already off to a good start.