This is indeed a loaded topic. Before I begin to discuss the science of exercise as a vehicle for “weight loss”, I feel compelled to inject an anecdotally-based observation from my own personal experiences with diet and fat loss, as documented in a bestselling book on evolutionary biology as a foundation for optimizing human health, “The Primal Blueprint” by Mark Sisson, published in 2011. Here is a link to the digital version of my story: http://www.marksdailyapple.com/just-shy-of-50-years-old-and-ive-never-felt-or-looked-better/. Both my anecdotal and professional experiences working with clients and patients as a Personal Trainer and Clinical Nutritionist for 10+ years have highlighted the true biochemistry of fat loss, later confirmed by my college studies. I have repeatedly explained to my clients that “you cannot exercise your way out of a bad diet” and that “80% of weight loss is to be found in the kitchen, not the gym. Granted, you have to have both components in place to achieve meaningful sustained fat loss, and by this I mean loss of body fat, not “weight loss” per say, which could consist from one day to the next of anything from water weight, depletion of intramuscular glycogen stores, or even muscle wasting caused by extreme caloric deficits. When the number on the scale nudges downward, it is not automatically a cause for celebration because one cannot know for certain how much of that weight decrement is actual loss of body fat. That said, the impact of exercise on decreasing the size of adipocytes (fat cells) is beyond doubt, but I see this mechanistically as more of a hormonal equation than a caloric one. You can be in a caloric deficit just be eating less and exercising more, but if the macronutrient composition and glycemic load of a diet does not induce lowered insulin and elevated glucagon levels that act upon HSL (hormone sensitive lipase) to cleave triglycerides from adipocytes, the opportunity for lean mass catabolism substituting for a healthy reduction of fat mass is always a danger. Fat loss is a thorny proposition indeed!
It would behoove us to factor in that the majority of body fat that is burned during even an optimally effective exercise session is in fact derived predominantly from intramuscular fat stores (the marbling we see in a fatty cut of steak), not circulating plasma FFA (Free Fatty Acids) from intra-abdominal or subcutaneous fat stores, i.e. the fat on our bodies that is visible to the naked eye when we walk on a beach in our bathing suits. Visceral or intra-abdominal fat in particular (the fat around our internal organs, such as our liver) is known to contribute to pathological insulin resistance manifesting as Type II diabetes and the various permutations of a condition known as Metabolic Syndrome, i.e. elevated blood sugar and blood pressure, increased triglycerides and heart disease risk. An individual who is dieting and exercising to lose weight could get a DEXA (Dual-energy X-ray Absorptiometry) scan done a few weeks after commencing an exercise-based weight loss regimen and they would find that the vast majority of reduction in their body fat stores was in fact attributable to the aforementioned intramuscular fat, not subcutaneous fat, following extensive bouts of aerobic exercise, or even resistance training for that matter, which can certainly elicit frustration in diligent dieters because the change that they see in the mirror is skinnier looking arms and legs due to loss of intramuscular fat while still carrying a significant amount of fat around their abdomen and/or hips (the “pear-shaped” hypothesis), as well as retaining substantial visceral fat stores, e.g. NAFLD (Non-Alcoholic Fatty Liver Disease), which is further exacerbated by standard low-fat high-glycemic weight loss diets. This is primarily why I am irritated by the meaningless term “weight loss”. Weight loss consisting of what? Apparently, many people do not care what their weight loss is comprised of, just as long as the scale shows a lower number than the day before. On the other hand, physically active people who have dialed in their hormonal health, stress reduction, sleep hygiene, and consume a diet that consists of a high proportion of healthy fats, adequate protein intake, and very low consumption of refined sugars tend to burn more subcutaneous fat while they are asleep, yes, I said asleep, and they wake up in the morning with slightly lower subcutaneous fat than when they went to bed the night before. Shocking but true!
It is even possible for someone, such as a hard-charging athlete, to lose 5 pounds of subcutaneous and intra-abdominal fat while gaining 5 pounds of muscle, hence they achieve no “weight loss” on the scale, yet look and feel better and have improved health outcomes. As with all aspects of human physiology, the proverbial “devil is in the details”. So make sure to be extremely wary of anything that identifies itself as a “weight loss” program. Oversimplifications are just pure marketing hype directed at emptying the pockets of unsuspecting consumers. Don’t go to the gym to “lose weight”. Go there to get stronger and fitter, to improve your flexibility and functional strength, to increase your bone density, to repair your hormones and improve your mood, and you should be skeptical of any Personal Trainer who tells you that they will help you “lose weight” or any book with a tagline such as “Lose 30 pounds in 30 days”. If you want to lose actual body fat, and do so safely, go home and clean out your pantry and refrigerator. Get rid of anything containing added sugar, anything that promotes inflammation in the body (such as vegetable oils and margarine), anything that spikes your blood sugar and leaves you feeling drained two hours later. Then you will finally start to fit into your skinny jeans and recapture your health and vitality, as I myself did 8 years ago, when I switched careers from obese software engineer to health coach and nutritionist.
The physique of the amazing young woman in the picture below is 50% genetics, 40% nutrition, and 10% exercise, but that 10% is just as important as the other 90% of the equation, because when it comes to looking and feeling your best, anything less than 100% amounts to 0%. We cannot choose our parents (genes), but we can choose what we put into our mouths and how we live our lives!