Kick boxing twice a week, walking the dog two miles every day – Clint Witchalls thought he was healthy. Then a visit to the doctor revealed he was dangerously overweight...

I've never really thought of myself as fat, but at a recent fitness assessment I was told I'm "pre-obese". I was shocked. How can I be pre-obese when my thighs don't chafe and I wear a medium size T-shirt? I feel perfectly healthy. I do two kick boxing classes a week, I regularly take the dog for brisk two-mile walks and I swing a 16kg kettlebell weight in my tea breaks.

I keep seeing headlines about the obesity crisis, usually accompanied by a picture of a gargantuan man or woman. When I read on, however, I discover that most of the article is in fact about overweight people – people like me – who don't look a bit like the person in the picture.

Who gets to decide who is obese or overweight anyway? A quick Google search reveals that a 19th-century Belgian astronomer and mathematician, Adolphe Quetelet, gets to decide.

He's the man who invented the body mass index (BMI). BMI is your weight divided by the square of your height. The resulting number, usually ranging from 12 to 42, tells you whether you're underweight, healthy, overweight, obese or extremely obese. In my case, 95.9kg divided by my height, 1.86m, gives me a BMI of 27.7. This number puts me at the top end of the overweight category – knocking on obesity's door.

BMI is widely used, yet despite its popularity not everyone is convinced it's a useful tool. One criticism is that it doesn't look at body composition. BMI can't tell what proportion of your weight is muscle, bone, water or fat. It just works on total weight. Under BMI, rugby players and bodybuilders would be classed as overweight or obese because of their muscle mass. BMI also starts to unravel for very tall people and pregnant women.

"In modern society, the obsession is with weight," says Professor Jimmy Bell of the Medical Research Council's Clinical Sciences Centre at Hammersmith Hospital. "Weight is meaningless. You can be thin. You can have a [healthy] BMI of 20 but still have lots of fat in your liver and be pre- diabetic. That is the problem with BMI."

Things have moved on since Quetelet's era. Today, there are a number of systems for determining body composition. Some of them are expensive, such as a magnetic resonance imaging (MRI) scan – which is Professor Bell's specialism . Others are cheaper, such as bioelectric impedance analysis (BIA). I don't think I can convince my GP I need an MRI scan, so I opt for the BIA.

A BIA device looks like bathroom scales. When you stand on it a low-level electrical signal is sent through the footplates and the resistance to the current, as it flows through your body, gives a reading of your body composition. Unlike muscle, there is not much water in fat, so the current is impeded when it passes through fat. The BIA monitor I used, a Tanita InnerScan50, measures total body water percentage, bone mass, muscle mass and total fat percentage.

The machine tells me that I'm 25 per cent fat – which is not good. Men shouldn't be more than 20 per cent fat and women shouldn't be more than 30 per cent fat. It rated my physique a "2". The manual that comes with the machine tells me this means: "Obese: This person has a high body-fat percentage with a moderate muscle mass level." So now I've been upgraded from pre-obese to obese. I am Mr Creosote – only I still don't feel it.

But maybe I shouldn't worry about what my BMI or some jumped-up bathroom scale says. A recent study suggests that being overweight when you're 40 years or older (I'm 44) might provide some benefit. Researchers at Tohoku University followed 44,000 people aged between 40 and 79 over a 10-year period, and found overweight people had the longest life expectancy.

At age 40, overweight men could expect to live 40.5 more years and women 47 more years. This compared with 38.7 years for normal-sized men and 46.3 for normal-sized women. Slim people did a lot worse. Underweight men only get 33.8 more years and women 41.1 years.

Then there was the National Health and Nutrition Examination Survey of 5,440 people, which found that more than half of overweight people and nearly one third of obese adults are metabolically healthy; that is, they don't have high blood pressure, high blood lipids (fats), insulin resistance or other signs that they are en route to a heart attack or diabetes.

