You already know how important a good pre-workout is to your training success. When it comes to increased energy, elevated focus, great pumps, and improved endurance, the right pre-workout can make all the difference in your performance.
For some of us, the right pre-workout is one that’s natural. After all, who wants artificial ingredients in their supplements? Especially when you can use a natural product that gives you everything you’re looking for.
There are not a lot of quality choices out there. Still, I dug deep to give you the details on the best natural pre-workout supplements on the market.
Let’s get started!
In a Hurry? Here is our top pick:
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It should be no surprise that NutraBio PRE Natural is what a real pre-workout is all about. There are no low-dosed proprietary blends here.
PRE Natural features extreme label transparency and effective dosing. We’re looking at a well-dosed four-complex formula that is nothing short of exceptional.
First, there are two forms of creatine. One is PharmaPure™ monohydrate, dosed at 3 grams. The other is MagnaPower®, which is creatine bound to magnesium. This is dosed at 500 mg.
There’s also betaine anhydrous. Both creatine and betaine promote cell volume or water-based pumps. Both of these ingredients also promote strength and power.
Next, CarnoSyn® beta-alanine is included to reduce muscle fatigue and enhance endurance.
From there, NutraBio PRE Natural includes citrulline, agmatine, and Nitrosigine® for increasing blood flow and promoting great muscle pumps. The increased blood flow is a function of increased nitric oxide (or N.O.) levels. This in turn relaxes blood vessels so blood can flow more freely.
For a good energy boost and elevated mental focus, there’s 350mg of PurCaf™ organic caffeine. Plus, there’s slower-releasing Infinergy™ DiCaffeine malate. It’s complexed with TheaPure™ theanine to help smooth the effects.
For focus, there’s tyrosine, choline, and huperzine-a.
Finally, for hydration, there’s calcium and sodium plus 2 grams of taurine. The downside? Creatine is dosed at the low end of the effective range.
No matter, the exceptional formula, and clear labeling make NutraBio PRE Natural the best overall natural pre-workout formula.
- Full Label Transparency
- Effectively Dosed
- The Best Pre-Workout Powder Bar None
- Creatine Is Dosed At The Bottom Of The Effective Range
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The Genius Brand Genius Pre is a balanced stim-free natural pre-workout supplement. It’s a good formula with natural ingredients that target pumps, focus, endurance, and strength.
Genius Pre emphasizes focus ingredients. In fact, it is one of the things that sets The Genius Brand apart.
There are several effectively dosed focus-enhancing ingredients, such as tyrosine (1 gram), AlphaSize® alpha gpc (600 mg), rhodiola (100 mg), and huperzine-a (10 mg).
There’s also CarnoSyn® beta-alanine to reduce fatigue and improve training endurance.
For pumps, there’s betaine, taurine, arginine, and citrulline malate. This combination promotes pumps in two ways: by pulling water into the muscles and by increasing N. O. levels. There are two ingredients you don’t see in a lot of pre-workouts. They are elevATP® and HICA.
First, elevATP® increases the body’s ATP production for more training energy. Next, HICA is a leucine metabolite that helps build muscle.
Overall, Genius Pre is one of the best pre-workout supplements available. Yet, not enough to topple NutraBio, our best overall pick.
Still, if you’re looking for a balanced stim-free pre-workout formula with a heavy emphasis on focus, this is the best natural Pre for you.
- Complete Formula
- Caffeine Free
- Good Focus Complex
- Fully Disclosed Label
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NutriChem Labs Nature’s Pump Plant-Based Pre-Workout is 100% natural. Plus, it’s vegan-friendly. This pre-workout supplement mainly uses patented plant-based ingredients that provide clean, sustainable, and effective results.
This is a four-complex pre-workout formula that provides exactly what you’d expect: pumps, performance, energy, and hydration.
Some of the ingredients may seem to be low in dosage. Yet they are clinically dosed based on the research supporting each of the patented ingredients.
For example, S7™ is a plant-based pump complex that has been shown to produce an impressive 230% circulating nitric oxide. That’s from a mere 50 mg!
This type of innovative approach is carried throughout this pre-workout formula with such ingredients as beet juice and InstAminos® BCAAs.
There’s also VegiSURGE® natural caffeine, Neurofactor® coffee fruit extract, and Aquamin minerals.
