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The Science Behind a Wrestler’s Diet

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I recently read an article about what and how wrestlers should eat for competition. As a personal fitness trainer, and a trained medical professional, I was frustrated to find, yet again, that this “guide” was devoid of any research, any reasoning and any real knowledge about diet, metabolism, and physiology, which prompted me to write this blog.

First, I think a little review of physiology and energy pathways is in order. The body’s primary energy source for cellular work is ATP (adenosine triphosphate) You can’t just eat ATP, though. It must be made inside your cells. And, the preferred source for ATP generation is glucose.

The body gets glucose in three ways:

  1. from the carbohydrates a person eats
  2. from glycogen stores, which the body makes following meals when blood glucose is high
  3. gluconeogenesis, the process by which the body makes glucose during times of fasting from protein and metabolites.

Whenever your body needs energy, the cells break down glucose by a process called glycolysis to pyruvate and 2 molecules of ATP. In an oxygen-rich environment, like during the “aerobic” phase of exercise or at rest, pyruvate enters the Krebs’ (citric acid) cycle. This cycle has the ability to generate an additional 34 molecules of ATP.

In an anaerobic environment for the body, like strenuous exercise, ie wrestling, instead of pyruvate moving into the Krebs’ Cycle, it is converted to lactate and an additional 2 ATP molecules. This provides a short burst of energy, with the trade-off of building lactic acid in the cells. It leads to a situation known as “oxygen debt.” Lactic acid is, itself, destructive to the muscle cells. It lowers the pH of the internal cellular fluid, which results in less effective contraction in muscle cells. For this reason, muscle fatigue is a real physiologic phenomenon, though most people feel “fatigued” long before they have reached the physiological limit of their muscles.

Two things mitigate lactic acid build up: decreasing production and increasing elimination. Lactate builds up when production of lactate is greater than its removal from the cell.  Having a higher VO2max decreases its production of lactic acid by helping an athlete maintain an aerobic environment. “Lactic acid training” helps the body increase the elimination of lactic acid.

How many calories are burned in a wrestling match?

It depends on how heavy the wrestler is. A 110 pound wrestler will expend 272 calories in a match, where a 220 pound wrestler will expend 460 calories.

What to eat after weigh-ins?

This question requires knowledge of the physiology of the digestive system. The mean gastric emptying time of a liquid meal is about 2½ hours, where as the mean emptying time for a solid meal is over 4 hours. Carbohydrates are further broken down in the small intestine and then absorbed fully over its passage through the small intestine-roughly 4-5 hours.

Physiologic changes occur after about 4 hours of fasting. So, when combining the physiology of the digestive tract, recommendations for a post weigh-in meal would be a liquid meal that contains multiple simple carbohydrates (which are absorbed quickly) in at least 5 to 1 ratio with simple proteins.

For our examples above our 110 pound wrestler needs 272 calories at the minimum which corresponds to 63 grams of a combination of carbs and protein-or 54 grams of carbs and 9 grams of protein at the minimum. While the 220 lb wrestler will require a minimum of 460 calories or 115 grams of carb and protein or 100 grams of carbs and 15 grams of protein. A liquid meal will get passed through the stomach twice as fast as a solid meal and a simple carbohydrate will be digested and absorbed faster than complex ones making energy readily available for use in competition.

Because each carbohydrate has a distinct absorption mechanism in the gut, eating a mixture of carbs (fructose, glucose, galactose) maximizes the rate of total carb absorption and transport into the blood stream. After the match, the athlete should have a snack to replete what was consumed during the intense workout in a ratio of 1 g/kg of carbs and .5g/kg of protein, or for our examples the 110 pound wrestler should have 50 grams of carbs and 25 grams of protein and the 220 pound wrestler should have 100 grams of carbs and 50 grams of protein. Chocolate milk is a favorite recovery drink with a 3 to 1 (carb : protein) ratio.

Wrestlers should have carb-rich meals with complex carbohydrates the night prior to, and following, competition. These meals will take 8-9 hours to digest and keep blood glucose levels stable over that span. There appears to be no advantage in high versus low glycemic carbohydrates taken prior to exercise. With post-exercise carbohydrate consumption metabolism has shown some slight differences. Consuming high glycemic carbs in the post-exercise phase is associated with increased muscle glycogen re-synthesis, while consuming low glycemic carbs yielded higher fat oxidation. No differences have been noted in recovery or subsequent performance, however.


