How Much Protein Do You Need to Build Muscle?
Introduction
Protein is often hailed as the cornerstone of muscle building, with gym-goers and athletes alike placing a huge amount of importance on their protein intake. Yet, amidst the sea of advice and rise of protein-packed products, misconceptions about protein intake are still common. There is a tonne of confusion around how much, the timing and the type you consume meaning that you could be missing out making progress on your muscle building journey. Understanding the role protein plays in building muscle is crucial for anyone looking to enhance their physique and optimise their workouts. In this article, we will equip you with the evidence on how much protein is needed to build muscle.
Muscle Growth and Protein
Muscle growth, or hypertrophy, is a complex process initiated by the stress and strain of resistance training. This physical demand causes microscopic damage to muscle fibres, necessitating repair and growth (1). Amino acids, the building blocks of protein, are critical in this repair process, facilitating the repair of muscle tissue and the synthesis of new muscle proteins. Thus, protein is foundation of muscle recovery and growth.
Mechanism of Muscle Growth
Muscle growth involves a complex interplay of cellular processes (2). Muscle hypertrophy can be considered distinct and separate from muscle hyperplasia.
- Hypertrophy: Contractile elements enlarge and the extracellular matrix expands to support growth
- Hyperplasia: An increase in the number of fibres within a muscle.
Hypertrophy is the primary process of increasing muscle growth in the body. Let’s break down the different types (3).
1. Myofibrillar Hypertrophy.
Myofibrillar hypertrophy involves the growth of muscle fibres, by increasing the number and density of myofibrils, which are the contractile units within muscle fibres responsible for force production.This type of hypertrophy is stimulated by lifting heavy weights that cause trauma and microtears in muscle fibres, prompting the body to overcompensate during recovery by increasing the volume and density of myofibrils.
2. Sarcoplasmic Hypertrophy
Sarcoplasmic hypertrophy involves an increase in the volume of sarcoplasmic fluid within muscle cells, which includes ATP, glycogen, creatine phosphate, and water. This type of hypertrophy increases muscle volume but does not increase the size or number of fibres, and thus does not result in improved performance.
3. Connective Tissue Hypertrophy:
Connective tissue hypertrophy refers to the growth and adaptation of connective tissues like tendons and ligaments in response to mechanical stress from resistance training. Strengthening connective tissues is crucial for providing structural support to muscles during intense workouts, reducing the risk of injuries associated with muscle growth and strength development.
Hypertrophy is thought to be mediated by the activity of satellite cells, which reside between the basal lamina and sarcolemma (4). These cells become active when a sufficient mechanical stimulus is imposed on skeletal muscle. Once activated, satellite cells proliferate and ultimately fuse to existing cells or among themselves to create new myofibres, providing the precursors needed for repair and subsequent growth of new muscle tissue.
Initiation Of Exercise-Induced Muscle Hypertrophy
It is hypothesised that 3 primary factors are responsible for initiating the hypertrophic response to resistance exercise (5):
- mechanical tension
- muscle damage
- metabolic stress
Mechanical tension refers to the force generated within muscle fibres during resistance exercise, leading to the activation of muscle fibres and the initiation of hypertrophic processes (2). Mechanical tension is a primary stimulus for muscle hypertrophy as it causes microtrauma within muscle fibres, triggering cellular responses that promote muscle growth and repair.
Muscle damage occurs when muscle fibres experience microtears due to intense physical activity, especially during resistance training. Muscle damage is a crucial component in initiating muscle hypertrophy as it activates satellite cells and inflammatory responses, leading to the repair and growth of muscle fibres (6).
Metabolic stress refers to the buildup of metabolites like lactate, hydrogen ions, and other byproducts of energy production during intense exercise. Metabolic stress plays a significant role in stimulating muscle hypertrophy by creating an environment within muscle cells that signals for adaptation and growth in response to the metabolic demands placed on them during exercise.
Now we have covered the science behind hypertrophy we need to include the next part of the puzzle, protein. To understand the relationship between protein and muscle growth, let’s start with muscle protein synthesis (MPS).
MPS is the process by which the body builds new proteins to repair and replace damaged muscle tissue. Several studies have highlighted the positive correlation between protein intake and MPS (7, 8). However, this is just one part of the equation. To grow muscle, MPS needs to outweigh muscle protein breakdown (MPB).
