Perhaps something that is not always acknowledged, is that while excess, dysfunctional adipose (fat) tissue has pathological and clinical manifestations, simply having excess adiposity does not guarantee the presence of metabolic complications.¹'² There are data to support the notion that weight and disease are not linearly related and that health and disease vary across the weight spectrum.³ In fact, approximately one-third of obese individuals (classified according to body mass index (BMI)) have been found to be free of metabolic abnormalities.¹'² For example, not all people who are obese develop type 2 diabetes mellitus(T2DM), and not all people with T2DM are obese.⁴ Why this is, we do not know. This could come down to genetic predisposition.

As ‘normal’ weight is not considered to be a risk factor for morbidities such as high blood pressure or T2DM, individuals who are metabolically unhealthy often go unidentified and thus are not treated for further disease prevention.¹ Moreover, a person with a higher weight maybe misdiagnosed as being unhealthy, though it has been shown that excess adipose tissue does not necessarily guarantee metabolic complications. Thus, the long-term effectiveness of the focus on weight reduction to improve the health of individuals has been questioned.⁵ Shifting the focus from fat mass to that of adipose tissue dysfunction would assist in identifying those at risk of metabolic complications.

PROSPECTIVE TRIALS SHINE LIGHHT ON TYHE EFFECT OF WEIGHT LOSS OVER ADULTHOOD

According to the National Health and Medical Research Council evidence hierarchy (2009), randomised controlled trials (RCTs) provide the best level of evidence for interventions, particularly when systematic reviews or meta-analyses of RCTs are provided. However, when it comes to the matter of aetiology (causation), prospective cohort studies provide the best level of evidence, especially when presented as a systematic review (NHMRC, 2009). Below we evaluate the data from large prospective cohort studies in terms of the relationship between weight and mortality.⁷

NHANES SURVEY

Chen et al. (2019) evaluated the data from the US National Health and Nutrition Examination Survey (NHANES), with 36,051 participants. A linear association was found between BMI stage 25 years and mortality, where overweight and obesity were significantly associated with an increased risk of all-cause mortality. However, an increase in age changed the association to a U-shape, with overweight having no association, and an increase in risk for those who were underweight. Stable obesity across adulthood and weight gain from young to middle adulthood was associated with an increased mortality risk. This is consistent with previous studies which have shown that a longer duration of obesity was associated with higher mortality risk. Furthermore, weight loss from middle to late adulthood was significantly related to increased mortality risk. This highlights the importance of maintaining ‘normal weight’ to ‘overweight’ across adulthood, and the need to prevent weight gain in early adulthood.

NURSES' HEALTH STUDY & HEALTH PROFESSIONALS FOLLOW-UP STUDY

Veronese et al. (2016) evaluated the data from the Nurses’ Health Study (74,582 participants)and the Health Professionals Follow-up Study (39,284 participants). This study also found a U-shaped relation between BMI and mortality. Furthermore, it highlighted the importance of considering diet and lifestyle factors when assessing the association between BMI and mortality. It was found that even people in the obese category had a lower risk of mortality if they had at least one low-risk lifestyle factor: high score on the alternate healthy eating index, high level of physical activity, moderate alcohol drinking, and not smoking. These factors likely provide health-protecting effects by inhibiting insulin resistance, inflammation, and oxidative stress and slowing the accumulation of cellular and organ damage.⁸

From this data, we can draw the following conclusions:
* Prevention of weight gain and obesity should be the primary goal for adults.
* ‘Normal weight’ does not guarantee that one is free of co-morbidities, as dysfunctional adipose tissue can occur at any weight. People in this category should still be screened and co-morbidities should be addressed accordingly.
* The classification of ‘overweight’ does not necessarily require the encouragement of weight loss, however, the presence of co-morbidities should be addressed accordingly.
* Weight loss in those with excess dysfunctional adiposity, should be encouraged early onin adulthood, as the longer the duration of obesity and potential co-morbidities, the higherthe risk for mortality. Furthermore, aiming for weight loss in late adulthood is associatedwith increased mortality.
* Promoting at least one positive lifestyle factor (e.g. healthy eating, increased physical activity, moderate alcohol drinking, and cessation of smoking) can have protective effects for those with obesity. This might be more appropriate than encouraging weight loss in later adulthood.

While weight loss from middle to late adulthood was significantly related to increased mortality risk, stable obesity across adulthood and weight gain from young to middle adulthood were also associated with increased mortality risk.⁷ This data makes it a ‘catch-22’ situation, in that obesity increases the risk of mortality, but losing weight also increases the risk of mortality. Thus, prevention of weight gain and obesity should be the primary goal for adults. This is ofcourse not realistic, so what does one do? The study by Veronese et al. (2016) provides some hope! A lower risk for mortality can be achieved if people with obesity have at least one low-risk lifestyle factor. This study highlighted the importance of considering diet and lifestyle factors when assessing the association between BMI and mortality. While there are associations between extreme weight and health problems, there is stronger evidence for the role of factors for health other than weight (e.g., vegetable consumption).⁹

In light of the above, it is proposed that body composition, including the distribution of excess body fat, is more important than the goal of achieving ‘normal weight’ according to BMI classification. Avoiding the accumulation of visceral fat and ectopic fat is paramount, as this is associated with morbidity and mortality.¹⁰ Encouraging the loss of this fat is also recommended. ‘Normal’ weight individuals presenting with T2DM or hypertension, with normal visceral adiposity, may have more of a genetic predisposition and would require interventions outside of weight loss.