Obese people are automatically thought to be unhealthy, but a significant proportion display none of the usual health problems. Finding out why can help us understand obesity's links to our immune systems, writes Clare O'Connell
THE FESTIVE SEASON has no doubt given waistlines around the country a good stretch, with too much food and too much drink putting the bathroom scales under new pressure.
We already know these extra kilos are not good for us, but a new study has identified a particular risk. Mounting evidence tells us that for the vast majority of people, obesity is linked with changes to the immune system, and not in a good way.
Irish researchers have shown that most obese patients have fewer immune cells in the blood to ward off viruses and cancer. But surprisingly, they have also found that a small number of severely obese patients remain "metabolically healthy" with a well-functioning immune system despite the condition.
Their findings could ultimately point the way towards handling obese patients on the basis of their metabolic health, allowing more effective targeting of treatments.
Doctors recommend that our body mass index (BMI), a measure of weight in relation to height, ideally should not exceed 25.
The study was sparked by a patient whose BMI was a full 41 points over the cut-off for obesity, explains consultant endocrinologist Prof Donal O'Shea, director of the weight-management clinic at St Columcille's Hospital in Loughlinstown, Dublin.
"He was in his late 40s with a BMI of 71, so you would expect him to have the full house of problems, but he had a nice blood pressure, blood sugar and cholesterol levels," Prof O'Shea recalls. "Then the next patient in to the clinic was still obese, with a BMI of around 45, but was younger and and had all the problems you would associate with obesity."
The unusual blood test results prompted some head scratching: "We asked what is protecting this guy? What is going on there?" says Prof O'Shea. "So we began to look in detail at our patient group and between 10 and 20 per cent of our patients would be protected like that, they would have severe obesity and yet none of the metabolic complications that go with it."
There were some clues that provided a lead though. Obesity is usually associated with low-grade chronic inflammation, and obese people tend to be at high risk of cancer and infection and often require longer antibiotic treatment than lean people, explains Dr Lydia Lynch, a post-doc at the Research and Education Centre in St Vincent's University Hospital. "It suggests that the immune system is involved," she says.
So the researchers compared circulating immune-system cells in blood samples from 52 metabolically healthy and unhealthy obese people and from 11 lean people. They made a startling discovery, published this month in the journal Obesity.
"One of the main cells involved in killing cancer and viruses is the natural killer cell," explains Dr Lynch. "And we found a huge decrease in natural killer cells in the metabolically unhealthy obese person compared to the lean and the metabolically healthy."
Not only that, but the few natural killer cells that were present in metabolically unhealthy obese people tended not to work very well, and their numbers of infection and tumour-busting T-cells were down too, explains Dr Lynch.
In a parallel study, samples of "visceral" or belly fat from the metabolically unhealthy obese patients looked inflamed under the microscope, and individual fat cells were swollen and linked with a poor ability to respond to insulin, which contributes to diabetes, she adds.
The findings highlight the active metabolic role of fat in the body beyond mere storage, says Prof O'Shea.
"If you were to have talked about fat function 10 years ago, people would have laughed you out of it, but the concept of fat function and fat failure is one that people are much more comfortable with now," he says. "And it's clear that in the metabolically healthy group, the fat is doing a much better job of dealing with the energy load."
In the longer term, Prof O'Shea believes that being able to identify the obese patients most at risk metabolically could help stratify and target treatment. And in the meantime, he hopes the emerging evidence will prompt people to realise that becoming obese can damage your immune system.
"I think if the medical profession starts viewing obesity as an immune-compromised state, they are much more likely to intervene than simply thinking of it as 'they are obese'. They are more likely to take it seriously," he says. "And if parents realise that kids putting on weight is immune-compromising them in our 'Coke and crisps' culture it might just make a few people have a different view of it, and that would be very positive."