You may think that MPB is a negative thing, but protein breakdown is essential for removing damaged or unnecessary proteins from muscle tissue (9). When protein synthesis exceeds protein breakdown, a state known as positive protein balance is achieved, leading to muscle hypertrophy or growth. This delicate balance between protein synthesis and breakdown is fundamental in regulating muscle mass, strength, and overall muscle health.
The correct balance requires adequate protein intake. Training will increase both MPS and MPB but without sufficient protein, you won’t build muscle. Protein is the building block of muscle and in the next section we will cover exactly how much protein you require.
Protein Basics
In this section, we'll delve into the fundamentals of protein and its pivotal role in muscle growth and repair.
What is protein?
Protein is a one of the three macronutrients alongside fat and carbohydrates and contains 4 calories per gram. It is made up of amino acids, which are the building blocks of proteins. The body uses amino acids to build and repair muscles and bones, make hormones and enzymes, and utilises them as an energy source (10).
Amino acids: the building blocks of protein
An amino acid is a molecule that serves as the building block of proteins. There are 20 amino acids that the body needs to function correctly, with nine of them classified as essential amino acids (EAAs) because the body cannot produce them on its own and therefore must be obtained through the diet (11). Amino acids are crucial for various bodily functions, including muscle growth, tissue repair, and energy production.
Branched-Chain Amino Acids (BCAAs) are a subset of essential amino acids that play a significant role in muscle protein synthesis and growth (12). The three BCAAs are Valine, Leucine, and Isoleucine. These amino acids are particularly important for muscle recovery, reducing muscle soreness, preventing muscle damage during exercise, and promoting muscle growth. Leucine, in particular, has a significant anabolic effect and is crucial for stimulating muscle recovery through pathways like mTOR (mammalian target of rapamycin).To create new muscle protein, all the amino acids must be present in adequate amounts (especially the essential amino acids) so taking BCAA’s in the absence of other amino acids isn’t going to support muscle growth (13).
Dietary Protein Types
Protein is abundant in many foods and knowing which foods contain dietary protein will help you decide your food choices.
Some dietary protein types include:
- Lean meats like beef, lamb, veal, pork and venison.
- Poultry such as chicken, turkey and duck.
- Fish and seafood like fish, prawns, crab, lobster, mussels, oysters, scallops, and clams
- Eggs
- Dairy products including milk, yoghurt (especially Greek yoghurt), and cheese (especially cottage cheese).
- Protein powders such as whey, plant based protein and casein.
- Nuts and seeds such as almonds, pine nuts, walnuts, macadamias, hazelnuts, cashews, pumpkin seeds, sesame seeds, and sunflower seeds.
- Legumes and beans like all beans, lentils, chickpeas, split peas.
- Tofu, seitan and tempeh.
Complete proteins contain all nine essential amino acids in consistent amounts, while incomplete proteins lack one or more of these essential amino acids (14).
Complete Proteins:
1. Chicken
2. Beef
3. Fish
4. Dairy products
5. Quinoa
6. Soy
Incomplete Proteins:
1. Nuts
2. Seeds
3. Grains
4. Vegetables
If you are plant based you just have to be more aware of getting a variety of sources in your diet to ensure you are getting all the aminos acids you need. The Marchon Plant Protein contains a blend of pea protein isolate, brown rice protein isolate and faba bean protein concentrate to create a complete protein source.
Factors Influencing Protein Requirements
Protein needs are not one-size-fits-all; they fluctuate based on numerous factors.
- Age and Life Stage. Growing adolescents and elderly individuals have different protein needs due to their unique physiological states (15).
- Physical Activity Levels. More active individuals require more protein to repair and build muscle (16).
- Workout Intensity: The more intense the workout, the greater the muscle repair needs, increasing protein requirements.
- Goals: Whether bulking up, cutting fat, or maintaining muscle mass, each goal comes with distinct protein demands.
Protein Intake for Different Goals
The current international Recommended Dietary Allowance (RDA) for protein is 0.8 g per kg of body weight (bw), regardless of age (17). These recommendations are derived as a minimum amount to maintain nitrogen balance and are not optimised for physical activity level (PAL).