The Fuzzy Math of Exercise

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There are many numbers that get tossed around pretty routinely in fitness. Calories are counted. Weight is measured. Repetitions are tracked. But, when you want to lose fat you need to think about the actual science behind the numbers.

Here’s one I hear all the time: it takes 3500 calories to burn a pound of fat.

That’s grand. But, is it correct?

I’m not going to dazzle you with math, but there are a few conversions that need to be made to get to the heart of this question.

Caloric information is reported in grams, so first let’s convert pounds to grams. There are 2.2 pounds = 1 kg and a 1000 grams = 1 kg…

So: Convert pounds to kg: 1kg/2.2lb = .455kg/lb

and then kg to grams: 1000g/1kg= 455 grams/lb rounded to the closest integer. This is the weight of one pound in grams.

There are 9 kcal/1gm of fat, so

455gm/lb x 9 kcal/1gm = One pound of fat contains approximately 4095 kcal.

So, a pound of fat is really almost 4100 kcals, not 3500. That’s a big difference!

Wang et al 2001 did a study where they measured the caloric expenditure of different types of resting tissue. Their results were reported in kgs. Christian Finn did a great article and summarized the results in pounds as:

Organ or tissue Daily metabolic rate
Fat 2 calories per pound
Muscle 6 calories per pound
Liver 91 calories per pound
Brain 109 calories per pound
Heart 200 calories per pound
Kidneys 200 calories per pound

As you can see, the daily metabolic rate of a pound of resting muscle is approximately 6 calories, while fat is 2 calories. There may be a few studies that may show some variety, but in general estimates of muscle metabolism is 5-10 calories per pound per day and fat is 2-3 calories per pound per day.

Any important point that should be made is that there is a difference between resting muscle, recovering muscle, and active muscle. The metabolic rate of each depends on protein turnover. The more turnover, the higher the metabolic rate.

Another topic that gets a lot of buzz, and mis-quoting: EPOC, or excess post-exercise oxygen consumption. Many trainers discuss this recovery phase as crucial for fat burning, going as far as saying half (or even most) of the fat calories burned during/after a bout of exercise fall into this category.

What science says? EPOC only accounts for an additional 6-15% of the calories burned during a workout

To get to the heart of this misunderstanding we need to think about some basic physiology.

See, stored fat can only be mobilized into energy through a process that requires oxygen—called lipolysis. So, if you’re working at an aerobic level, well those calories can be accessed. If not, your body finds alternative fuels that don’t require oxygen. At rest your body can access fat stores easier, but once you hit a threshold of 65% of your VO2Max, your body looks to  first carbs and then protein for quick sources of energy.

HIIT exercise, because it operates near to the 65% VO2Max threshold for most of the workout, it will glean fat energy —and then again so will your normal daily activities—so what is to be gained in EPOC? Relatively little, but let’s look at an example.

More math….

If a person burns 500 calories in an aerobic workout, the majority of these calories will be from fat stores. If EPOC is roughly 10% of the total caloric expenditure, then you can figure 50 calories will be consumed in the post exercise phase. The calories consumed from fat during EPOC alone are not more than rest or even during the workout, assuming the workout maintained sub-anaerobic threshold levels for a considerable portion of the workout.

Is HIIT it?

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The most common recommendation for adult activity is 150 minutes of continuous moderate intensity aerobic activity every week and 2 or more days of muscle strengthening activities. Unfortunately, few people have that kind of time available for exercise. Also, people with chronic health issues couldn’t tolerate exercise for long enough periods to derive full benefit from it.

You might hear a lot of trainers touting a training modality called HIIT, or High Intensity Interval Training, as the best way to burn fat. HIIT is a training program that involves intervals of intense exercise followed by periods of active rest-or moderate exercise run in cycles.

The scientific community considered an alternative in HIIT. Tremblay et al 1993 found that when young adults performed either Continuous Moderate Training for 20 weeks or HIIT for 15 weeks, it was the HIIT program which induced a more pronounced reduction in subcutaneous adiposity (under-skin fat) compared with the CMT program. This spawned the notion that HIIT was superior for fat loss in any modality. Trainers will advocate many training routines that they will call HIIT, but what does the research show and how does that apply to the average person, the sick person, and the athlete? What routines have been shown to be effective?  After reading some really bad articles that either show no research or a misunderstanding of it, I decided to share it with you.

