Sheena Meredith is a doctor, writer and healthcare communications consultant. The opinions in this article belong to the author.

(CNN) There are two major factors that influence life expectancy : smoking habits and weight. And they're heading in opposite directions.

These two trends are not unconnected. The average tobacco quitter gains 11 or 12 pounds over the long run . Although the benefits of not smoking are calculated to offset damage from the increased risk of obesity, the obesity tide is eroding some of the gains in life expectancy from reduced smoking.

Obviously, therefore, physicians are enthusiastically advising their heavier patients to lose weight , hoping for the same success as they have achieved with smoking, right? Well, no. In fact, even when doctors inform patients that their body mass index (BMI) is too high, few offer constructive guidance about how to lose weight, citing barriers such as lack of time, training and confidence.

Why is it that the public health approach applied so effectively to tobacco fails to translate to being overweight? Doctors have little hesitation about telling their smoking patients to quit, but a certain ambivalence seems to creep in when it comes to talking to patients about being overweight.

This is mirrored by official guidance. On smoking, the Surgeon General's advice to physicians is blunt: "Talk to your patients. Tell them that quitting smoking is the most important step they can take to improve their health. They will listen to you ."

In contrast, six pages of advice from the National Institutes of Health on talking to patients about weight loss describes obesity as a "sensitive topic," advises speaking "caringly," opening discussion "in a respectful way" and using the patient's preferred terms to describe being overweight.

Being overweight is less obviously a choice, and not a yes-no lifestyle issue where doctors can confidently say, "just stop." Moreover, official guidelines on diet and weight are notoriously inconsistent, and environmental influences on weight are far more widespread.

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Unlike smoking, a person's flab is very much a part of them. Saying "you're fat" comes to seem almost on a par with saying "you're ugly."

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Unlike the yoga-posturing, vegetable-munching Hillary Clinton, Trump doesn't exercise. He eats buckets of fried chicken. He knows he should lose weight but his lifestyle gets in the way. He says he's always been overweight.

In other words, he's like an awful lot of Americans. So perhaps Trump is doing us a favour, making it fair game to talk about his size as a way to open discussions about obesity without upsetting those of a more sensitive disposition.