What is obesity – is it merely about BMI?

Introduction

In my earlier post “Is obesity a disease or a risk factor for other conditions”? I had highlighted that now obesity is recognised universally as ‘a chronic, relapsing, progressive disease process.’ However, misconceptions still abound about ‘what is obesity’, not only in the public at large but even among the health professionals. Traditionally, when talking about what is obesity, we tend to erroneously confuse how we measure obesity with the definition of obesity. One such popular and well-known measure of obesity, which will be discussed subsequently, is Body Mass Index (BMI). BMI However, obesity is not about BMI; at its most basic, the term obesity describes the presence of ‘excess’ fat in the body. 

Definition of obesity

In the year 1998, WHO defined obesity as under:

“A condition of abnormal or excessive body fat accumulation, to the extent that health may be impaired.”

In the year 2010, Scottish Intercollegiate Guidelines Network, part of the NHS Quality Improvement Scotland, described obesity as under:

“Obesity is defined as a disease process characterised by excessive body fat accumulation with multiple organ-specific consequences.”

What is the composition of the human body?

To be able to understand obesity better, it’s prudent to understand the composition of the human body. As is evident from the definition of obesity, excess body fat, not excess weight per se, is responsible for the various adverse health effects associated with obesity. In view of the same, assessment of body composition is of great relevance to determine a person’s disease risk. Traditionally the human body has been partitioned into two chemically distinct compartments, viz. fat mass (FM) and fat-free mass (FFM). The term FFM encompasses all tissues minus the fat, including water, muscle, bone, and internal organs.

With the advent of newer technologies, assessment of various components in FFM has become possible. Therefore, now a more valid four-compartment model of body composition analysis has emerged; the sum of the four components equals the body mass. According to this model, the human body has been partitioned into four chemically distinct compartments viz. fat mass (FM), water, bone mineral, and residual (which predominantly comprises of protein in muscles and other tissues). However, in the context of the current topic, the discussion will be limited primarily to the fat component.

Where does body store fat?

The total body fat exists in two separate storage sites or fat depots – essential fat and storage fat. Essential fat, as the name indicates, is the fat which body requires for its normal physiologic functioning. This includes fat present in the heart, liver, lungs, spleen, kidneys, intestines, muscles and certain tissues in the central nervous system and bone marrow. In addition, in females, essential fat also includes sex-specific essential fat. This additional sex-specific essential fat in women is needed for some biologically important functions related to childbearing and other hormone-related functions. The average essential fat in males is about 3% of the body mass, whereas it is about 12% in females – four times the value in males.

FFM plus essential fat is known as ‘lean body mass’ (LBM). Storage fat depot primarily includes fat stored in the adipose tissue. Adipose tissue is a loose connective tissue, composed primarily of specialised cells called adipocytes in medical jargon (adipose ~ denoting body tissue used for the storage of fat), which store fat; in addition, it also contains a variety of other cells which perform various metabolically important functions in the body. In common parlance, it (adipose tissue) is often referred to as body fat or simply fat. Although fat is the main component, it is not the only component found in adipose tissue. Storage fat in average males and females is similar – 12% of the body mass in males and 15% in females.

Where is the adipose tissue located in the body?

In humans, adipose tissue is located beneath the skin (subcutaneous fat), within and around internal organs in the abdominal cavity (e.g. intestines, liver, kidneys) and thoracic (chest) cavity (e.g. heart, lungs) (‘visceral adipose tissue’ [viscera ~ any large interior organ in any of the three great body cavities, especially those in the abdomen]), between muscles (inter-muscular), and in the bone marrow (yellow bone marrow). Subcutaneous fat accounts for almost up to 80% of the total body fat.

What is the role of adipose tissue?

Main role of adipose tissue is to store energy in the form of lipids; however, being a poor conductor of heat, subcutaneous fat layer acts as an insulator, reducing the loss of body heat through the skin and provides mechanical support and protection (cushioning) in the form of padding around and between certain organs such as the heart, lungs, liver, spleen, kidneys, and intestines.

What is the average percentage of body fat in healthy people?

In healthy, young adults, the average percentage body fat ranges from 12% to 15% of the body mass, in males, and 25% to 28% of the body mass, in females. The difference between males and females in the amount of body fat is primarily on account of the additional sex-specific essential fat in females. However, data indicates that the body fat, in both men and women, tends to increase steadily with advancing age; age between 25 to 44 years being the ‘danger’ period when adults tend to gain excess body fat. Though the reasons for this “creeping overfatness” are not clearly understood, this should neither be considered as a normal ageing process nor desirable. As will be discussed in subsequent posts, it is possible to prevent this body fat accretion with age by following a physically active lifestyle lifelong.