There has been considerable research into HIIT, so let’s highlight some of those studies.

Rognmo et al., 2004 found that when cardiac-diseased patients performed either continuous moderate training (CMT) or HIIT for 10 weeks that VO2Max increased by 10% more in the HIIT group. HIIT Training regimen used: 4 intervals of 4 minutes at 80-90% VO2Max followed by 4 minutes of active recovery at 50 to 60% VO2max, for three sessions per week.

Warburton et al., 2005 found that when cardiac-diseased patients performed either CMT or HIIT that VO2max increased by 5% more in the HIIT group.  HIIT Training regimen used: 9 intervals of 2 minutes at 90% VO2R [difference between resting and maximal VO2] followed by 2 minute active recovery at 40% VO2R twice a week for 16 weeks.

Wisloff et al., 2007 found that when cardiac diseased patients performed either CMT or HIIT that VO2max increased by 30% more in the HIIT group. HIIT Training regimen used: four intervals of four minutes at 90 to 95% HRmax followed by three-minute active recovery at 50 to 60% of HRmax for three sessions per week for 12 weeks.

Tjonna et al., 2008 found that when people with metabolic syndrome performed either CMT or HIIT that endothelial (blood vessel) function, blood glucose control and fat metabolism improved more in the HIIT group. HIIT Training regimen used: four intervals of four minutes at 90% HRmax followed by three-minute active recovery bouts at 70% HRmax for three sessions per week for 16 weeks.

Little et al., 2011 found that when people with Type II diabetes performed HIIT that subjects’ blood glucose control was significantly improved after just six total exercise sessions. HIIT Training regimen used: 10 intervals of 60 seconds at 90% HRmax followed by 60 seconds of passive rest or very-light active recovery for three sessions per week for 2 weeks.

Fex et al 2014 found that when pre-and type II diabetic patients performed HIIT elliptical workouts that they had significant improvement in fasting blood glucose, waist and hip circumference, appendicular fat mass, leg lean body mass and appendicular lean body mass, systolic blood pressure, resting heart rate and VO2 max. HIIT Training regimen used: elliptical 3 times a week for 12 weeks.

Keating et al 2014 found that when inactive overweight subjects performed either and CMT of 45 minutes or HIIT  that similar improvements were noted in both groups, however only the CMT group noted improvements in total body fat. HIIT Training regimen used:  6 intervals of 60 seconds of 120% VO2peak followed by 2 minutes of low intensity) 3 times per week for 12 weeks.

Sijie et al 2012 found that when overweight young women performed either CMT (50% VO2Max) or HIIT that body composition, left ventricular ejection fraction, heart rate at rest, maximal oxygen uptake and ventilatory threshold were more improved in the HIIT group. HIIT Training regimen used:  intervals of 85% VO2Max followed by brief periods of low activity 50%-VO2Max) 5 times a week for 12 weeks.

Peake et al 2014 found that in a randomized and counterbalanced order, 10 well-trained male cyclists and tri-athletes that performed both a CMT session 66% VO2Max and a HIIT session 81.6% of VO2Max and demonstrated that the HIIT session oxidized more carbohydrates and oxidized less fat than the CMT session.

Trapp et al 2008 found that when relatively fit young women performed either CMT (60% VO2Max for 40 min) or HIIT that significant reductions in total body fat, subcutaneous leg and trunk fat, and insulin resistance were found in the HIIT group, but not in the CMT group.  HIIT Training regimen used:  60 intervals of 8s sprints followed by 12s of active recovery 3 times a week for 15 weeks.

By reviewing the research, there are a few things to take away. Subject groups can be very different from the general population. If we ignore that for a moment, we can clearly see that HIIT is at least as effective at improving fitness (VO2Max) as continuous moderate exercise, but is more time-efficient. It can also be clearly argued that HIIT is actually more effective at increasing fitness than CMT. Likewise, HIIT is at least as well tolerated as CMT. Despite conflicting studies, the evidence still supports HIIT effectively lowering body fat, even if it is not more effective than CMT at lowering body fat.

  • In summary, HIIT will significantly increase aerobic and anaerobic fitness, decrease fasting insulin, increase insulin sensitivity, and reduce abdominal and subcutaneous fat

Important information to know about HIIT.