This makes NutriChem Labs Nature’s Pre-Workout the best pre-workout powder using only plant-sourced ingredients.
- Vegan Friendly, All Natural Formula
- Features 6 Patented Ingredients
- More Bioavailable Than Other Pre-Workout Supplements
- Only 20 Servings
- Caffeine May Be Too Low For Some Users
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Legion Athletics Legion Pulse is a simple pre-workout that contains just nine ingredients. That includes the electrolyte minerals calcium, potassium, and sodium.
It’s well dosed and includes CarnoSyn® beta-alanine, overdosed at 3.6 grams. There is also betaine and citrulline malate, which is well-dosed at 8 grams.
For an energy boost, there’s 350 mg of caffeine anhydrous.
Legion Pulse also includes theanine to help smooth out the harshness of caffeine. To support focus, there’s also 300 mg AlphaSize® alpha-gpc.
If this pre-workout supplement has a downside, it’s a lack of ingredients. If you think about it, most ingredients work best when they are part of a complex.
Is a lot of caffeine enough for energy? Or is it better to feature several properly dosed stimulants with caffeine? For some users, it’s better to have several compounds working together toward one goal. That makes more sense than just one compound doing all the work.
The other downside is that this product contains several sweeteners. It may be too sweet, natural sweeteners or not.
Still, the nutrient profile makes Legion Pulse the best natural pre-workout for users seeking a no-frills formula.
- Only 9 Total Ingredients
- Several Overdosed Ingredients
- Fully Disclosed Label
- Only 21 Servings
- May Be Too Sweet For Some Users
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Nutrex Research Outlift Natural Pre-Workout may seem like a fairly simple pre-workout supplement. Yet, it’s one of the few pre-workout supplements in this guide that contains a 6-gram BCAA (branched-chain amino acid) complex. Branched Chain Amino Acids are critical because they support protein synthesis.
Beyond that, it’s a well-rounded pre-workout powder. It contains a total of ten pre-workout ingredients that are fully disclosed and effectively dosed.
The formula includes citrulline malate and CarnoSyn® beta-alanine. Plus, there’s patented Creapure® creatine monohydrate.
It also contains 350 mg of caffeine. Yet, unlike other pre-workout supplements, there’s nothing to smooth out caffeine’s effects.
That means that there may be too much caffeine for some users.
Still, the 6-gram BCAA complex makes Nutrex Research Outlift All-Natural the best natural pre-workout that supports protein synthesis.
- Clinically Dosed
- Fully Disclosed
- Features a 6g BCAA Complex For Protein Synthesis
- May Not Contain Enough Stimulants For Some Users
- May Be Too High In Caffeine – Nothing To Smooth Its Effects
- May Not Be Enough Focus Ingredients For Some Users
- Only 20 Servings
Transparent Labs Stim-Free Pre-Workout is a complete pump, focus, and endurance pre-workout that features extreme label transparency.
To increase endurance and boost performance, there’s beta-alanine and BetaPure™ betaine anhydrous. Also, there is elevATP®. This ingredient promotes ATP production for more muscular energy.
This is another pre-workout with branched-chain amino acids. There are 4 grams of BCAAs to help the body maintain an anabolic state during your training sessions.
For pumps, there’s citrulline malate. This is dosed at 6 grams, and it’s the only pump ingredient. Since this blend results in only 3 grams of pure citrulline, your pump may suffer a little. Still, there is betaine, which will promote cell volume.
This formula contains well-chosen, well-dosed natural ingredients. They are in a fully transparent label to make it the best stim-free natural pre-workout supplement.
- Fully Disclosed Label
- Effectively Dosed
- Complete Formula
- Gluten-Free – non-GMO
- Focus Complex Could Be Stronger
- Creatine Could Be Dosed A Little Higher
Transparent Labs Pre-Series Bulk Pre-Workout is an interesting deviation from their Stim-Free version.
The primary difference is that this version contains a testosterone complex that consists of 75 mcg of vitamin D3, 30 mg of zinc, and 3 mg of boron. This complex is designed to support the growth of lean muscle.
There’s also a mild 180 mg caffeine anhydrous and 30 mg Infinergy™ Diaffeine Malate to boost energy levels. It’s supported by 50 mg of theobromine, which helps smooth the effects of caffeine.
