Healthy while overweight: A fat error

21. April 2015
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Overweight, yet blood pressure, cholesterol and blood sugar levels are normal – so everything is fine, right? Unfortunately not. More and more study results indicate that the concept of being a metabolically healthy obese person could soon be an obsolete one.

Looking at the worldwide picture about 39% of people are overweight and 13% are obese – this means more than 1.9 billion overweight and more than 600 million obese people in total. The number of people affected has in recent decades so rapidly increased that the WHO has for several years been talking about there being a global obesity epidemic. It is indeed considered certain that too high a BMI increases the risk of diabetes mellitus, cardiovascular disease and endometrial, colorectal and breast carcinomas. Yet one group from among the overweight appears, at least from a metabolic aspect, to be healthy – this phenomenon being labelled metabolically healthy obesity. Whether this state can hold up over the longer course of time is something now being studied by a research group led by Professor Mika Kivimäki at University College London (UCL).

In their study the researchers investigated the health of 2,521 people from 39 to 62 years of age. Over a period of 20 years, they kept track of the participants’ BMI and HDL cholesterol levels, blood pressure, fasting plasma glucose levels, triacylglyceride and insulin resistance as well as their intake of anti-hypertensive or anti-diabetic drugs. In addition, participants were divided into obese (BMI ≥ 30) and non-obese groups (BMI <30). The proportion of healthy-obese individuals at the beginning was 36.5%. Already after 5 years health in this group had deteriorated significantly: 31.8% of the formerly healthy-obese were considered to be unhealthy-obese. After 10 years, the proportion already made up 40.9% and reached 51.5% after 20 years.

Over the same period the level of health of the healthy non-obese had also deteriorated, although to a significantly lesser degree: healthy obese individuals demonstrated after twenty years an 8-times higher likelihood of belonging to the group of unhealthy-obese than did healthy non-obese individuals. “Healthy obesity is more a temporary phase than something that lasts over time”, declares Joseph Bell, a Ph.D. student at UCL and lead author of the study. “It is important to look at healthy obesity over a longer period of time and to consider the long-term tendencies. All types of obesity require treatment, even those who appear healthy, as they carry a high risk of future deterioration”.

Invisible Danger

In addition to the danger that healthy obesity might only be an intermediate step on the way to unhealthy obesity – despite the apparent absence of pathological indicators such as dyslipidaemia and insulin resistance being observed during healthy obesity periods – there may be other risk factors which remain unknown. In one study of almost 15,000 metabolically healthy adults, researchers led by Dr. Yoosoo Chang compared the coronary calcification of metabolically healthy obese and metabolically healthy normal weight individuals by using a cardiac CT. The calcium score was used to determine the extent of sub-clinical coronary atherosclerosis – a high calcium score as such is associated with serious adverse cardiovascular events.

The group of metabolically healthy adults with a BMI of over 25 had a significantly higher prevalence of coronary atherosclerosis than the metabolically healthy adults of normal weight (BMI 18.5 to 22.9). “Obese individuals who are being categorised as healthy because they have no cardiac risk factors should not be considered by their doctors to be healthy”, believes Dr. Chang, first author of the study. “Our research shows that the presence of obesity is sufficient to increase the risk of future heart disease. It is important that such people find out that changing their diet and exercise habits is able to prevent future cardiovascular events while they still have a chance”. Studies like these show that elevated BMI, despite apparent metabolic health, seems to be by no means a harmless or even favourable condition.

The long dispute over the healthy obese

The concept of metabolically-healthy obesity has been a controversial subject since its genesis. On the one side there are the proponents of the theory, in whose opinion being healthy and obese is not only harmless but even helpful. They lean on studies like those of Dr. Katherine Flegal, epidemiologist at the US National Center for Health Statistics. In 2013 she determined that overweight individuals (BMI 25-29.9) had a significantly lower mortality than did normal weight persons (BMI 18.5-24.9). While the mortality rate of less obese people (BMI 30 to 34.9) was not statistically different from that of normal-weight people, people with more severe obesity (BMI ≥ 35) showed a significantly increased mortality.

This phenomenon is not new: already in 1985 Dr. Reubin Andres, then director of the US National Institute on Aging, determined that the relationship between mortality and body weight is not linear but U-shaped. The lowest point of the curve differed between age groups: Whereas for younger people it was found more to the left, ie. at lower body weights, it moves with increasing age towards higher body weights. If there is indeed a causal relationship between weight and mortality, the usual recommendation of a weight reduction would specifically for older people be counterproductive. Possible explanations for the lower mortality of those people considered overweight range from cardioprotective metabolic effects arising from increased body fat to advantages of greater metabolic reserves and a higher probability of receiving optimal medical treatment.

All just an artefact?

Opponents of the healthy overweight movement complain that in many studies the effect of confounding variables is not being taken into consideration to a sufficient extent. In their opinion, there needs to be an awareness with the analysis of the data that serious diseases in particular are often associated with weight loss, which could lead to the erroneous impression that low body weight increased mortality. Similarly, the effects of smoking were undervalued in many studies: smokers have on average lower relative body weight and die sooner than non-smokers.

If one correctly takes into account the two parameters disease and smoking, the apparent U-shaped distribution of the mortality rate turns out to have been a mere statistical aberration, the opponents of healthy-obesity theory state. And they can back up their claims with their own study analyses: Dr. JoAnn Manson was able, for example, to show that the BMI of women increases linearly with mortality. The lowest mortality was shown by the women whose BMI was below 22. The data on smokers was however not considered for this analysis. If one eliminates from among the non-smokers those who died within the first four years of the start of the study (as an indicator of a pre-existing disease at the beginning of the study and thus a disease-related weight loss), the women with a BMI below 19 demonstrated the lowest mortality. In addition, the researchers were not able to detect any age-dependent shift of the optimum BMI.

Another analysis, this time of 1.46 million adult men and women, showed that the lowest mortality occurred with a BMI of 22.5 to 24.9 – even when smoking and ill persons were factored in. If one analyses only the data of healthy non-smokers, the optimal BMI was 20 to 25. Only among healthy non-smokers with a BMI below 18 – considered underweight – was increased mortality observed; in this group, the mortality rate increased with decreasing BMI. At the same time the mortality rate increased in the group with a BMI over 25 alongside increasing BMI. A high BMI was linked to a significantly higher risk for healthy non-smoking young people (age 20-49 years) than it was for healthy non-smokers over 50.

Dangerous research results

The question about what is the cause and what is effect will probably still keep the researchers busy for a while. However, the main basis for the heated debates about the healthy-obese seems to have less to do with the question of whose statistical analysis method is actually the correct one, but rather what message the results are conveying. The fear that positive reports about healthy obesity could be taken by some people as a free ticket for weight gain leaves many health experts wiping sweat from their brow. They see their efforts to curb the obesity wave undermined by these results.

Meanwhile, the critique about something completely separate is growing: the concept of BMI itself. The relationship between body size and weight alone is not in fact the decisive prognostic value. “Whether obesity threatens health depends crucially on how the fat is distributed and how much muscle is present”, stresses Prof. Matthias Blüher, endocrinologist at UKM Leipzig and head of the obesity clinic. Visceral fat in particular has become targeted by doctors and researchers – a waist circumference greater than 88 cm in women and 102 cm for men is considered risky. The reason for this is the strong hormonal activity of the adipocytes in this tissue. “Meaningful values are therefore only acquired when we measure more data”, says Prof. Blüher. “These include in particular the hip and waist circumference and, in more recent times, the neck circumference”.

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