  • Every single HIIT study was done using a cardiovascular exercise apparatus-a treadmill, an exercise bike, or an elliptical machine.
  • There have been no studies looking at circuit workouts, cross fit workouts, or other form of weight training as an HIIT modality, for good reason as those workouts are unsafe to maintain at high intensity.
  • HIIT has been used either as 8-30s sprints/maximal effort or as a timed exercise where heart rate was measured.
  • Circuit training is circuit training, not High Intensity Interval Training, and have not been used in any study protocols.
  • Every study protocol has used an active rest period of at least equal, if not double, the high intensity time period. Going from exercise to exercise as fast as you can or with minimal rest is not something that has been tested. Nor has it been recommended in any way.
  • Weight lifting, on its own, has been shown to lower body fat as has both CMT and HIIT.
  • Weightlifting should probably not be used in HIIT, but if it is going to be used as a HIIT modality, heart rate should be measured as sprints + weights do not mix. An appropriate rest period must be given to assess HR.
  • Weightlifting absolutely needs to be part of every person’s training program, however, it is not ideal for HIIT.
  • HIIT seems to be a time-efficient way to improve a person’s cardiovascular fitness, although it need not completely replace CMT.
  • At the present time there remains no optimal interval, duration or frequency nor modality for HIIT and much more research is needed to offer specific recommendations
  • At present, HIIT has not been associated with a metabolic increase for any period of time in comparison to any other form of training and there is no known difference in excess post-exercise oxygen consumption (EPOC) compared to any other form of exercise.
Table 1. HIIT Recommendations for Clinical Populations
Program Component Program Modification 
Frequency Two HIIT sessions per week
Exercise Intensity 85–95% of maximal heart rate (HRmax)
Bouts/Session Inversely related to duration of HIIT bouts:

• 4 bouts/session if duration of HIIT bout is 4 minutes
• 8 bouts/session if duration of HIIT bout is 2 minutes
• 10 bouts/session if duration of HIIT bout is 60 seconds

Duration of Bouts 4 minutes, 2 minutes or 60 seconds
Recovery Length Greater than, or equivalent to, the duration of each HIIT bout. For example, if the duration of the HIIT bout is 60 seconds, the recommended recovery length is 2 minutes; if the duration of the HIIT bout is 4 minutes, the recommended recovery length is 4 minutes.
Type of Recovery Active recovery at low intensity (e.g., 50–70% HRmax) is strongly recommended over passive recovery.

How to Hire Trainer (Part 3)

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In the previous posts we discussed the WHY and the WHAT about personal trainers that make them a valuable part of your fitness plan. This post is all about deciding between trainers.

Ultimately, people hire trainers to achieve measurable fitness goals. In order to do this, a professional trainer should actively measure your progress. This should be done at regular intervals and tracked to ensure you are progressing.

Typical measurements:

Weight. Expect to be put on a scale at every session. In your own home, you should check your weight daily. Forget about your anxiety—you need to know where you stand, and recognize that day-to-day fluctuations may vary but the trend should always arrow downward. Women, particularly, should get a sense of how their monthly hormonal cycles causes weight to spike on different days.

Fat. Body fat assessments can be done through skin fold measurements with calipers, MRI imaging, bioimpedence, or hydrostatic weighing. Caliper assessment and bioimpedence measures are easy to perform and should be made at least every 6-8 weeks. Some scales even include bioimpedence monitors, so you can do this yourself at home.

Strength testing. As you train, your muscles should get stronger. This is yet another reason to weigh yourself regularly because increasing muscle mass increases appetite. If you don’t watch your diet, you can easily overeat your caloric needs and add fat. Your trainer should be tracking your increasing strength, and challenging you to increase the weight and repetitions you lift.

Easy assessments to evaluate your increasing strength:

Endurance assessment. Regular training should increase your exercise tolerance and ability to perform. Treadmill tests, like the Bruce Protocol Stress Test, will assess the maximal rates of respiration (VO2 max) and heart rate (HR max). The Harvard Step Test is good at measuring levels of aerobic fitness. The Orthostatic test for cardiovascular fitness is less activity related, but gives good insight into levels of fitness while at rest. Expect to be assessed every 6-8 weeks to measure your developing endurance.

If your trainer of interest does not assess their clients, this is a bad sign. How can this trainer tailor a fitness program to your needs if they do not know your health and fitness status? Trainers who don’t assess their clients are not results-driven, and will not make efficient use of your time or money.

