There are a range of mechanisms associated with the health costs of becoming fat, some of which are much more iron-clad than others when it comes to the supporting evidence. On the leading edge, there is the comparative lack of exercise usually associated with gaining weight. It is also probably the case that increased levels of methionine intake that come with a larger calorie intake lead to an unfavorable adjustment of metabolism. Once you have excess visceral fat tissue, the inner fat tissue that accumulates around your abdominal organs, this acts to further alter metabolism in harmful ways - it is much more active in this respect than other fat tissue in the body. It also leads to increased levels of chronic inflammation, and is associated with higher mortality and a raised risk of suffering most of the common fatal age-related conditions.
This visceral fat is so damaging to health that you can pick out its effects just by looking at measures of body shape that preferentially capture visceral fat over other aspects of weight gain. Body mass index is useful but a little too insensitive to weight distribution, for example, which is why researchers tend to come up with other proposed categorization systems for epidemiological studies such as the body shape index noted last month. But even very simple approaches such as measuring waist circumference can distinguish the impact of visceral fat on health in large studies:
Having a big belly has consequences beyond trouble squeezing into your pants. It's detrimental to your health, even if you have a healthy body mass index (BMI). Men and women with large waist circumferences were more likely to die younger, and were more likely to die from illnesses such as heart disease, respiratory problems, and cancer after accounting for body mass index, smoking, alcohol use and physical activity.
The researchers pooled data from 11 different cohort studies, including more than 600,000 people from around the world. They found that men with waists 43 inches or greater in circumference had a 50 percent higher mortality risk than men with waists less than 35 inches, and this translated to about a three-year lower life expectancy after age 40. Women with a waist circumference of 37 inches or greater had about an 80 percent higher mortality risk than women with a waist circumference of 27 inches or less, and this translated to about a five-year lower life expectancy after age 40.
Importantly, risk increased in a linear fashion such that for every 2 inches of greater circumference, mortality risk went up about 7 percent in men and about 9 percent in women. Thus, there was not one natural "cutpoint" for waist circumference that could be used in the clinic, as risk increased across the spectrum of circumferences. Another key finding was that elevated mortality risk with increasing waist circumference was observed at all levels of BMI, even among people who had normal BMI levels.
All in all getting fat seems like a poor plan and staying fat a worse one. Medical science may well advance rapidly enough to rescue you from the consequences at some point later in your life, and almost certainly so if you are presently young, but why roll the dice and endure those consequences at all if you don't have to?