What are the criteria for defining obesity…how fat is too fat?

            Obesity can be defined on the basis of three different criteria, as under:

1. Total percentage of body mass composed of fat
2. Regional distribution or ‘fat patterning’
3. Size and the total number of fat cells

Total percentage of body mass composed of fat

Generally accepted criteria for defining obesity, based on body fat percentage, is body fat percentage above 20% of the body mass in males and above 30% of the body mass in females.

Regional distribution or Fat patterning: Pears vs. Apples

Apart from assessing the percentage of body mass composed of fat, assessment of distribution (or patterning) of body fat at different anatomic regions is another useful method for measuring obesity. Large-scale evidence has linked obesity to the development of various metabolic abnormalities, which further increase the risk of various diseases including heart disease and diabetes and will be discussed in detail in subsequent posts. However, obesity has been found to be quite complex and diverse in character; equally overweight or obese individuals having the same amount of total body fat may present with markedly different risk factor profile. For e.g. while some obese individuals present with a spectrum of various metabolic disorders and diseases, some equally obese persons may not suffer from any of these obesity-related health hazards.

Research has revealed that this metabolic heterogeneity of obesity and its related cardiovascular risk is primarily on account of individual variation in regional body fat distribution. Over the years, starting from the early 1980s, many research studies confirmed the notion that rather than the excess body fat per se, regional distribution of body fat at different anatomic sites was much more important in driving the risk of cardiovascular diseases and other health hazards linked to obesity.

Is the human body programmed to store fat differently?

As will be discussed in the subsequent post on how obesity develops, calorie intake over and above the body’s requirement is stored in the body as fat. It has become evident now that our ability to store fat in various adipose tissue compartments/sites could markedly differ from one individual to another. Often, the ‘extra’ energy in diet is stored as fat in the subcutaneous adipose tissue; however, in many cases the ‘energy surplus’ gets deposited in undesirable sites such as the visceral adipose tissue (mainly in the abdominal cavity), the liver, the heart, and the skeletal muscles; a phenomenon described as ‘ectopic fat deposition’. A number of factors, including genetic susceptibility to visceral obesity, and smoking are responsible for this ‘preferential’ accumulation of visceral fat.

What is abdominal obesity?

Visceral adipose tissue in the abdominal cavity is known as ‘intra-abdominal fat’; individuals with excess intra-abdominal fat are often labelled as ‘abdominally obese’. An excess amount of intra-abdominal or visceral adipose tissue has been found to be quite harmful as it gives rise to a constellation of metabolic abnormalities, which will be discussed in subsequent posts.

Does disease risk vary with the fat storage site?

Robust and convincing evidence from large-scale studies, such as the Framingham Heart Study and the Jackson Heart Study, that have extensively used computed tomography (CT scan) for assessment of body fat, show that excess visceral adipose tissue, along with excess fat in other ectopic sites such as the liver and heart etc. is responsible for the various metabolic abnormalities and related diseases associated with excess body fat, independent of the total or subcutaneous body fat. This is on account of the fact that ectopic fat is metabolically more active than the subcutaneous fat and represents the malignant form of obesity. Individuals with high levels of subcutaneous body fat (subcutaneous obesity) but with low levels of visceral adipose tissue often have a normal metabolic profile and have been labelled as ‘metabolically healthy obese subjects.’

What is fat patterning?

Fat patterning refers to the distribution of body fat on the trunk and extremities. Distribution of fat around the trunk and upper body, mainly in the abdominal area, is known as ‘central’ or ‘android-type’ obesity. This pattern of fat distribution may lead to an “apple-shaped” body and is more common in males than females. In contrast to the android type of obesity, distribution of fat around the gluteal (buttock) and thigh region is known as ‘peripheral’ or ‘gynoid’ type of obesity. This pattern of fat distribution may lead to a “pear-shaped” body and is more common in females.

Fig. Fat patterning – Pear vs. Apples

Factors affecting total body fat and fat patterning.

Both, the total body fat and the regional distribution of body fat are influenced by gender, age and ethnicity.