Whoever you decide to hire, make sure that your trainer keeps your fitness and health as his/her priority. You are paying for a professional service, not a workout buddy.

How to Hire a Personal Trainer (Part 2)

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What makes a good trainer?

Many people believe they know how to train themselves, but creating an appropriate nutrition and training program to achieve their fitness goals takes real knowledge. For those clients who do seek help, they often seek out short cuts and cheap advice. Personal training is more expensive than group training, exercise classes, and online advice columns, but skimping on your fitness leads to a steep cost.

Obesity and poor fitness increase the incidence of health problems like diabetes, high blood pressure, or back, knee and hip issues that hamper your lifestyle. It may cause treasured pastimes to be set aside, or affect intimacy with your partner. The self-esteem costs are immeasurable, but you cannot put a price tag on fitting into your favorite clothes and being comfortable in public. Poor fitness can also increase depression and make you feel old undesirable. In the long run, seeking a true fitness professional, who will helps you reach your goals, is a good investment.

My trainer’s thin, so he/she must be good.

Trainers can look fit for many reasons and, don’t forget:  genetics play a major part.  Just as with all people, some trainers who routinely eat fast food and junk food still looked ripped.  Others may not even follow their own workout plans.  The way your trainer appears may have little to do with the type of advice he or she may give you.

Sometimes, the fittest trainers are also some of the best, but many times that is not the case. In the rare instances that a person may want to be a competitive bodybuilder, then finding a trainer who is actively competing may be desirable as they are actively following a strict diet and lifting regimen, but also has some great tricks of the trade.


The most important aspect you should consider when choosing a trainer is education. Anyone can label themselves a “fitness coach” or some equivalent but does that mean they are competent and capable?

Qualified personal trainers have had education in fitness training, and are certified through any of several national certifying organizations.  Not all certification programs are created equal, but ACSM, NASM, CSCS, and ACE are top-level with very good certification and re-certification programs.  Usually, the more certifications a trainer has from these governing bodies, the more knowledgeable and better they are.  Rule of thumb:  look for letters after the trainer’s name not the muscles on his or her body.

Be wary of people who push DVD workouts, weight-loss shakes, extreme fad diets, and fad workouts; or even, people who capitalize on their own attractiveness to sell their wares/programs.  Often, they don’t have a ton of knowledge in training or weight loss, and certainly know nothing about you. First and foremost, find a trainer who is interested in helping you, not making a lot of secondary supplement income. My suggestion when you encounter these people is to smile politely, nod your head and then run as fast as you can to a certified trainer, if you need one.

How To Hire A Personal Trainer (Part 1)

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Why do you need a trainer?

Most simple answer: You want to get fit. Perhaps you were once fit, but life got in the way. Perhaps you have never been fit, but recognize the need to be more health conscious due to medical issues. For whatever reason, hiring a competent, professional personal trainer is the best path toward increasing your fitness.

Here are the facts: When the average sedentary person starts to exercise there will be weight loss and fat loss. When the average person transitions to a healthy diet, there is weight loss. You do not need specific workouts or diets to accomplish some weight loss.

Unfortunately, most people have more than five or ten pounds to lose. And, their expectations for weight loss can be unrealistic. A good training program will usually cause a person to lose one pound a week. For a person who is 40-50 pounds over their ideal weight, they should expect to see full results in 9-12 months.

Why so long?

It takes time to build good dietary habits, a good routine, and muscle into a sedentary person. How many people do a Weekend Warrior-type workout and end up wrecked for days—or even injured? It took time to add the weight; it takes time to lose it, too.

Looking for a short cut or a cheap fix will cost you more in the long run. Take a look around your house for discarded fitness equipment, exercise DVDs, weight-loss shakes, and fad diets and now think about how much you spent. And for what? Have they brought you to the state of fitness you desire?

Committing to yourself

Fitness programs should be created with your needs in mind, by a professional trainer—one who has extensive education and experience. Your personal trainer should be a partner on the road to fitness. They need to take into account any lingering injuries, postural issues, current fitness state, and underlying health issues that will affect the manner in which you exercise.

Above all, fitness training should not be a “cookie cutter” approach. Anyone can put a person on a treadmill then walk him, or her, through a circuit workout of low resistance machines. But, is this the best, most efficient, manner to achieve your weight loss goals? Probably not.