Note that, contrary to popular belief, the RDA doesn’t represent an optimal intake. Instead, it represents the minimum intake needed to prevent malnutrition. Unfortunately, the RDA for protein was determined from nitrogen balance studies, which require that people eat experimental diets for weeks before measurements are taken. This provides ample time for the body to adapt to low protein intakes by down-regulating processes that are not necessary for survival but are necessary for optimal health, such as sexual function, protein turnover and immune function (18).
The general equation for protein intake is a function of your body weight and your physical goals. The table below is a collection of the latest research for recommended protein intake in g/kg of bodyweight based on your goal, your output and current weight (19, 20, 21, 22).
In short, if you are within these ranges you will see progress. If you want to get more detailed we can break these recommendations down even further.
Protein needs for beginners in strength training
When you commit to doing strength training and you haven’t trained that much before or you have had a long break you can experience something called (in lay terms) newbie gains.
The mechanisms behind newbie gains involve several key factors that contribute to the rapid progress beginners make in strength and muscle mass during the initial phase of their training (23):
- Neurological Adaptations: In the first three months of dedicated training, newbie gains are partly attributed to significant increases in strength, with a substantial portion coming from neurological adaptations. These adaptations involve improvements in neuromuscular coordination and efficiency, allowing beginners to recruit muscle fibres more effectively during exercises.
- Muscle Protein Synthesis (MPS): Higher levels of muscle protein synthesis play a crucial role in newbie gains. The body responds to new training stimuli by increasing MPS, which is essential for repairing and building muscle tissue. Untrained individuals experience a robust spike in MPS in response to resistance training, contributing to muscle growth and adaptation.
- Hyperresponsiveness of Muscles: In the initial stages of training, muscles are hyperresponsive to new loads and intensity. This hyperresponsiveness leads to rapid increases in muscle size and contractile power as the body adapts to the stress of resistance training. Newbie gains are facilitated by this heightened sensitivity of muscles to training stimuli.
In light of this you can get away with a lower amount of protein due to the beneficial effects of strength training. Making sure you at 1.4g/kg/bw is crucial. However, having your intake sit around 1.8g/kg/bw would be advised. This will provide more amino acids to support recovery and muscle gain.
Protein requirements for someone bulking
Before we get into the specifics for protein, when it comes to bulking it is important to look at the timeline you want to achieve this in. It is a tradeoff between how much fat you want to gain compared to how quick you want to gain muscle.
The faster you want to build muscle, the more bodyfat you are likely to gain. The smaller the surplus, even though muscle growth will be slower, it will be accompanied by less fat gain. For example, here are the results of a recent study that over a 4 week period gave participants different calorie targets (higher energy intake vs lower energy intake) (24).
By gaining 0.5kg per week instead of 1kg, the participants cut their rate of muscle growth in half, but only gained a fifth as much fat. So even though their rate of muscle growth is only 50% as good, their ratio of muscle-to-fat gain is twice as good.
G1 - Higher energy intake G2 - Lower energy intake - Study link
In relation to protein here are some recommendations if you are looking to bulk (25).
- Consume ~0.4 g/kg body mass per meal to maximally stimulate MPS following a period of rest or exhaustive resistance exercise.
- Spacing protein-containing meals ~3–5 h throughout the day maximises MPS rates over the course of a 16 hour period.
- Practise pre-sleep protein ingestion (1–3 h prior to sleep) to offset declines in MPS that would occur during an overnight fasting period.
- To maximise muscle protein accretion with resistance exercise, daily protein intakes should be ~1.6 g/kg/day and up to 2.2 g/kg/day. This intake can be achieved by ingesting 3 meals, each containing ~0.53 g/kg protein, or 4 meals containing ~0.4g/kg protein.
Protein intake during fat loss phases
It is important to clarify that while shifting macronutrient ratios towards higher protein intake can aid in fat loss on a eucaloric diet (a diet matching calorie intake with expenditure), transitioning to a hypocaloric diet (consuming fewer calories than burned) is necessary for sustained weight loss.
Higher protein consumption is beneficial for preserving lean mass, particularly in lean individuals. For optimal body composition during calorie restriction, athletes following a hypocaloric diet are advised to consume 1.6–2.4 g/kg of body weight, with a preference towards the upper range as they become leaner or increase their caloric deficit through reduced food intake or increased exercise.