Although the study found that people with a normal BMI were more likely to have good metabolic health indicators (76.5 per cent), it showed that plenty of fat people are healthy. Maybe I fall into the half of overweight people who are actually healthy. But I need to find out for sure.

The next day, I headed to the Human Performance Centre at the South Bank University in London, for a full MOT. Pat Fox, a sports scientist, ran a number of tests. My waist-to-hip ratio proves normal. My blood pressure is normal, and my fasting blood glucose level (an indicator of insulin resistance) is normal. In fact, it's lower than the lowest end of the normal range. And my triglycerides – the fat in my blood – are low. I was just starting to feel a little bit smug when Pat gave me the bad news: my HDL cholesterol (good cholesterol) was very low whereas my LDL cholesterol (bad cholesterol) was high.

More tests followed. Steve Hunter, a lecturer in exercise physiology, did a skin-fold test. He grabbed bits of flabby skin at carefully measured locations on my body, pinched them with giant callipers and keyed the results into a computer. Although the skin-fold test looks like something from Victorian medicine, it was developed in the 1970s and is quite accurate at determining total body fat percentages. Unfortunately, the calculations put my body fat at 29 per cent – four percentage points higher than the BIA scales.

The final test was a fitness test called VOMax. It sounds like a fizzy drink, but it's more like a mild form of torture. You run or cycle with a mask on until you reach your maximum heart rate. The mask measures your oxygen use. The more oxygen your body is able to use, the fitter you are. "Not bad," Steve said, reading the computer screen. "You're fitter than the average person your age." I mentally totted up the tests that had given good results and the tests that had negative results. I decided that I was among the 50.3 per cent of overweight people with no serious health risks. Pat soon deflated my optimism.

"You're fit, but not healthy," he said. My body fat percentage and poor cholesterol ratio put me firmly in the "at risk" group. Pat prescribed lots more vigorous exercise to raise my HDL cholesterol, and a change of diet to lower the LDL.

"Just losing weight won't necessarily change your cholesterol level," he said. "You have to lower your saturated fat intake." The worrying thing is, if I'd taken only some of the tests: say, waist-to-hip ratio, blood pressure and aerobic fitness, I'd have looked a picture of health.

The following day, I called Professor Bell for a second opinion. "Your high LDL is something to worry about because it's a very strong predictor of coronary heart disease and atherosclerotic plaques," he said. "It's not so much that your LDL is high, which is bad, but that your HDL is so low." He says that if I want a thorough understanding of my fat, I should have an MRI scan.

It's all very well knowing about your body composition (from the BIA scales) or your significant cardiometabolic markers (like blood lipids), but I should really know where the fat resides to know how "at risk" I am. "Fat distribution is the game at the moment," he says. I may fall into the group that is known to doctors as "Tofi" (thin outside, fat inside) – putting me at much higher risk than someone who is "fat-fit", the professor explains. "To get a clean bill of health, you require more than the measures you've done."

More tests? I've had a BMI test, a BIA test, a skin-fold test, a VOMax fitness tests, cholesterol tests, a triglycerides test, a fasting glucose test, blood pressure tests, a waist-to-hip ratio test and lung function test, and still I don't have a conclusive answer about my health. Fat is a complex thing. There has been a lot of BMI bashing lately and I was ready to put the boot in too after I was declared pre-obese. I was sure I would be in the 50 per cent of overweight people who are in fact healthy, but I'm not. If it wasn't for my BMI rating, I wouldn't have started digging and discovered that I have a terrible cholesterol ratio. But a cholesterol reading, on its own, still doesn't tell me everything. It's just an early warning sign.

"There are no short cuts," Professor Bell says. "If you want to know how healthy you are, you have to have an [MRI] image, or you have to be fit. You don't need to have an image if you jog every day and have a healthy lifestyle."

And that's the thing. I could keep on digging, trying to discover just how unhealthy I am, or I could just get on with doing something about it.

Time to get my running shoes on.