Phil,
I’m glad you liked my post on AeT. I hope it’s all accurate.
Here is my 2 cents on METs
Metabolic Efficiency Tests (METs) were developed to help optimize treatment of Metabolic Syndrome, which is a constellation of medical diagnoses, including diabetes, high blood pressure, high cholesterol, and heart disease. Commercialized programs, including MAF, were developed and modified that utilize the data and metrics found on METs to help improve health and treat these above medical ailments with diet and exercise. The endurance training industry has also started using these same MET metrics to customize training and diet plans, which has surged in popularity recently due to the interest in low carb diets and ultra-endurance events.
Note that the term ‘metabolic efficiency’ is inherently flawed. Burning more fat vs carbs is not more efficient, or healthier, per se. One can be quite efficient at burning carbs, and vice versa. Your body tends to just burn what you eat, and after about 3-5 days of eating a low carb diet, for example, you acclimate to the new stimulus by becoming more efficient at burning fat and less efficient (maladapted) at burning carbs—however, your metabolism is likely no more efficient as a whole. For example, for endurance and ultra-endurance events that don’t require much anaerobic zone work, studies performed over the last 3 decades or so show no advantage of high vs low carb. Add an anaerobic component and carbs win. This does not reflect whats popular now, just the science. Perhaps with more evaluation this may change.
The standard MET includes the measurement of oxygen consumption (VO2) and carbon dioxide production (VCO2). Ancillary data collected include: HR, pace, and perceived effort (RPE). VO2max testing and subsequent establishment of training zones based on population based percentages can also be performed with the same machine. The ratio of VCO2/VO2, or respiratory efficiency ratio (RER), can be calculated and determines what percent of your energy at that given moment is being fueled by carbs vs fats. Through a calculation this yields a curve for percent carbs and percent fats, which can then be correlated to HR and pace, and eventually to your training zones derived from your VO2max. Crossover is when the percent fat and carb lines cross—they’re at 50/50. FatMax is when your percent fat curve is the highest, which is by definition below the crossover point, and often far below. Some patients do not have a crossover point, meaning that the HR or pace where this is occurs is perhaps below the HR range which is measured during the test. This could be minimized by starting the MET with the athlete at rest—which also has the added benefit of yielding ones basal metabolic rate (using the Weir equation).
Patients with Metabolic Syndrome, it was proposed, could benefit from low fat diets and exercise that burned fat. They were prescribed aerobic exercise at FatMax or below Crossover to maximize fat burning. MAF HR also targets this zone. Over time the low fat diet became less popular while low carb diets became more popular, and with this METs have also become more popular because they can be used to confirm your carbs/fat macro ratio is in fact fueling your aerobic training with predominantly fats. As we know, aerobic energy systems can use either fat or carbs for fuel, with the efficiency of both types of fuel being nearly equal per unit oxygen (carbs have a slight advantage). Therefore, the robustness, or efficiency of one’s aerobic system cannot be tested or determining one’s crossover point, ie, percent fat vs carbs. Nor is it possible to determine one’s AeT with the RER. In general, aerobic system development is independent of fuel source—its development depends almost exclusively on two things: genetics (eg, percent slow twitch muscle fibers) and volume/intensity of exercise.
Nevertheless, if you use a low carb training methodology a MET can be used to confirm that you are indeed using mostly fats for lower zone work, and that your crossover point coincides with your AeT HR, with the latter determined by another method, like percentage of your VO2max. Please note that a low carb diet is not a prerequisite for having a high crossover point, ie, a crossover point near or at AeT. A more balanced diet, say of 50/50 carbs/fats by calorie, could also achieve this, especially if your training program has high enough volume, which is the case for many serious athletes. This is because with high volume, even without fasted training, one often doesn’t have enough time to consume enough carbs to fully replete their glycogen stores every day—most serious endurance athletes inadvertently maintain a mild-moderate glycogen deficit, at least until they taper and perhaps carb load. This is how and why fat adaption, although easier to achieve at lower training volumes with a high fat diet, can still be achieved with most diets containing both carbs and fats. In fact, achieving a crossover point near AeT on a 50/50 fat/carb diet is probably more indicative of a metabolically efficient athlete than one who achieves this on a low carb diet, because the crossover point will shift to a higher HR based on diet alone, at least to some degree (its kind-of a shortcut that perhaps defeats the purpose).
So in summary, when should one obtain a MET? This depends on the type. The first kind is a VO2max test (maximal MET), which can provide ones basal metabolic rate, VO2max, and heart rate training zones by population based percentages. VO2max is an excellent and traditional method of quantify ones aerobic energy system, including cardiac output and oxygen uptake and utilization in the muscles (including mitochondrial density and local muscle vascularity). VO2max testing is not usually what people are looking for when they receive a MET—I believe currently the UA community is looking more for their Crossover point and Fatmax, which is evaluated with a submaximal MET.
The submaximal MET (the one testing RER, crossover point, and Fatmax, and not VO2max) can be obtained to confirm one has achieved a ‘fat adapted’ state, either through diet or exercise, or a combination of both. If you subscribe to this training philosophy, and its potential pros and cons, the submaximal MET can confirm your have achieved this state (ie fat as >50% of your fuel), both at rest, and during training (even at levels near your AeT). Note that maximal and submaximal METS are optimally done on different days, and the results of a submaximal MET only reflects your metabolism after fasting for 12hrs and running on a treadmill in a controlled environment.
Exercise does amazing things to your metabolism and aerobic system, and in most cases, regardless of the macro nutrient ratios you decide to present it with. Homeostasis works.
