In April last year, I found out that I was morbidly obese. While my Body Mass Index (BMI) was only slightly higher than normal, my percentage of body fat (PBF) was a shocking 29 percent. Anything above 25 percent for males is considered dangerously high. In addition, my blood pressure was borderline high and my triglycerides (a type of fat in the blood) was off the charts.

I was 86kg then. The doctor who saw me told me that I’m “morbidly obese” and recommended that I shed at least 10kg over the next year. I thought she was mad; I had not been below 80kg for nearly 20 years. So I told myself I would try to lose 5-6kg.

I have always had a self-image as a fit and healthy person. In school, I was a competitive runner; in my 20s and 30s, I took part in road races and triathlons. In the first two years of the pandemic, I hiked or ran at least twice a week. What I hadn’t realised was that my body’s metabolism had slowed and I was eating and drinking too much for my age (I turn 50 this year).

My initial reaction to the health shock was that I simply needed to step up the frequency (and intensity) of my exercise regime. Intuitively, I knew that I would need to cut down on my calorie intake. And as a behaviourist who used to be fit, I thought I had the requisite knowledge to lose weight easily.

Reality was a bit more complicated, and progress occurred more haphazardly than I had expected. In the process, I learned that some of the conventional wisdoms around weight loss were not only questionable, but may be counterproductive. I also had to (re-)learn a few behavioural insights for sustaining change. Perhaps most importantly, I found that nudges work better than shoves.

Nudge is a term popularised in Nudge: Improving Decisions About Health, Wealth, and Happiness, a book by Richard Thaler, an economist and Nobel Prize laureate, and Cass Sunstein, a legal scholar and best-selling author. It refers to an intervention which changes the “choice architecture” (the way choices are structured or presented to us) without relying on coercion or altering (financial) incentives significantly. Behavioural economists (like me) argue that nudges are often more effective than mandates and incentives in sustaining behavioural change. We also suggest that the best way to alter our behaviours is to counter one cognitive bias with another.

Consider changing defaults from opt-in to opt-out. This is often discussed in terms of organ donation. Countries with an opt-in system, such as the US, see much lower rates of participation than those with an opt-out system, such as Singapore. Changing defaults from opt-in to opt-out is a powerful nudge because it leverages our natural inertia to increase compliance or participation.

I’ll describe my weight-loss journey through five behavioural insights: the use of a commitment device; the need to counter our (natural) tendency to be overconfident with objective measures and external accountability; the recognition that small changes, accumulated over time to develop into habits, are probably more effective than major overhauls of one’s diet; the benefit of starting with achievable near-term goals; and the value of a social approach to individual change (because humans are inherently social creatures).

In particular, the BMI does not say anything about the distribution of a person’s weight: what percentage is from fat, muscle, or bone. So muscular people may have a BMI that puts them among the obese even though they have little fat. Conversely, someone who has little muscle may have a normal BMI, but still have too much body fat. As my physician pointed out, ageing might cause one to lose muscle and bone mass and gain fat around the waist, a change in body composition that would be concerning for health but might not be noticed if one focused on the BMI.