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

As 2025 draws to a close, it is traditional to look back at the year that was and forward to what may come next. Many of us will be considering New Year’s resolutions, such as getting fitter, eating better and boosting our immune system – but how do you know if your new habits are truly working?

To start with, “boosting” your immune system is a misnomer, since an overactive immune defence would be a bad thing, but as new research is revealing (see page 26), it is possible to assess our ability to fend off infection by measuring levels of certain immune cells. From this, your “immune grade” can reveal if you are fighting fit.

Tests alone aren’t very useful, though, if you don’t know what you are measuring. We are increasingly aware of the need to have a diverse gut microbiome, with numerous DIY faecal tests now for sale. The trouble is, no one agrees which microbes give you a high score. That should soon change, thanks to a study using the Zoe health app, which can now score the overall health of your microbiome from 0 to 1000 (see page 11).

That said, it is important not to be blinded by statistics. Body mass index (BMI), for example, is one of the most commonly used measures of health, but it is heavily flawed. It remains popular because it is a simple calculation of a person’s weight relative to their height, yet this fails to take into account whether a person’s weight is high because of excess fat, which can reflect poor health, or more bone and muscle mass. This is why researchers proposed a whole new definition of obesity earlier this year.

There are two things we can take from all of this. The first is that if you are trying to make a change in your life, be sure that you have the right numbers to measure that change. It’s no use pledging to get up early to exercise each day if you measure success by what time your alarm goes off. The second is that the science of what works is always changing, and you should aim to keep up with the best evidence available. Of course, if you’re reading this, you are already off to a good start.