Like Pre-Series Stim-Free, there are BCAAs, beta-alanine, and BetaPure™ betaine anhydrous.
Also, like their Stim-Free, there are 6 grams of citrulline malate. Both betaine and, in this case, taurine, promote cell volume. Yet, another N.O. booster would make this pre-workout even better.
Still, Transparent Labs Pre-Series Bulk is the best natural pre-workout to help build muscle.
- Fully Disclosed Label
- Effectively Dosed
- Features Testosterone Boosting Ingredients
- Creatine Taken Out Of This Version
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Naked Nutrition Naked Energy is a pre-workout that’s ideal for beginners. There are only four active ingredients, plus B vitamins and calcium. That gives beginners good physical and mental performance without overloading them with ingredients.
Naked Energy is designed to enhance athletic performance with CarnoSyn® beta-alanine, creatine, and Ajipure® arginine. Boost energy with 200 mg of caffeine from unroasted coffee beans.
Naked Energy is vegan-friendly, gluten-free, and non-GMO. It’s also unflavored so it can be mixed with water, a sports drink, Naked BCAAs, or the beverage of your choice.
If there is a downside, some of the ingredients are dosed low. Advanced users will want to double-scoop it. At two scoops, that’s 400 mg of caffeine. That’s too high of a caffeine intake for beginners.
Still, when used as directed, this is a good choice for new users. Finally, it’s 3rd party tested for quality, potency, and purity.
- Good Choice For Beginners
- 200 mg Natural Caffeine For A Natural Energy Boost
- Only 4 Active Ingredients – Vitamin C – B Vitamins – Calcium
- Gluten-Free – non-GMO
- Some Ingredients Are Low-Dosed
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Here’s an exclusive two-stage pre-workout. Old School Vintage Blast is designed to let you finish your training session strong.
Stage 1 lets you get a good start with citrulline malate and arginine. There’s also l-carnitine l-tartrate, and 150 mg of caffeine.
Stage 2 helps you finish strong with 100 mg of microencapsulated caffeine. This is a special type of caffeine that releases slowly. This keeps energy levels up and avoids the usual mid-workout crash.
A unique ingredient is d-aspartic acid. This is a testosterone-boosting ingredient. It’s not something you usually see in a pre-workout.
One downside is that several ingredients are dosed low. Some experienced users will probably double scoop it for more effective doses.
Even so, the label is fully disclosed. Also, Vintage Blast contains zero artificial sweeteners, flavors, or colors. It’s sweetened with stevia. It’s also keto and vegetarian-friendly.
Finally, Old School Vintage Blast is 3rd party tested for quality.
- Unique Two-Stage Pre-Workout
- Two Forms Of Caffeine
- Fully Open Label
- Several Ingredients Are Dosed Low – Advanced Users May Want To Double-Scoop It
Some users do not want to use pre-workouts, or anything else, with artificial ingredients.
For these users, a natural pre-workout powder provides a cleaner, healthier option.
For pre-workout products to qualify as “natural”, there should be no artificial anything. Any pre-workout supplements you’re considering should be 100% naturally sweetened, flavored, and colored.
Next, there’s no sense in buying a product that’s dramatically underdosed, or one that uses proprietary blends.
Think about it: a pre-workout needs to provide energy, focus, pumps, and performance. Exceptional products also support hydration.
For natural pre-workouts to work well, they should contain effectively dosed natural ingredients that support each of these areas. Not to mention, you should be able to see how much of each ingredient you’re getting.
That’s how the best pre-workout supplements enhance physical and mental performance to help you reach your fitness goals.
When most users think of pre-workouts, they typically think of stimulants. Yet a solid pre-workout should be balanced, not over-loaded with stims.
Natural pre-workouts that are effectively dosed provide the benefits:
- Increased Energy & Focus – This is one of the main benefits of good pre-workout supplements. Most of the pre-workout supplements in this guide depend on caffeine for energy.Also, there are several ingredients that elevate focus. These ingredients help create an “in the training zone” mindset.
- Increased Motivation – Besides common pre-workout ingredients for focus, some pre-workouts improve your motivation to train. These are ingredients that help you achieve the mind-muscle connection.