Recent studies suggest that lean resistance-trained athletes aiming to minimise muscle loss should target a protein intake of 2.3–3.1 g/kg, especially as leanness and caloric deficit rise (26). This recommendation is supported by the International Society of Sports Nutrition (ISSN) and bodybuilding contest preparation guidelines (27). These guidelines are tailored for individuals already at a relatively low body fat percentage seeking to shed additional fat while safeguarding muscle mass.
For those with overweight or obesity, meta-analyses propose a daily protein intake range of 1.2–1.5 g/kg to optimise fat loss. This recommendation aligns with the European Association for the Study of Obesity, which suggests up to 1.5 g/kg for elderly adults with obesity (28,29). It is essential to note that these guidelines are based on total body weight rather than lean mass or ideal body weight considerations.
Protein Timing and Distribution
The debate around protein timing—pre or post-workout—continues, but the consensus leans towards the importance of overall daily intake and consistent distribution throughout the day to optimise muscle protein synthesis. Below is a diagram depicting the hierarchy of importance.
You may have encountered people suggesting that around 30 grams of protein is the maximum amount assimilated from a single meal, with the excess considered 'wasted' and eliminated from the body (30). The origin of this protein absorption limit is challenging to pinpoint, likely stemming from early studies linking increased nitrogen excretion in urine with higher protein intake, initially interpreted as protein wastage (31).
The concept of a "protein intake ceiling" was recently challenged in a study that showed the ingestion of 100 g protein results in a greater and more prolonged (>12 h) anabolic response when compared to the ingestion of 25 g protein (32). This research should be taken with a hefty dose of context. The participants consumed the protein post exercise and the make up of the meal was 80% casein protein, which we know is slower digesting than other sources of protein (33). It does open up the possibility of consuming larger feeds of protein more infrequently and still maximising protein synthesis. However, until more research is done it is still advised to try and ensure that you get an even distribution in your meals across the day.
Protein Quality and Sources
Deriving protein from food is relatively easy if you know what foods contain protein. The bioavailability and amino acid profiles of these sources vary, influencing their efficacy in muscle building. A dietary protein’s quality is determined by its protein bioavailability and amino acid profile. Animal proteins often provide a complete amino acid profile, making them particularly effective for muscle synthesis.
Some plant based foods will contain an incomplete amino acid profile which means that to get all 20 amino acids, combining foods is necessary. Plants also contain trypsin inhibitors, phytates, tannins, and other antinutrients that prevent some of the protein you ingest from being digested (34). Antinutrients can be reduced, but not entirely removed, by cooking. Plant-protein powders, being mostly free of antinutrients, are about as bioavailable as animal protein (35). Our plant protein contains a blend of pea isolate, brown rice isolate and faba bean to provide a complete amino acid profile and 3g of leucine.
The Digestible Indispensable Amino Acid Score (DIAAS) is a protein quality method proposed in March 2013 by the Food and Agriculture Organisation to replace the current protein ranking standard, the Protein Digestibility Corrected Amino Acid Score (PDCAAS). Below are some common foods ranked on the DIAAS scale with 1.18 being the most bioavailable.
Adapted from Phillips, et al, 2017.
Balancing Macronutrients
Achieving the right macronutrient ratio is key to muscle gain. Protein is essential for building and repairing muscle tissue, with a recommended ratio of around 30% in the diet. Carbohydrates provide energy for workouts and muscle recovery, typically making up about 40% of the diet. Healthy fats, comprising approximately 30%, are important for hormone regulation and overall health. Balancing these macronutrients optimally supports muscle growth and recovery.
However, there is considerable nuance around this. Higher carbohydrate intakes (45-60%) of total daily intake have been shown to be beneficial for muscle growth compared to lower intakes (36, 37). The issue in the first study was that on a low carbohydrate diet the participants struggled to eat enough overall calories which will have impaired their muscle growth potential as we know that overall calories and protein are the two key nutritional components to a successful muscle building diet.
In summary it comes down to personal preference regarding carbohydrate and fats. However, be aware that lower carbohydrate intakes may make it harder to eat enough to gain muscle.
Protein Supplements
For those struggling to meet their protein needs through food alone, supplements like whey, casein, and plant-based proteins offer a convenient solution. They are not needed, you can derive all the protein you need from whole foods, however if somebody’s protein intake is fairly high, using a protein supplement may be useful.
