Being sedentary and consuming fast food are habits that have stealthily entered our fast-paced Western lifestyles and are common contributors to obesity. In response, most of us have either aimed to become healthier or at least known someone who has pursued this goal for themselves. Technological and social advancements have increased our risk of developing a range of diseases and disorders, while simultaneously increasing our awareness of the need for healthy living. Availability of healthy food alternatives at grocery stores, podcasts on healthy living, YouTube tutorials on home exercises and health-tracking apps are some instances of how healthy living is now more accessible than ever before. The age-old idea of a healthy mind in a healthy body is also picking up traction once again as we become more conscious of mental health issues being commonplace in our society. All of this converges at one question: how do we make ourselves healthier?
But, before we get ahead of ourselves, how do we determine if we are indeed healthy? For a long time, a person’s body mass index, or BMI (kg/m2), and their waist to hip circumference ratio have been primary measures of their body weight status and are still popularly used to categorize people as underweight, overweight or obese. While these indicators are extremely useful, they only scratch the surface of the issue. Obesity is a complex metabolic condition that is chronic, debilitating and deeply intertwined with the increased risk and occurrence of several metabolic disorders. Weight loss and weight management for people who are obese becomes imperative not because they need to look shapely and meet societal beauty standards but because they need to avoid unnecessary, and sometimes deadly health complications.
A notion that is common to any kind of weight loss program is creating a caloric deficit. This simply means — consume fewer calories than you burn. Crudely put — “Eat less, exercise more.” This sounds like a sure, easy formula for success. However, it does not always translate into weight loss for everyone. There are many questions to be answered. For starters, how many calories do you burn every day? This is called the total daily energy expenditure or TDEE. While an average adult burns 18002100 kCal/day, this number is an average estimate and can vary from individual to individual. Knowing the number of calories that you burn every day would help determine how many calories you would want to consume each day such that you lose weight.
While it is rather simple to determine your calorie intake, it is significantly more challenging to determine your resting metabolic rate. Resting metabolic rate, or RMR, is the total calories burned by your body when you are completely at rest. This is the energy your body requires to keep itself functioning and is the major portion (6075%) of your TDEE. Each physical action requires energy and hence, burns calories. Deliberate physical exercise generally contributes only 1530% of your TDEE unless you are an endurance athlete or an Olympic swimmer who undergoes regular rigorous training.
There are several questions around the RMR that have long puzzled researchers, doctors and fitness enthusiasts alike. How widely does the resting metabolic rate vary from person to person? Can it be increased through long-term physical activity? Does your gut microbiota impact your resting metabolic activity? How significant is this impact?
A public health perspective published in 2015 consolidated data from the existing scientific literature to conclude that “no single value of the RMR is appropriate for all adults”. The average estimates available online are likely an overestimation of one’s actual RMR. Several factors including age, sex, body weight and height can impact your RMR. Many mathematical equations have been devised over the years to calculate this ever-elusive magical number of the RMR. Two of the popular ones are the Harris and Benedict equation and the Mifflin St Jeor Equation. We do not need to worry about the complexity of their names or their mathematical nature as several free online calculators are readily available and can give us relatively accurate measures of our RMR and TDEE.
RMR is calculated based on several factors. Interestingly, while one has no control over factors such as their age, sex, or height; physical activity is a factor that can be regulated at will. People aiming for weight loss usually follow a regimen that is a combination of dietary interventions and physical activity. While short-term physical activity has been shown to increase your RMR, the effect of long-term physical activity on RMR is more convoluted. Put simply, the RMR can be most closely correlated to the lean mass or fat free mass (FFM) in the body. As one exercises or cuts down on their calories, their overall body weight decreases, and so does their FFM. A reduction in FFM translates to a reduction in RMR, which is undesirable. Hence, it is optimal to design your workouts and nutritional plans such that you lose weight while the inevitable loss of FFM is minimized. Reducing the ratio of carbohydrates to proteins in your diet and weight training have been shown to help achieve this goal.
While all of this may seem too excessive and unnecessary, obesity is indeed quite complex by design. There are several genetic and environmental factors at play. A key environmental factor that has historically been overlooked, but has now been speculated to regulate the RMR, is the gut microbiota.
The gut plays a central role in metabolism and is home to a plethora of microbes that are important for our survival. In mice, depletion of the gut microbiota has been shown to reduce the amount of weight loss achieved upon calorie restriction. It has been shown to cause an attenuation in the metabolic rate, a reduction in lean mass (or FFM) and an increase in fat mass. Conversely, calorie restriction has been shown to cause a shift in the microbiota towards more probiotic, healthy bacteria such as Lactobacillus and Bifidobacterium along with a concomitant decrease in harmful bacteria such as Helicobacter.
Another study involving mice treated with risperidone, an antipsychotic drug, showed drug-dependent weight gain. Further probing into this apparent obesity of the mice revealed the underlying reduction in their resting metabolic activity. There was a whopping 16% reduction in the RMR of these mice which was later confirmed to be caused by a shift in their gut microbiota. This 16% reduction is roughly equivalent to a human adult consuming an extra cheeseburger every day. This study was repeated in humans and a similar effect was observed. This was also attributed to microbiota as a reduction was observed in the ratio of Bacteroidetes to Firmicutes, two types of bacterial phyla commonly found in the gut. The microbiota in our gut, thus, may be actively involved in regulating our resting metabolism.
Our bodies work in mysterious ways – from the food on your plate to the microbes in your gut, everything is a part of your weight loss journey. While there is no shortcut for hard work in life, sometimes shifting your perspective can help make the goal look more achievable. Obesity is not a “lazy fat person” problem. Being “fat” and “lazy” may both be symptoms of this multifaceted issue, but the cause lies deeper – it may be your lifestyle, it may be your genes and it very well may be the invisible microbes in your gut!
References:
- McMurray RG, Soares J, Caspersen CJ, McCurdy T. Examining variations of resting metabolic rate of adults: a public health perspective. Med Sci Sports Exerc. 2014;46(7):1352-1358. doi:10.1249/MSS.0000000000000232
2. https://blog.nasm.org/nutrition/resting-metabolic-rate-how-to-calculate-and-improve-yours
3. Stiegler,Petra, and Adam Cunliffe. “The role of diet and exercise for the maintenance of fat-free mass and resting metabolic rate during weight loss.” Sports medicine 36.3 (2006): 239-262.
4. Tremblay, Angelo, Jean-Pierre Després, and Claude Bouchard. “The effects ofexercise-training on energy balance and adipose tissue morphology and metabolism.” Sports Medicine 2.3 (1985): 223-233.
5. Bahr SM, Weidemann BJ, Castro AN, et al. Risperidone-induced weight gain is mediated through shifts in the gut microbiome and suppression of energy expenditure. EBioMedicine. 2015;2(11):1725-1734. doi:10.1016/j.ebiom.2015.10.0186.Wang, Shuo, et al. “Gut microbiota mediates the anti-obesity effect of calorie restriction in mice.” Scientific reports 8.1 (2018): 1-14.
Manjula Kamath
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