Sometimes in order to save time and avoid mental strain we have a tendency to oversimplify. However, like many situations, over simplification to save time comes at the expense of accuracy and relevant nuance. The concept of being “fat, but fit” or “The Obesity Paradox” meets this criterion.
Generally, being overweight or obese is bad for you. Obese individuals have a higher risk of developing cardiovascular disease and type 2 diabetes than lean people. Likewise, being unfit is considered an independent risk factor for developing the same conditions. At first glance, our initial impression of those who are overweight or obese is that they are unhealthy and, particularly from the standpoint of the fitness professional, unfit.
The concept of “healthy obesity” was born out of research conducted as a part of the Aerobics Center Longitudinal Study (ACLS) completed in 1989. Two researchers followed up on 25,714 men for 10 years. The first observation made was that men with a BMI indicative of obesity (30 m/kg2), but demonstrating high levels of cardiorespiratory fitness reduced (>3x) their risk of cardiovascular disease mortality. Essentially, if you are fat and fit, it can mean you have a reduced risk of cardiovascular disease.
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This finding was supported, by a second observation that looked specifically at body composition (i.e. body fat %), which demonstrated similar findings of fat, but fit-healthy individuals. Similar longitudinal studies (research involving gathering data on the same subjects repeatedly over a long period of time, often years or even decades) have been conducted in women also demonstrating the protective effect of fitness in obese individuals. The author of the Obesity Paradox, Carl J. Lavie, MD, demonstrated that obese people actually have better survival rates than lean people with congestive heart failure. However, Dr. Lavie does stress the importance of a lean metabolically healthy profile with an emphasis on avoiding a low fitness.
Another health concern for obese individuals is the potential for developing the cluster of high blood glucose and poor blood lipid profile called Metabolic Syndrome. However, cardiorespiratory fitness has been shown to reduce the risk of developing metabolic syndrome by 50%. What this means is that CV fitness in obese people reduces metabolic syndrome risk.
So yes, you can be overweight or obese and 1) be fit and 2) be healthier as a result of being fit.
The other side of the coin, and another important take home message, is that you can be normal weight, unfit, and be at a greater risk of dying from a heart attack than your obese, but fit, counterpart. This is a sobering fact for many that consider their lean body composition to be completely protective.
The unifying concept in this paradigm is that fitness is vitally important to decrease the risk of cardiovascular disease and metabolic syndrome regardless of body composition. Exercise stimulates metabolically active tissues, where fuel is stored and utilized, to adapt in a manner that imparts metabolic health on those with less than ideal body composition.
The merits of muscle
For most, the concept of healthy muscle makes sense with improved fitness. Since muscle is the primary site for energy utilization during exercise it would be logical to suggest that having healthy muscle would protect someone from the metabolic complications that often come with obesity. Two major adaptations that occur in the muscle as a result of exercise training are increased insulin sensitivity and increased mitochondrial density/function. Muscle is a major site of glucose disposal after glucose levels increase after we eat. That glucose is converted into glycogen (or oxidized/burned). This process is stimulated by the action of insulin on the muscle. This process works much more efficiently in well trained muscle. In particular, higher intensity exercise has been shown to increased insulin sensitivity better than moderate intensity exercise acutely.
The mitochondria are the major site of energy production in the muscle and the only site for the breakdown of fat. Obese individuals that are physically fit have been shown, while not in all cases, to have normal mitochondrial density and function when compared to normal weight counterparts. At one point, it was thought that longer duration aerobic exercise was needed to increase mitochondrial density and function. However, brief intense exercise has also been shown to elicit similar adaptation when compared to the traditional long duration aerobic exercise.
Most fitness professionals are concerned about reducing their clients body fat. And for good reason. Reduction in body fat is one of the tried and true methods of improving overall health. However, now that we have established that obese individuals can be metabolically healthy there must be other adaptations at play, not only within the muscle, but also the fat and liver.
The helpful hormones of fat: leptin and adiponectin
The discovery of leptin, a hormone secreted by fat cells that regulates energy intake, was a game changer for fat. Fat, which was once thought of as just an inert storage depot has become understood to be an important endocrine organ that impacts metabolic health. The more recently discovered adiponectin is an adipokine, a type of small protein hormone released from fat cells which regulates glucose levels and fatty acid breakdown, has been shown to be reduced in insulin resistant obese people.
Interestingly, adiponectin is increased in those who are “fat, but fit”. A review of studies that measured adiponectin in response to exercise reported it to be increased by 38%. These findings may explain how someone who is obese, but exercises, can maintain metabolic health. They have healthy fat that produces substances that help preserve metabolic health.
The dangers of the fat you can’t see
Another important consideration is the location of fat. The fat that we are most familiar with is called subcutaneous (under the skin) fat. This is the fat that is measured when doing skin fold measurements to estimate body fat percentage. There is also a depot of fat called the visceral fat, which as the name indicates, is stored between the visceral organs. Visceral fat produces inflammatory substances that can directly interfere with insulin sensitivity. The accumulation of visceral fat is highly associated with insulin resistance. Simply put, the fat that accumulates around the abdominal wall, also called android obesity, is considered more harmful.
Obesity and the liver
The liver is a major metabolic hub. The liver receives a massive amount of blood flow and is therefore exposed to the carbohydrate, fat, and protein we eat. Recently, non-alcoholic fatty liver disease (NAFLD) has been added to the list of negative phenomena associated with obesity. In NAFLD, there is an imbalance in fatty acid accumulation and fat oxidation, whereby fatty deposits are embedded in the liver. Obese individuals do appear to develop NAFLD, with 45% of metabolically healthy obese individuals having NAFLD as reported in one large study. The role of fitness and in particular high intensity exercise plays in regulating this condition is not known.
What type of exercise has been shown to benefit the obese?
It is important to note that most of the studies that have investigated MHO (Metabolically Healthy Obesity) look at fitness through the lens of cardiorespiratory fitness. Rarely is muscular strength and/or the engagement in strength training considered. In fact, some of the only data regarding the topic is just a slight positive association between relative body strength (1-RM leg press divided by body weight in kg) and MHO. It appears the attribute of strength is associated with protection in the obese state. However, the data are currently lacking on types, intensity, and frequency of strength training that may protect obese people from metabolic dysfunction.
Important to note, these analyses are longitudinal in nature and do not focus on specific behaviors of MHO, but rather the attributes that are associated with the phenotype (physical state). Approaches, like HIT (high intensity strength training), have definite cardiovascular effects, however, the method of measuring cardiovascular fitness (VO2max test on a treadmill or bike) are highly specific (see Body by Science; chapter 2) and may not be able to detect the cardiovascular adaptations acquired through HIT. However, high intensity interval training (HIIT) is a method that has shown to increase cardiorespiratory fitness with a reduced time commitment that aligns naturally with the HIT paradigm.
HIT may play an important role in maintaining health and longevity of obese clients. The concept of a healthy physique is changing. This is not to say that reducing body fat in obese and overweight clients is not a worthwhile goal. However, it does speak to the importance of maintaining an exercise program that will increase production of healthy factors coming from the muscle, fat, and liver.
Take home message for HIT practitioners
- Not all obese people are metabolically unhealthy.
- Increasing the fitness of your client, regardless of a reduction in body fat, will likely improve their overall metabolic health and reduce the risk of cardiovascular disease.
- Consider a variety of interventions when working with overweight/obese people to improve all components of fitness.