Whey Protein
Whey protein is celebrated for its complete amino acid profile and rapid absorption, making it a favourite among those who exercise regularly for post-workout recovery. There are different types of whey protein.
- Whey Protein Concentrate (WPC): Contains around 70-80% protein and retains some lactose and fat, offering a creamy texture and good flavour.
- Whey Protein Isolate (WPI): Typically comprises 90% protein or higher, with lower levels of lactose and fat compared to concentrate, making it a purer form with fewer calories.
- Clear Whey Isolate: This type is an isolate that has been hydrolysed, resulting in a clear-looking shake with a finer texture and fruity flavors.
- Whey Protein Hydrolysate: This form can be either a concentrate or isolate that has undergone hydrolysation, making it easier to digest and absorb quickly, aiding in faster muscle recovery.
MARCHON Whey Protein Powder contains a blend of concentrate, isolate and hydrolysate to provide an incredible tasting complete protein.
Casein Protein
Casein is another milk-derived protein, however due to its structure, digests slower than whey providing a sustained release of amino acids. It is a complete protein source and is more commonly used before bed.
Studies show protein ingestion prior to sleep may represent an effective dietary strategy to inhibit muscle protein breakdown, stimulate muscle protein synthesis, facilitate the skeletal muscle adaptive response to exercise training and improve exercise training effectiveness (38, 39).
MARCHON Micellar Casein provides a convenient source of protein that can be used a shake or made into a mousse like dessert.
Plant-based Protein Powder
Plant-based protein powders offer a convenient solution to anyone who struggles with digesting a milk based protein or follows a plant based diet. A crucial element to look out for is to ensure there is a blend of proteins to ensure a complete amino acid profile is present. If it is a single ingredient such as pea or brown rice then it will have an incomplete amino acid profile.
MARCHON Plant Protein has a blend of pea isolate, brown rice isolate and faba bean to ensure it provides 24g of protein with 3g of leucine.
Conclusion
Understanding your protein needs is pivotal in the quest for muscle growth. It's not just about quantity; quality, timing, and distribution also play crucial roles. Tailoring your protein intake to your individual goals, lifestyle, and dietary preferences can optimise your muscle-building progress.
Frequently Asked Questions (FAQs)
Can I build muscle without increasing my protein intake?
Increasing protein intake is generally recommended for muscle growth, but total caloric intake and exercise regimen are also crucial factors. The most important factor is to eat at least 1.4g/kg/bw. Increasing protein beyond this to 2.4g/kg/bw may offer a benefit to increase muscle growth, but beyond this there is minimal extra benefit.
How soon after a workout should I consume protein?
Rushing to chug your protein shake immediately after your workout is not needed. Recent research proposed the theory that there isn’t so much an anabolic window, but more of a garage door of opportunity to positively impact performance, recovery and nutrient absorption (40).
If there is a short period of time between exercise bouts (under 12 hours), consuming protein and carbohydrates in the initial post workout period of 0-2 hours should be prioritised (41). But if there isn’t this time restraint then ensuring overall daily intake is the most important factor.
Is it possible to consume too much protein?
Firstly, from a practical standpoint, it is challenging to overconsume protein. This is due to it being high satiating macronutrient. Numerous studies that have gone upward of 3g/kg/bw have had a high attrition rate to the study with people struggling to eat the required amount (42,43). The Protein Overfeeding (PROOF) study explored the metabolic changes to low and high protein diets, and this sub-study examined the changes in body composition and blood lipids when eating high and low protein diets during overeating (44). This concern with this paper is that the high protein intake was only 25% of total daily intake which isn’t actually that high. The previously mentioned study where they fed participants 5.5 times the recommended daily allowance of protein had no effect on body composition. This was the first study to demonstrate that consuming a hypercaloric high protein diet does not result in an increase in body fat.
High protein intake in the long term may result in bone and calcium disorders. Excessive protein intake can lead to disorders related to bone health and calcium metabolism. In addition to this, high protein diets may impact renal function, potentially leading to kidney issues (45). There is an incredible amount of nuance around this in relation to the type of protein consumed, whether the person is resistance training, do they have any other underlying health issues and what the rest of their dietary habits consist of.
Can I get enough protein on a vegetarian or vegan diet?
Yes, with careful planning, plant-based diets can meet protein requirements for muscle building. As the article alluded to earlier, you need to ensure that you are getting a complete amino acd profile in order to get enough quality protein in your diet. It just takes a little more awareness.
