It’s estimated that there are over 2+ million scientific papers published each year, and this firehose only seems to intensify.

Even if you narrow your focus to fitness research, it would take several lifetimes to unravel the hairball of studies on nutrition, training, supplementation, and related fields.

This is why my team and I spend thousands of hours each year dissecting and describing scientific studies in articles, podcasts, and books and using the results to formulate our 100% all-natural sports supplements and inform our coaching services. 

And while the principles of proper eating and exercising are simple and somewhat immutable, reviewing new research can reinforce or reshape how we eat, train, and live for the better. 

Thus, each week, I’m going to share five scientific studies on diet, exercise, supplementation, mindset, and lifestyle that will help you gain muscle and strength, lose fat, perform and feel better, live longer, and get and stay healthier. 

This week, you’ll learn the multiplex ways mental fatigue enfeebles your performance, if weightlifting helps you lose visceral fat, whether caffeine suppresses your appetite, and more. 

Mental fatigue reduces your athletic performance in surprising ways.

Source: “Does mental fatigue affect skilled performance in athletes? A systematic review” published on October 14, 2021 in the journal PLOS One.

Most research on athletic performance focuses on physiological factors like glycogen, enzyme, and hormone levels, muscle mass, Vo2 max, and so on, and while these ingredients are important, they’re only part of the recipe. 

There’s a new school of thought, largely pioneered by Tim Noakes and Samuele Marcora, that believes the brain gets the final say in our athletic performance (either subconsciously or consciously or both). In other words, although physiology may determine your absolute potential at any given moment, your headspace determines how far you can push the envelope. 

While this is somewhat self-evident–you don’t need a study to know that feeling frazzled makes for less productive workouts and an upsurge of enthusiasm does the opposite–we still don’t fully understand the underlying mechanisms behind this effect or what we can do to counter them. 

To help demystify this murky field of research, scientists at the Universiti Putra Malaysia reviewed 11 studies that had soccer, basketball, and table tennis players complete cognitively draining tasks prior to workouts or competitions. 

Most studies found that mentally demanding tasks caused significant cognitive fatigue, and that their performance dipped as a result. No surprise there.

What’s more interesting is how this drop in performance manifested. 

Mental fatigue doesn’t seem to have much of an impact on physiology. Things like heart rate, aerobic capacity, and maximal strength don’t really change if you’re feeling ragged.

Instead, mental fatigue saps our athleticism by disrupting decision making and our perception of effort.

In a sense, “good technique” can be thought of as just a series of nearly instantaneous conscious and subconscious decisions about how to move your body, and just like writing a book, allocating investments, or designing a building, this stream of decision-making drains our cognitive gas tank. 

Specifically, mental fatigue . . .

  • Scatters our attention. We become more easily distracted and less goal-directed, and have trouble distinguishing between trivial and crucial details in our environment. 
  • Gives us tunnel vision. Ironically, although cognitive fatigue fragments our attention, we also have a tendency to miss the forest for the trees. For instance, we might become fixated on a particular opponent and lose sight of the rest of the field. 
  • Makes us clumsier. We become less coordinated and make more mistakes that we otherwise wouldn’t if we were mentally fresh. 
  • Slows down our decision-making. In order to avoid making mistakes, we have to move slower, which is a liability in any sport (basically, we’re forced to choose between speed or accuracy). This is even true in weightlifting, where having to slow down during a rep means your muscles work harder which means you probably get fewer reps.
  • Disrupts our ability to plan and prepare. Most sports pivot on our ability to anticipate the movements of opponents, and this skill is also undermined by mental fatigue.

Other research has shown that cognitive fatigue tends to spike our rating of perceived exertion (RPE)–it makes exercise feel harder, which makes our workouts less productive and enjoyable. 

By my lights, the main takeaway from all of this is simple:

Limit mentally and emotionally draining activities before and during your workouts. Resist the temptation to check social media, answer email, study, or ruminate on thorny decisions while working out. You’ll probably feel and perform better and enjoy higher quality decision making to boot.

What’s more, you should also adjust your expectations if you go into a workout feeling mentally bushed. Treat cognitive fatigue with the same respect as physical fatigue, and don’t hesitate to adjust your weights, reps, or sets as needed to allow for more recovery

That said, it’s worth noting three caveats about this research.

First, most of the studies on this topic have involved arbitrary mental exercises designed to make your mental gears grind, such as the Stroop test (which you can try for yourself here). I wouldn’t be surprised if mentally difficult but enjoyable and fulfilling activities (writing, practicing music, reading, drawing, drafting a business plan, etc.) had either a less negative, neutral, or even bracing effect on our performance (especially if you’re able to enter a flow state).

Second, I’d also be curious to know if this effect cuts both ways. Do intense workouts or competitions make it more difficult to complete cognitive tasks later in the day? Should you periodize your training around big projects at work or important personal decisions?

Third, can we train ourselves to better resist the enervating effects of cognitive fatigue? For example, some endurance coaches, such as Steve Magness, have experimented with having their athletes solve math problems in the middle of workouts to enhance their mental resilience. Maybe working out after a long day at work can have a similar antifragile effect over time.

