A recent analysis, pooling together 57 studies involving 900,000 participants from four continents, confirmed “once and for all... obesity shortens lifespan”.

Obesity, according to World Health Organisation criteria, is the condition in males with over 25 per cent body fat and women with over 35 per cent body fat.

To measure body fat, you just have to sit inside the BodPod, a human-size, computerised egg-shaped pod that uses the Archimedean ‘Eureka’ principle— how much air you displace.

After I wrote recently about how the pod is big enough for a sumo wrestler in The Straits Times, a reader posed this question: These wrestlers look obese but are highly conditioned athletes. Can obese be healthy?

Beginning their careers in late adolescence, these professional sportsmen build up their muscles with intensive exercise. Their body size—fat mass included—is bulked up through an extraordinarily high-calorie diet of 5,500 calories in two huge meals a day. By contrast, the average Japanese diet has only half the calories.

Sumo wrestlers train for three hours before breakfast, burning reportedly 1,400 calories. This is followed by a huge breakfast consisting of a special stew that is super-rich in protein, usually chicken, fish, tofu, beef and vegetables. To maximise weight gain, afternoons are spent resting.

On average, today’s professional sumo wrestler is 177cm in height and 114.5kg in weight. Unlike in Western wrestling, there are no weight categories in sumo wrestling, so a bigger bulk offers an edge that could be crucial in matches, which may last only 10 to 15 seconds each time. One is instantly defeated when any part of one’s body crosses the ring boundary.

The initial impact of a push to force one’s opponent, especially a heavier one, out of the ring can often be determinative. While punching, choking and kicking the upper body are not allowed, thrusting blows are.

The larger one’s lean muscle mass, the greater the momentum of one’s thrust. And the greater one’s total body mass, the larger one’s inertia, so it will be harder for the opponent to lift one out of the ring. Thus, training aims to build up muscle bulk and power - though the diet transforms them into chubby mountains.

A 2000 study showed that these wrestlers have muscles that are 18-35 per cent larger than that of non-wrestlers. These bigger muscles also burn up more energy, even at rest. A 2007 study by Japanese researchers showed that, because their muscles were metabolically far more active at rest, sumo wrestlers expended, on average, 2,300 calories at rest a day against 1,500 calories in non-wrestlers at rest.

Thus, when their strenuous training is taken into account, sumo wrestlers may well be taking in only about 800 calories more than non-wrestlers do every day. But that would still pack on the weight, of course.

Calories apart, a major health concern should be over their hearts. After all, a lot of obesity-related mortality comes from heart disease.

In all of us, the left side of the heart is always thicker than the right because it works harder to pump blood throughout the body compared to the latter, which only pumps blood through the lungs.

Research shows that the left side of the heart of sumo wrestlers—like most top athletes—is more muscular than most people’s.

Unlike other elite athletes, though, the cavities of their left hearts are very capacious as well. This condition afflicts the flabby hearts of the very obese too. Unlike the latter though, sumo wrestlers have normal heart functions.

This unusual combination of a very capacious left side of the heart (seen in the very obese) and normal heart function (which the very obese do not have) because of over-developed left heart muscles (seen in top athletes) was first reported in the American Journal of Cardiology in 2003.

So while they may be overweight, they are not obese and are, in fact, healthy athletes? This would generally be true of the top-ranked sumo wrestlers.

Sumo wrestlers fall into six divisions, not by weight but by skill level. A 1999 study showed that those in the top two divisions had significantly less fat compared to those in the lower divisions. The former had, on average, just 23.5 per cent body fat, so they were not obese at all. In one wrestler, it was even as low as 10.3 per cent.

Overall, though, based on body fat ratios, 55 per cent of sumo wrestlers were in fact obese.

Unsurprisingly, then, a 2003 study published in the British Journal of Sports Medicine reported a far higher prevalence, on average, among sumo wrestlers than the general population of Syndrome X—the combination of high levels of blood lipids, diabetes and hypertension that raises the risk of heart problems.

The study also found their average life expectancy to be 10 years shorter than that of Japan’s general population, with very high mortality rates from the ages of 35 to 74. (Most sumo wrestlers retire before their late 30s.) A study published in Prevention in 2006 put that difference even higher—at 20 years.

In retirement, their strenuous training schedule and extreme diet are both set aside, so their bodies drop in size. What impact this has on their health is not really known. In other elite athletes, retirement sees their left heart muscles lose their increased bulk over time. But since their left heart capacity was normal all along, their heart function is not compromised.

In retired sumo wrestlers, the same left heart muscle mass reduction presumably also ensues. But what about their extra-capacious left hearts? It is not known how major a role this plays in their higher mortality rates after 35.

Overall, however, it seems likely that no one gets away scot-free after subjecting the body to extremes for decades. (By Andy Ho in Singapore/ The Straits Times)