- Promotes Great Pumps – For many, if not all users, the pump is important. After all, what’s better than looking in the mirror and seeing huge, pumped muscles? The best pre-workout formulas contain several good pump ingredients.As noted, there are two kinds of pumps. First, there’s nitric oxide-based caused by increased blood flow. Second, there are water-based caused by ingredients that pull water into the muscles.
- Reduced Fatigue And Improved Endurance – The main ingredient that slows fatigue is beta-alanine. This ingredient extends endurance. You will have more intense workouts if you can knock out some more reps. This means you’ll increase muscle growth. That fact alone makes a pre-workout worth using.
- Enhanced Strength & Power Output – Some users try to progressively increase strength when they train. This in turn will lead to more muscle mass. The best natural pre-workouts help do this by containing strength-enhancing ingredients.
- Enhances Hydration – Staying hydrated helps you achieve peak performance. That’s because it prevents the effects of dehydration, such as muscle cramps. Other symptoms of dehydration include decreased cognitive function and muscular fatigue.Many pre-workouts contain electrolytes for hydration. Also, cell volumizing ingredients such as creatine, betaine, and taurine pull water into the muscle cells. This, in turn, improves hydration.
Virtually every pre-workout contains at least one nitric oxide, or N.O., boosting ingredient. These ingredients increase blood flow, which promotes great pumps.
There are several good N. O. boosting ingredients. Besides citrulline, these include S7™, agmatine sulfate, norvaline, Vaso-6™, Nitrosigine®, and arginine.
The best pre-workouts contain several of these. An example of this is our best overall, NutraBio PRE Natural. This product also has the highest citrulline content.
Citrulline is arguably the best N.O. boosting ingredient. It is an amino acid that changes into arginine once it’s in the body. It needs to be understood that back when N.O.-boosting products first came out, the only ingredient was arginine.
So why not just feature arginine as the only pump ingredient? The reason is that citrulline absorbs better than arginine. Not only that, citrulline does a better job of stimulating N.O. production.
In contrast, some pre-workouts contain citrulline malate.
This is a blend of citrulline and malic acid. That’s an ingredient that has a role in ATP production. ATP is what your muscles use for energy. Therefore, citrulline malate supports endurance and pumps. Still, the amount of pure citrulline you’ll get in any pre-workouts that use this form will be cut in half. (1, 2)
We mentioned other N.O. boosters, let’s take a closer look at these.
Agmatine Sulfate – This is a common ingredient that increases blood flow and stimulates increased N.O. production. Plus, it enhances muscle glycogen storage.
Nitrosigine™ – This is a patented blend of arginine and silicon that increases levels of nitric oxide. It’s known for its long-lasting effects and can last in the body for 6 hours. Also, it’s fast-acting and begins to act within 30 minutes of taking.
S7™ – This is a low-dose, patented blend consisting of 7 herbal ingredients. They are green coffee bean extract, green tea extract, turmeric, tart cherry, blueberry, broccoli, and kale. S7™ stimulates your body’s production of N.O.
Vaso-6™ – This is a patented form of green tea that stimulates N.O. levels in the body. Like other branded ingredients, there are studies that support its effectiveness.
This is one of those natural ingredients that are as famous for its side effects as its performance effects.
Not that it has serious side effects. It will cause short-term, harmless tingling and itching. Some users do not like this effect. In fact, some sports nutrition manufacturers under-dose it for that reason.
Yet, companies such as Huge Supplements, to name a brand, advertise that they have the strongest pre-workout. For this reason, they overdose it. For reference, the clinical dose is 3.2 grams. Huge Supplements Wrecked claims to be the strongest pre-workout and dose it at 3.5 grams.
Why is this product not in our guide? Because it’s not natural.
How does beta-alanine work? First, it buffers acid buildup in the body but does it indirectly. It does this by increasing levels of carnosine in your body. It’s actually carnosine that buffers lactic acid.
To get the full effects, you need to use the full clinical dose. At that dose, it will help you perform more reps and sets. (3)
There can be no doubt that caffeine is the most widely used energy ingredient on the planet. You’ll find caffeine as the main stimulant in your energy drink and pre-workout. Other supplements with caffeine will include fat-burning products. There are also formulas based on amino acids with caffeine for energy.
Caffeine is always the first choice for energy, focus, and alertness. It also stimulates thermogenesis which burns fat. Finally, it improves in-the-gym performance.