We’ll just have to wait for more research to see. 

TL;DR: Mental fatigue scatters your attention and ability to prioritize, impairs your ability to focus, create plans, and make decisions, and makes exercise feel significantly harder.

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Knee sleeves don’t work for the reasons you may think they do.

Source: “Effect of a Neoprene Knee Sleeve on Performance and Muscle Activity in Men and Women During High-Intensity, High-Volume Resistance Training” published on December 1, 2021 in Journal of Strength and Conditioning Research.

Knee sleeves are tight-fitting cloth tubes usually made of neoprene that you wear over your knees while you lift weights.

They’re popular among powerlifters and recreational weightlifters, who feel that they help them lift a little extra weight during exercises like the back squat, front squat, and lunge by keeping their joints warm and stable. Many wackadoodle powerlifters intentionally buy knee sleeves so tight-fitting they need a friend to help pull them on, hoping this will add extra “rebound” to their squat. 

That’s the idea anyway, but how much do they actually help? 

Scientists at the University of Rhode Island set about testing this idea by having 20 experienced male and female weightlifters do 6 sets of the leg press to failure at 80% of one-rep max with 3 minutes rest between sets with and without knee sleeves.

The researchers measured the weightlifters’ peak and average power, muscle activation, RPE, heart rate, blood lactate, and total reps performed in each set, but found essentially no difference when people did or didn’t wear knee sleeves. (There were some minor variations in blood lactate levels and muscle activation, but nothing significant). 

This doesn’t mean knee sleeves are entirely useless, though. Other research shows they can improve joint comfort and stability and increase coordination, which may improve your form (and they seem to be most helpful when squatting with low reps). 

If we connect the dots of these different studies, it’s fair to say that knee sleeves probably don’t directly improve performance in the same way wearing a belt does. Instead, they may help you feel more balanced and stable, which could allow you to use heavier weights with good form.

TL;DR: Knee sleeves don’t directly improve performance on lower-body exercises, but they may indirectly improve your performance by helping you feel more balanced and stable. 

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Want to reduce visceral fat? Just lift weights.

Source: “Effect of Resistance Training with and without Caloric Restriction on Visceral Fat: A Systematic Review and Meta-Analysis” published on May 16, 2021 in Obesity Reviews.

A 2012 systematic review and meta-analysis conducted by scientists at the University of Sydney concluded that weightlifting is ineffective at reducing visceral fat—the fat that surrounds your internal organs and is most associated with your risk of all-cause mortality. 

As a result of this review and others like it, many health authorities recommend calorie restriction and cardio as the ticket for rooting out visceral fat, and have pigeonholed weightlifting as purely beneficial for building muscle.

One everpresent confounder in weight loss research, though, including this study, is diet. Some of the studies included in this meta analysis had people maintain a calorie deficit and some didn’t, which muddles the results. For example, if someone follows an aggressive weight loss diet, lifting weights may not offer that much additional benefit in terms of visceral fat loss, but would obviously still be beneficial for improving overall health and body composition.

Taking these issues into account, scientists at the University of Tehran conducted another systematic review and meta-analysis on the effects of weightlifting and visceral fat loss.

They analyzed 34 studies involving a total of 2,285 participants. About a third of the studies compared weightlifting+calorie restriction to calorie restriction alone, and the rest compared weightlifters to non-weightlifters. 

And to the pleasure of weightlifting maximalists like myself, the results showed that jacking steel significantly reduced visceral fat in obese, non-obese, middle-aged, and elderly participants who weren’t restricting their calories.

In studies where people were restricting their calories, lifting weights didn’t seem to offer much additional benefit, but this may be due to some statistical knottiness rather than a limitation of weightlifting, per se. 

Two studies were primarily responsible for this finding, and in both cases, the groups that only restricted calories (and didn’t lift weights) started the study with about 50% more visceral fat than those who did lift. In both studies, the weightlifting+calorie restriction and calorie-restriction-only groups also lost the same amount of visceral fat relatively speaking (~30%), but the calorie-restriction-only groups lost more in a relative sense since they started out a lot chubbier. 

This last point is moot, really, since there are many, many other reasons to lift weights aside from a slight boost in visceral fat burning (bigger, stronger muscles and bones, longer life expectancy, and improved glucose control, quality of life, and so on).  

What’s the best way to reduce visceral fat, then?

If you want to maximize visceral fat loss (or fat loss in general), combining weightlifting, calorie restriction, sufficient protein, and cardio is the winning formula. And if you want an exercise program and diet plan that works synergistically to help you gain muscle, lose fat, and get healthy fast, then check out my best-selling fitness books Bigger Leaner Stronger for men, and Thinner Leaner Stronger for women. (If you aren’t sure which book is right for you, take this quiz and find the perfect Mike Matthews book and program for you).

TL;DR: Although some health experts pooh-pooh weightlifting for losing visceral fat, research shows it will likely help. 

Protein isn’t “less anabolic” for older folks.

Source: “Protein Requirements for Master Athletes: Just Older Versions of Their Younger Selves” published on September 13, 2021 in Sports Medicine.

Some researchers have put forward the idea that your body becomes desensitized to the muscle-building effects of protein as you get older.

