How Much Protein Do Athletes Need?

You’re an athlete, pushing your body to the limit, striving for peak performance. Whether you’re a seasoned competitor or a dedicated gym-goer, you’re constantly seeking ways to optimize your training, recovery, and overall athletic potential. And one of the most crucial nutrients for achieving your goals is protein for athletes.

The Building Blocks of Strength and Recovery

Protein isn’t just a dietary staple – it’s the fundamental building block of muscles, tissues, and even hormones. Think of protein like the bricks that make up your body’s magnificent machine. When you exercise, you break down muscle fibers, and protein provides the essential amino acids needed to rebuild those fibers stronger and bigger. This process, known as muscle protein synthesis, is the foundation of muscle growth and repair.

Beyond Muscle: Protein’s Crucial Roles in Athletic Performance

While muscle building is a primary benefit, protein plays many other crucial roles in athletic performance. Here’s a closer look:

• Hormonal Regulation: Protein influences the production of crucial hormones like insulin and growth hormone, which directly impact muscle growth, repair, and recovery.
• Energy Production: While carbohydrates are your primary fuel source during exercise, protein can also be used as energy when your carbohydrate stores are depleted, particularly during endurance events.
• Immune Function: Protein helps support a healthy immune system, which is vital for athletes who constantly push their bodies and may be more susceptible to illness.

Understanding Your Individual Protein Needs

So, how much protein is actually needed to fuel your athletic journey? The answer isn’t a one-size-fits-all solution. In this article, we’ll break down the science behind protein, exploring key factors that influence your individual protein needs, such as the intensity of your training, the type of sport you participate in, your body composition, and your overall calorie intake. We’ll also delve into the best protein sources and how to time your protein intake for optimal results.

Get ready to unlock the power of protein and learn how to optimize your nutrition for maximum athletic performance!

The Science Behind Protein for Athletes

Now that we’ve established the importance of protein in athletic performance, let’s delve deeper into the science behind its impact on your body. Understanding how protein works at a cellular level will help you appreciate its significance and make informed decisions about your protein intake.

Muscle Growth and Repair: Building a Stronger You

Think of your muscles as constantly being rebuilt and remodeled. Every workout, especially strength training, puts stress on your muscle fibers, causing microscopic tears. This breakdown is crucial for growth, as it triggers a repair process that leads to stronger, more resilient muscles. Protein plays a starring role in this process.

When you consume protein, your body breaks it down into amino acids, the building blocks of muscle tissue. These amino acids are then used for:

• Muscle Protein Synthesis (MPS): The process of creating new muscle protein, leading to muscle growth and repair. The rate of MPS is directly influenced by your protein intake, training intensity, and other factors.
• Muscle Repair: Amino acids are also crucial for repairing damaged muscle fibers, helping to reduce muscle soreness and improve recovery time.

Essentially, protein acts as the fuel for your muscles’ growth and repair cycle, enabling you to push your limits and recover effectively from your training.

Hormonal Regulation: The Chemical Messengers of Muscle Growth

Protein doesn’t just provide building blocks; it also influences the production and release of key hormones that play a vital role in muscle growth and repair.

• Insulin: This hormone is crucial for transporting nutrients, including amino acids, into muscle cells, facilitating muscle protein synthesis.
• Growth Hormone (GH): This hormone promotes muscle growth, reduces fat, and enhances recovery. Protein intake can stimulate GH release, particularly after exercise.

By influencing these hormones, protein indirectly enhances the muscle-building process, promoting a more efficient and effective adaptation to your training.

Energy Production: Fueling Your Performance

While carbohydrates are the primary fuel source for exercise, protein can also be used as energy, especially during prolonged endurance activities. This process, called gluconeogenesis, involves the conversion of amino acids into glucose, the body’s preferred energy source.

While not as efficient as carbohydrates for energy production, protein provides an alternative fuel source when your carbohydrate stores are depleted, helping to maintain your energy levels and performance during extended workouts.

Immune Function: Protecting Your Body

Protein is crucial for maintaining a healthy immune system, which is vital for athletes who are constantly pushing their bodies to the limit and may be more susceptible to illness. Protein is involved in producing antibodies, white blood cells, and other components of the immune system that protect your body from infection.

Ensuring adequate protein intake helps to strengthen your immune system, minimizing your risk of getting sick and allowing you to train consistently without interruptions.

In the next section, we’ll explore the factors that influence your protein needs, helping you personalize your protein intake for optimal athletic performance.

Factors Affecting Protein Needs

Now that we’ve explored the science behind protein’s impact on athletic performance, let’s dive into the factors that determine your individual protein needs. There’s no one-size-fits-all answer when it comes to protein for athletes because your body’s demands vary based on your training regimen, sport, and other factors. Understanding these factors is key to tailoring your protein intake for optimal results.

Training Intensity and Volume: The More You Train, the More You Need

Imagine your muscles as a construction site. The more you train, the more “building materials” (protein) you need to repair the breakdown and build stronger, more resilient structures. The harder and longer you train, the greater the demand on your muscles, and the more protein your body requires to recover and rebuild.

Think about it this way: A weekend warrior who lifts weights twice a week will have different protein needs than a professional athlete training for a marathon or a competitive bodybuilder. The higher the intensity and volume of your training, the more protein you’ll need to support muscle growth and repair.