Here’s a look at the usual types of caffeine you will find in pre-workout supplements. (4, 5)
- Caffeine Anhydrous – This is caffeine with all the moisture removed. This makes the caffeine more concentrated.
- PurCaf™ Organic Caffeine – This is a highly water-soluble powdered extract standardized to 95% pure caffeine from green coffee beans.
- Infinergy® DiCaffeine Malate – This is a trademarked complex of 75% caffeine and 25% malic acid. The caffeine is absorbed more slowly, reducing jitters and any “crash” effect.
- VegiSURGE® Natural Caffeine – This is a natural source of caffeine from green tea or green coffee beans.
- Neurofactor® Coffee Fruit Extract – This is coffee made from the entire coffee plant.
- Green Tea – This is a natural form of caffeine that also contains the antioxidant EGCG.
- Guarana – Here’s a form of caffeine that originates in Brazil.
The best pre-workouts offer an energy complex that contains other stimulants. These usually include theanine or theobromine, which smooths the harsh effects of caffeine.
None of the pre-workouts in this guide contain exotic stimulants beyond those mentioned here.
In any pre-workout, the focus complex goes hand in hand with stimulants. Typically, a focus complex includes choline and tyrosine. Some products may include additional focus ingredients.
Here’s a closer look at several common focus ingredients.
Tyrosine – This is an amino acid with a key role in the manufacture of dopamine and norepinephrine. These are two neurotransmitters that improve focus. They also improve mood and relieve anxiety.
Choline – This ingredient is involved in the synthesis of phospholipids. These compounds support cell signaling and cell structural integrity. Also, choline is a precursor of acetylcholine. That’s a neurotransmitter involved in mood elevation.
Perhaps most important, choline is involved in promoting a mind-muscle connection. This is the mental connection you should achieve with your working muscles when you train.
Alpha GPC – This ingredient occurs naturally in the brain. It’s 40% choline by weight. Alpha GPC is superior to other forms of choline. That’s due to its enhanced absorption.
Huperzine A – This is an ingredient that improves focus and indirectly affects acetylcholine levels. It works by blocking the enzyme acetylcholinesterase, which controls the breakdown of acetylcholine. By inhibiting this enzyme, more acetylcholine becomes available.
These focus ingredients help improve mental health as it relates to working out. Beyond focus and motivation, these natural ingredients improve memory and mood.
This means you will achieve optimal mental function when using pre-workouts containing these natural ingredients. (6, 7, 8, 9)
This ingredient is naturally occurring in beets. Betaine is also an osmolyte. That means it helps pull water into the muscles. This creates cell volume or water-based pumps.
Probably, most people think of pumps in terms of levels of N.O. Yet, the best pre-workouts combine osmolytes like betaine anhydrous with N. O. boosters such as citrulline.
Plus, betaine, like creatine, supports strength and power. In fact, it’s believed to work much like creatine does. The two are often combined in pre-workouts and other natural supplements. (10)
Taurine is common in many energy drinks. Like betaine and creatine, taurine is an osmolyte. It also supports cognitive function. Taurine has proven itself and become a common ingredient in many pre-workouts.
This is the most studied of any natural ingredient in this guide. Creatine can be found in red meats, chicken, and fish. Plus, the body produces small amounts naturally.
Still, to get the full benefits of cell volume, muscle mass, and more strength, you need extra supplementation.
Many pre-workouts do not contain creatine. This is because some users cycle it. If it’s in your pre-workout, you can’t properly cycle it.
Even so, it should be in your pre-workout. Cycling is not a requirement, so you can take it daily. Even if you use it separately, there’s nothing wrong with a little extra.
Our best overall, NutraBio Pre Natural, has over 3g of two types of creatine.
Like creatine, only a handful of pre-workouts contain BCAAs or other aminos. There are nine total essential aminos, and the BCAAs make up three.
The reason they should be in your pre-workout is because the BCAAs stimulate protein synthesis. This is a direct trigger for the growth of lean muscle.
The best example of a pre-workout with BCAAs is Nutrex 100% Natural Outlift.
One thing that’s evident is that many of the products in this guide use branded ingredients. These are ingredients that are backed by several studies. Plus, they are higher quality than non-branded ingredients. Finally, absorption is also superior over their generic counterparts.