A skilled personal trainer will be able to devise an alternating exercise regimen that will strengthen your muscle groups in a way that realigns your posture and reduces joint/spine injury. A professional personal trainer can provide you with a detailed diet plan to help compliment your workouts and increase your weight loss. Beginning a partnership with a personal trainer is a commitment to yourself to get fit.

P90X, Insanity, Cross-Fit: Fitness Trend or Fitness Fad?

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p90x photo: p90x p90x.jpg

In recent blogs, we have outlined a few important points:

There are three key times to burn fat.

  1. At rest/when not working out.  Fat burning during this time period constitutes the majority of not only time but also amount of calories.  At rest, the average person burns 2000 calories, while the average high intensity workout lasts for an hour and burns 500 calories.  It’s important to ask: Is my workout program allowing me to burn more calories per hour when I am not working out?
  2. During the workout.  High intensity workouts burn more calories, expend more stored energy, and trigger more a pronounced post-exercise recovery phase.  As we previously discussed, high intensity anaerobic workouts have been shown to catabolize the body’s own protein, especially Leucine.  Leucine is a key amino acid, which has many functions, including hunger control and prevention of muscle degradation.  The important question to ask is, Does my workout cause protein to be consumed or build muscle? 
  3. Post-exercise recovery phase.  High intensity workouts have been shown to burn fat at an elevated rate for several hours after the workout.  It’s important to plan a workout that will trigger a significant post-exercise recovery phase fat burn.  Does my workout cause a post-exercise recovery phase that burns fat?

P90X, Insanity and Cross-fit are similar workouts.  Their vertical progression workouts utilize high volume weight training in an intense format that minimizes recovery.

Now, back to the first question:  Do these programs raise a person’s metabolism? 

Exercise, naturally, has been shown to raise a person’s metabolism.  High volume training adds muscle, and increasing muscle is highly correlated to increased metabolism. Yes, these programs will help a sedentary person speed up their metabolism.

Do these programs cause muscle or fat to be consumed or build muscle during the workouts?

To burn fat, workouts need to be aerobic, and consume energy at a rate that matches the lipolysis (fat-splitting) pace. This has been determined to be at, or near, 65% VO2.

P90X, Insanity and Cross-fit workouts run at a much higher intensity, near that of the anaerobic threshold, and thus anaerobic for most people.  As such, the preferred energy source is carbohydrates.  Sure, all workouts are catabolic, but strength training is the most catabolic activity.  Rest between sets facilitates energy utilization through an aerobic pathway. When muscle recovery is delayed—as is what happens in the P90X/Insanity/Cross-fit models—the muscle operates in an anaerobic state. In this state, as much as 15% of the energy used comes from protein.

Additionally, the lack of rest results in rapid fatigue and may increase the risk of injury, at a cellular and whole muscle level. With fewer amino acids available (because they got turned into ATP) the muscle cells are less able to repair local damage and build bigger (stronger) muscle cells. Lack of amino acids is also a strong trigger for hunger, resulting in an increased drive to eat in the post-recovery phase.

So although these high volume weight-training workouts will create a situation where a person can add lean mass, the intensity of the workout will lead to more muscle loss and decreased muscle repair when anaerobic. The stage is then set for high risk for overtraining and injury and in a nitrogen-negative (protein poor) environment will eventually lead to a loss of metabolism.

Finally, does the high volume/anaerobic style of P90X/Insanity/Cross-fit workouts result in a post exercise recovery phase where fat-burning is promoted? 

Yes. Due to the huge calorie burn in the workout, the post-exercise phase is where those fat calories will get consumed.

Are the P90X/Insanity/Cross-fit workouts a fitness trend, or a fitness fad?

While these workouts have some good points—namely incorporating weight training and periodization—they are little more than glorified circuit workouts, a training modality used for decades.

Also, they aren’t the most efficient method to build muscle, speed up a metabolism or burn fat.  Their anaerobic pace promotes rapid fatigue and can lead to injuries resulting from poor form and overtraining—particularly in the home setting where proper supervision is absent.  Without nutritional guidance, people using these training styles run the risk of slowing their metabolism in the long run.

In my opinion, a fitness trend needs to incorporate more periodization with a combination of low volume training, eccentric training, and appropriate interval training components. I have seen personal trainers use components of P90X/Insanity/Cross-fit to add variation within challenging and progressive workouts, but, in general, these programs are fads and will likely fall out of when the next generation of circuit workouts strolls along.