Gender

Gender differences in the deposition of body fat are evident even in the foetal stage, but they become much more pronounced during puberty. Men tend to have greater total lean mass and bone mineral mass and a lower body fat mass compared to women, who have substantially more total body fat than men. Apart from these whole body gender differences in total lean, fat and bone mass, there are major differences in tissue distribution between the two sexes. Men tend to have greater muscle mass in the arms, heavier bones, less fat in the limbs, but the relatively greater amount of central body fat; women, on the other hand, have a more peripheral distribution of fat in early adulthood. Differences in body composition between the two sexes are primarily on account of the sex steroid hormones viz. androgens and oestrogens.

In men, with a decline in the levels of testosterone, most important derivative of androgens, with age, there is a reduction in muscle mass and increase in fat mass. In women, parity (number of times a woman gives birth to a child) is an important contributor to changes in body composition and body shape. Following a childbirth, there is a redistribution of body fat;  women who have given birth have less lower-body fat and greater central body fat, which increases with the number of births, resulting in lower hip and thigh circumferences and increases in waist circumference. Menopause (also known as the climacteric, is the time in most women’s lives when menstrual periods stop permanently, and they are no longer able to bear children) is also associated with an increase in body fat and a redistribution of fat to the abdominal area. It is not clear whether such changes are due to hormonal changes or to the ageing process.

Age

While the total percentage of body fat may remain constant or increase with age, the major effect of ageing is a substantial redistribution of fat tissue among the various fat depots. Starting from the late middle age until the 80s or later, the volume of subcutaneous fat declines and there is a redistribution of fat from subcutaneous to visceral depots. This redistribution of fat is accompanied by accumulation of fat in muscles, liver and bone marrow and loss of lean body mass.

Ethnicity

Ethnicity also has an important correlation with body composition and regional distribution of body fat. Various studies have found a higher percentage of body fat in Asians at lower BMI as well as increased prevalence of truncal fat, compared to Caucasians. In addition, individual differences in regional body fat distribution have also been found between people of different ethnicities. For instance, blacks have less visceral adipose tissue than whites, while Asians, particularly people in China and South-East Asia, including India, display a greater amount of visceral adipose tissue for a given waist circumference or BMI, compared to the western population. This characteristic obesity phenotype (observable characteristics of an individual), of central obesity but relatively lower BMI has been referred to as the “Asian Indian Phenotype or Paradox.”

Size and the total number of fat cells

The number of fat cells or adipocytes in the body can be estimated by drawing a small sample of subcutaneous fat through a needle inserted directly into the fat depot. Following chemical treatment, first, fat cells are isolated and then average fat content in each fat cell is determined; then, total body fat divided by the average fat content in each fat cell gives the total number of fat cells in the body. This may sound to be merely of academic interest to an average reader, however, some basic knowledge of the subject will help understand some important aspects related to weight gain and weight loss.

Adipose tissue mass can increase in two ways. Firstly, through fat cell hypertrophy (enlargement of an organ or tissue resulting from an increase in size of its cells), wherein the existing fat cells enlarge in size as they are filled more and more with fat, and secondly through fat cell hyperplasia (enlargement of an organ or tissue caused by an increase in the reproduction rate of its cells), wherein the total number of fat cells increases. When excess calorie intake, over and above the body’s requirement, persists for a significant period of time, the body starts gaining weight, mainly in the form of excess body fat. In the beginning, this weight gain is facilitated by the enlargement of the existing fat cells. As the weight gain continues, fat cells reach a limit beyond which they cannot further increase in size; hereafter, the further gain in weight is facilitated by an increase in the number of fat cells. The major structural difference between the severely obese and nonobese persons is the number of fat cells. 

Does weight loss reverse the increase in the number of fat cells?

When calorie intake is reduced to create a calorie deficit with respect to the body’s requirement and maintained for a substantial period of time, the body loses weight and total body fat starts reducing. When the body loses weight, fat cells shrink in size; however, the number of fat cells does not reduce. So, in terms of the number of fat cells, it can be said that weight loss in obese persons does not really “cure” their obesity.

Conclusion

Misconceptions about ‘what is obesity’ abound in both, the general public as well as among the health professionals. Obesity is not just a cosmetic consideration; it is a chronic, relapsing, progressive disease process. Contrary to the general perception, obesity is not about BMI; at its most basic, the term obesity describes the presence of excess fat in the body. BMI, the most common measure of obesity, fails to identify large sections of the population who meet the criteria for obesity by % body fat. In 2000, the World Health Organisation (WHO) labelled obesity as the most blatantly visible, but most neglected public-health problem worldwide.