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The obesity challenge to the NHS looms large. The costs of treating related health conditions, such as type 2 diabetes, are rising relentlessly. Policymakers are anxious to find answers. Prevention is the goal, but what sort of nudge is most effective to get people to look after themselves better?

A new study suggests that the fear of losing something is a better incentive than the prospect of gaining a benefit.

Health and Social Care Secretary Matt Hancock is speaking on Wednesday at a conference convened by the global insurer Vitality which will unveil new research on incentives to get healthier.

The work has not been formally peer-reviewed but the intention is to do so in preparation for publication in a medical journal. At this stage it does make interesting reading.

More than 400,000 people in the UK, USA and South Africa on rewards schemes run by the company were tracked over two years. Typically treats such as cinema tickets or coffee shop vouchers are offered to insurance customers who make regular trips to the gym verified by swiping membership cards.

This study attempted to measure what happened when, on top of those incentives, customers were given an Apple watch to wear with its built-in exercise monitoring capability. About 100,000 of them took the watch offer. Customers paid a minimal amount for the watch and no more after that, so long as they took regular exercise.

Researchers from the organisation RAND Europe compared the Apple watch data - which records actual physical activity - with the gym swipe data - which only tells you if someone has visited the gym, not what exercises they have done in it.

Based on an assumption that gym-goers did do some vigorous exercise during their visits, the data appeared to show a notable increase in activity levels among the smartwatch users.

It would appear, according to the researchers, that the fear of having to pay more for the watch was a good incentive to boost activity and was more effective than the traditional rewards system incentivising gains.

Of course there are caveats. The data was from those who had individual or corporate insurance policies and so with a bias to middle-income groups. They may have had a greater inclination to take exercise than those without insurance policies. But the report's authors argue that the customer base included a wide range of ages and body mass index readings.

It's not a watertight piece of scientific research, but the scale of the group covered, at more than 400,000, is such that policymakers will take a close interest. The economist Richard Thaler will be at the conference. He first coined the expression "nudge theory" which means helping people make decisions in their best interest. The preliminary conclusions of those involved in the study is that behavioural programmes and incentives make a difference.

So what does this mean for the NHS? It's hardly credible to think that smartwatches will be handed out at GP surgeries. But there are incentive schemes in the NHS, including offers of shopping vouchers by Greater Manchester Health and Social Care to pregnant women who give up smoking.

The idea that people are incentivised by being given something for free as long as they keep up an exercise programme, and which they will have to pay for if they fall short, will not go unnoticed as talk of the obesity crisis continues.