One important aspect of a natural pre-workout is that it does not contain artificial sweeteners. Also, no pre-workout should contain sugar. Look for these natural sweeteners instead:
- Stevia – This is a zero-calorie sweetener that comes from the stevia plant. Like any natural sweetener, this does not increase blood sugar or cause insulin spikes.
- Monk Fruit – Like stevia, monk fruit contains zero calories and does not raise blood sugar.
- Erythritol – This is a sugar alcohol that does not cause a rise in blood sugar. It may cause gastric discomfort at higher doses.
- Coconut Sugar – This is a sugar that will impact blood sugar levels to a point. Yet it has a lower glycemic index rating than regular sugar. This ingredient is on the edge of acceptable natural sweeteners.
- Sugar – Most health-conscious individuals will want to avoid sugar. The average pre-workout will not include any meaningful sugar content.
- Artificial Sweeteners – For many users, sweeteners such as sucralose are avoided. An all-natural product should never contain this ingredient.
- Artificial Flavors – Some pre-workouts combine natural and artificial flavoring. It’s only natural if it uses natural flavoring exclusively.
- Artificial Colors – Any coloring should be natural. Usually, artificial colors mean dye has been added.
Are you using a stim-based natural pre-workout? Check first with a medical professional, especially if you have any pre-existing medical conditions.
Next, don’t take it late in the day. Otherwise, your sleep quality will be impaired. Most users use their stim-based product earlier in the day. If they have to train later, they switch over to a non-stim product.
From there, make sure you follow all label directions.
Most natural pre-workouts suggest taking one serving 15-30 minutes before you train. Are you using a high-stim pre-workout? Have you experienced gastric discomfort? Drink more water with your serving. The extra water will ease potential gastric effects.
A pre-workout by itself is generally not designed for fat burning. Still, stimulants promote thermogenesis. This is especially true of caffeine.
If you want intense workouts, yes. You have to be at the top of your game to achieve peak performance. Plus, you will have better pumps, better focus, more energy, and more strength by using a pre-workout.
Some pre-workouts, such as Naked Nutrition Naked Energy, are unflavored. Others, such as NutraBio Pre Natural, come in delicious natural flavors such as green apple and sweet tea.
As you can see, there are not a lot of good choices available in a natural pre-workout.
If you’re looking for the best overall, look no further than NutraBio PRE Natural. It’s a complete formula that’s clinically dosed and fully disclosed. Every training need is covered by PRE Natural.
If you’re looking for a stim-free option, Transparent Labs Pre-Series is a well-rounded formula with superior label transparency.
For users that want a vegan-friendly option, there’s NutriChem Labs Nature’s Pump. This is a fully disclosed, effectively dosed formula that uses only plant-based ingredients.
These are the best natural pre-workouts on the market. Now it’s up to you to choose the one that fits your needs the most.
- Figueroa, A., Wong, A., Jaime, S. J., & Gonzales, J. U. (2017, January). Influence of L-citrulline and watermelon supplementation on vascular function and exercise performance. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/27749691
- Breuillard, C., Cynober, L., & Moinard, C. (2015, April). Citrulline and nitrogen homeostasis: an overview. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/25676932
- https://www.healthline.com/nutrition/beta-alanine-101
- https://www.healthline.com/nutrition/caffeine-and-exercise#section4
- Baggott, M, et. al; “Psychopharmacology of theobromine in healthy volunteers”; Psychopharmacology; February 2013; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3672386/
- Poly C, Massaro JM, Seshadri S, et al; “The relation of dietary choline to cognitive performance and white-matter hyperintensity in the Framingham Offspring Cohort”; The American Journal of Clinical Nutrition; 2011; 94(6):1584-1591; https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3252552/
- Deijen JB, Orlebeke JF; “Effect of tyrosine on cognitive function and blood pressure under stress”; Brain Res Bull. (1994); http://www.ncbi.nlm.nih.gov/pubmed/8293316
- Ved, H; “Huperzine A, a potential therapeutic agent for dementia, reduces neuronal cell death caused by glutamate”; Division of Biochemistry, Walter Reed Army Institute of Research; 1997; http://www.ncbi.nlm.nih.gov/pubmed/9141073
- https://www.ncbi.nlm.nih.gov/pubmed/23236741
- https://academic.oup.com/ajcn/article/80/3/539/4690529