What's the role of protein in muscle recovery?
Protein plays a crucial role in muscle recovery by providing the necessary amino acids for repairing and rebuilding muscle tissue after exercise. When you consume protein post-workout, it supplies your muscles with the essential building blocks needed to reverse damage caused by exercise, promote muscle growth, and prepare your body for subsequent workouts. The amino acids in protein aid in repairing the micro-tears that occur in muscles during exercise, helping them heal and grow stronger.
Should I be concerned about protein intake while cutting?
The key to a successful cut is to maintain as much muscle mass as possible with a high protein being crucial to preserving muscle mass. When cutting you will have to elicit a calorie deficit, therefore ensuring protein intake is around 2g/kg/bw will help with increasing satiety and fullness, which will ultimately help be more successful with your cut.
Can I replace whole food protein sources with supplements?
While supplements are convenient, whole foods offer additional nutrients and should be the primary protein source. If you are strugglnig to get to your daily protein intake through whole foods, a protein supplement may be a useful addition, but it shouldn’t be the sole source of your intake.
Is timing more important than total daily protein intake?
Total daily intake is more important, but distributing protein intake throughout the day can maximise muscle protein synthesis. If you are finding that your recovery is poor or you are struggling to get your overall intake then focusing on protein around your workouts may help.
How do I calculate protein intake if I'm overweight?
There are two ways to do this. The first is to base your intake on your target weight or you can base it on lean body mass rather than total body weight. This is more accurate, especially in extremely overweight individuals. Calculating your lean body mass can be tricky. Using the target weight is an easier method and will still give you a good starting point.
- https://pubmed.ncbi.nlm.nih.gov/20847704/
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8075408/
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2174086/
- https://pubmed.ncbi.nlm.nih.gov/37382939/
- https://journals.lww.com/nsca-jscr/fulltext/2010/10000/the_mechanisms_of_muscle_hypertrophy_and_their.40.aspx
- https://pubmed.ncbi.nlm.nih.gov/19765243/
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7400877/
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5872778/#B28-nutrients-10-00360
- https://journals.physiology.org/doi/full/10.1152/japplphysiol.00081.2009
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5852756/
- https://pubmed.ncbi.nlm.nih.gov/26041391/
- https://www.mdpi.com/1422-0067/23/7/4022
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5568273/
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3905294/
- https://pubmed.ncbi.nlm.nih.gov/30235845/
- https://pubmed.ncbi.nlm.nih.gov/29182451/
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5872778/#B29-nutrients-10-00360
- https://pubmed.ncbi.nlm.nih.gov/2407099/
- https://pubmed.ncbi.nlm.nih.gov/28642676/
- https://pubmed.ncbi.nlm.nih.gov/30235845/
- https://onlinelibrary.wiley.com/doi/10.1002/jcsm.12922
- https://sportsmedicine-open.springeropen.com/articles/10.1186/s40798-022-00508-w
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6950543/
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6942464/
- https://pubmed.ncbi.nlm.nih.gov/29182451/
- https://pubmed.ncbi.nlm.nih.gov/24092765/
- https://pubmed.ncbi.nlm.nih.gov/29182451/
- https://pubmed.ncbi.nlm.nih.gov/23097268/
- https://pubmed.ncbi.nlm.nih.gov/16469983/
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5828430/
- https://pubmed.ncbi.nlm.nih.gov/2407099/
- https://pubmed.ncbi.nlm.nih.gov/22330017/
- https://pubmed.ncbi.nlm.nih.gov/27916799/
- https://fppn.biomedcentral.com/articles/10.1186/s43014-020-0020-5
- https://pubmed.ncbi.nlm.nih.gov/23107545/
- https://pubmed.ncbi.nlm.nih.gov/33530512/
- https://pubmed.ncbi.nlm.nih.gov/15107010/
- https://pubmed.ncbi.nlm.nih.gov/22330017/
- https://pubmed.ncbi.nlm.nih.gov/27916799/
- https://pubmed.ncbi.nlm.nih.gov/32629950/
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7796021/
- https://pubmed.ncbi.nlm.nih.gov/24834017/
- https://pubmed.ncbi.nlm.nih.gov/29399253/
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7763540/
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4045293/