Specifically, some studies have found that people getting long in the tooth don’t enjoy as much muscle protein synthesis as younger folks after eating a protein-rich meal, and that they have to eat a lot more protein to compensate for this (some studies say as much as ~50% more).

Scientists aren’t sure why this is, although a few potential culprits include:

  • Chronic inflammation
  • Reduced amino acid delivery to the muscles
  • Reduced ribosomal protein content or activity (ribosomes are responsible for protein synthesis)

It turns out, though, that this may be a case of correlation being confused with causation. 

According to this narrative review published in the journal Sports Medicine, the reason many old folks don’t benefit as much from eating protein isn’t because they’re getting on in years, but because they’ve spent too many years not getting in the gym.  

While there isn’t any direct evidence to support this theory, the author points to studies showing that older Dutch men (who typically walk or cycle two hours per day on average) enjoy a similar anabolic response to younger folks, probably because of their high levels of physical activity.

Other research also shows that the biological milieu associated with “anabolic resistance” (an impaired ability to gain muscle) is significantly reduced or absent in older people who train regularly.

For instance, couch potatoes generally have worse blood flow and capillary density in their muscles, which is associated with fewer satellite cells (which aid muscle growth), less nutrient delivery to muscles, and reduced muscle protein synthesis versus folks who work out consistently.

What’s more, research also shows that older trainees have less systemic inflammation than untrained middle-agers.

At bottom, this review shows that before the age of 60, and perhaps even beyond, our bodies don’t slip into decrepitude unless we allow them to. And if you want a fitness program that’s specifically designed to help middle-agers and golden-agers get in the best shape of their life, check out my newest fitness book for men and women, Muscle for Life.

TL;DR: As sedentary people age, their bodies don’t build as much muscle in response to eating protein, but this isn’t true of people who stay active. Keep lifting weights!

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Don’t expect caffeine to kill your appetite.

Source: “Caffeine Transiently Affects Food Intake at Breakfast” published on July 19, 2018 in Journal of the Academy of Nutrition and Dietetics.

Caffeine is one of the most popular fat loss supplements for several reasons.

It increases metabolic rate, improves your strength, endurance, and power (which tend to dip when dieting) and helps offset some of the ennui that sets in when you restrict calories. 

Anecdotally, many people find it takes the edge off their appetite, too, but this last point isn’t well studied.

To take a stab at this quandary, scientists at SUNY University had 50 participants aged 18-to-50 years old report to their lab once per week for 3 weeks and drink a glass of cold juice containing either zero, one, or three mg of caffeine per kg of body weight (this would be the equivalent of about one or three strong cups of coffee for the caffeinated drinks). 

Thirty minutes after drinking their jitter juice, the researchers invited the participants to eat as much as they wanted from a breakfast buffet, and surreptitiously jotted down how many calories they ate. 

The researchers also had the participants rate several dimensions of their appetite (hunger, fullness, thirst, desire to eat) before drinking their juice, 30 minutes after, and immediately before the breakfast using a scale from 1-to-5, with 1 being “not at all” and 5 being “extremely.”

After breakfast, the participants were allowed to leave the lab and resume their normal eating habits, but they had to write down everything they ate for the rest of the day.

The results showed that caffeine didn’t seem to have much of an impact on the participant’s calorie intake. They all ate about the same number of total calories regardless of whether they had caffeine before breakfast or not, and didn’t report feeling less hungry after consuming caffeine. 

Strangely, most people ate a bit less at breakfast after they took the smaller dose of caffeine (1 mg/kg vs 3 mg/kg), but also ate a little more throughout the rest of the day, wiping out this small benefit. 

Before we cut caffeine from our fat loss supplement roster, though, we need to put these results in context.

Most studies on this topic have found that caffeine only modestly reduces appetite for a brief period, and that people generally eat more after this benefit wanes. Of course, this is true of all fat loss supplements and forms of exercise for that matter–they only help if you control your calories. Thus, you can’t expect to knock back a few shots of espresso and start shedding pounds without changing anything about your diet. 

If you do control your calories, though, then caffeine could still help you achieve your weight loss goals in a small but meaningful way.

This would be in addition to all of the benefits of caffeine you saw a moment ago, such as increased metabolic rate, improved workout performance, higher energy levels, and more. Just don’t expect it to make a major dent in your appetite.

If you’d like a 100% natural and delicious source of caffeine that also contains five other ingredients that will boost your workout performance, try Pulse.

(And if you’re wondering what other supplements might be a good fit for your budget, circumstances, and goals, then take the Legion Supplement Finder Quiz! In less than a minute, it’ll tell you exactly what supplements are right for you. Click here to check it out.)

One more point: I’d also be curious if there might be any interactions between caffeine, diet, and exercise on appetite. While caffeine doesn’t seem to reduce appetite much on its own, many people find it helps them eat less, and it’s possible that it works better when combined with a proper diet and exercise program. Just idle speculation for now, but an interesting question to ponder.

TL;DR: Caffeine is only a mild appetite suppressant, and this effect wears off quickly, but it still offers a number of other weight loss benefits that make it worth taking.

+ Scientific References