Sport-Specific Demands: Fueling Your Performance

Different sports place different demands on the body, leading to varying protein needs. Here’s a breakdown of how sport-specific requirements impact protein intake:

• Endurance Athletes: Runners, swimmers, cyclists, and other endurance athletes typically require slightly less protein than strength athletes. While their muscles are still working hard, endurance athletes primarily rely on carbohydrates for fuel, and their protein needs are mainly focused on supporting muscle repair and recovery.
• Strength and Power Athletes: Weightlifters, CrossFit athletes, powerlifters, and other strength-focused athletes need more protein to support muscle growth and repair. Their training emphasizes building muscle mass and strength, so they require a higher protein intake to facilitate those processes.

Body Composition: Muscle Mass Matters

A higher percentage of muscle mass equates to a higher protein requirement. If you have a more muscular physique, your body needs more protein to maintain and build that muscle mass. This is why strength athletes, with their higher muscle mass, typically have higher protein needs compared to individuals with a lower percentage of muscle.

Energy Needs: Matching Your Calorie Intake

Your overall energy needs, determined by your activity level and calorie expenditure, also play a role in your protein requirements. If you burn more calories, you’ll need more protein to support your overall energy needs and maintain a healthy body composition.

In the next section, we’ll move on to specific protein intake recommendations for different types of athletes, providing a framework for understanding your individual needs.

Recommended Protein Intake for Athletes

Now that you understand the factors that influence your protein needs, it’s time to talk numbers. While the exact amount of protein you need will vary based on your individual situation, here are some general guidelines to help you get started. Keep in mind that these are just starting points, and personalized recommendations are always best.

General Protein Intake Recommendations for Athletes

Here are some general ranges based on athlete type:

• Endurance Athletes: 1.2 to 1.4 grams of protein per kilogram of body weight per day (0.5 to 0.6 grams per pound of body weight).
• Strength and Power Athletes: 1.6 to 2.0 grams of protein per kilogram of body weight per day (0.7 to 0.9 grams per pound of body weight).

For example, a 150-pound endurance athlete would need about 68 to 85 grams of protein per day, while a 150-pound strength athlete would need about 85 to 113 grams of protein per day.

The Importance of Personalized Assessment

It’s important to remember that these are just general guidelines. The best way to determine your individual protein needs is to consult with a registered dietitian or sports nutritionist. They can take into account your specific sport, training volume, body composition, energy needs, and other factors to provide personalized recommendations tailored to your goals.

They can also help you create a nutrition plan that includes a variety of protein sources, ensuring you’re getting all the essential amino acids for optimal muscle growth, recovery, and performance.

In the next section, we’ll explore the optimal timing of protein intake for maximizing its benefits.

Optimal Protein Timing for Athletes

You know how much protein you need, but when should you consume it? The timing of your protein intake can have a significant impact on muscle growth, recovery, and overall performance. Let’s explore the best times to fuel your body with protein.

Pre-Workout Protein: Setting the Stage for Success

Consuming protein before a workout can help stimulate muscle protein synthesis, giving your muscles a head start on repair and growth. Having protein in your system before you hit the gym can also provide energy for your workout, especially if you’re following a low-carb diet. Aim for 20 to 30 grams of protein about 1 to 2 hours before your workout.

Post-Workout Protein: Repairing and Rebuilding Muscles

This is arguably the most important time for protein consumption. Immediately after exercise, your body is primed for muscle repair and growth. Consuming protein within 30 to 60 minutes of your workout helps provide the building blocks your muscles need to rebuild and recover. This is known as the “anabolic window.” Aim for 20 to 30 grams of protein, ideally in combination with carbohydrates, after your workout.

Protein Throughout the Day: Consistent Muscle Support

It’s not just about pre- and post-workout protein. Consuming protein throughout the day is crucial for maintaining a steady supply of amino acids for muscle growth and repair. Aim to spread your protein intake evenly across your meals and snacks, ensuring a consistent supply of building blocks for your muscles.

• Breakfast: Start your day with a protein-rich breakfast, like eggs, Greek yogurt, or oatmeal with protein powder.
• Lunch and Dinner: Include lean protein sources, such as chicken, fish, tofu, beans, or lentils, in your meals.
• Snacks: Choose protein-rich snacks like nuts, seeds, protein bars, or cottage cheese to maintain a steady protein intake throughout the day.

By following these guidelines for protein timing, you can optimize your muscle growth, recovery, and overall athletic performance. In the next section, we’ll dive into the best protein sources to fuel your journey to athletic greatness.

Top Protein Sources for Athletes

You’ve figured out your protein needs and the best times to consume it, but now comes the fun part: choosing delicious and nutritious protein sources to fuel your athletic journey. From lean meats to plant-based powerhouses, there’s a wide range of options to satisfy your protein requirements and keep your diet interesting. Let’s explore some of the best protein sources for athletes.

Lean Meats: A Classic Choice for Protein

Lean meats are a staple in many athletes’ diets, offering a high-quality source of protein with a good balance of essential amino acids.

• Chicken: A versatile protein source, chicken is lean and low in fat, making it a great choice for both strength and endurance athletes.
• Turkey: Similar to chicken, turkey is a lean protein source packed with amino acids, and it’s often a good option for those looking to reduce their fat intake.
• Fish: Fish is a fantastic source of protein and omega-3 fatty acids, which are essential for brain health and may help reduce inflammation. Opt for fatty fish like salmon, tuna, or mackerel for a boost of healthy fats.
• Beef: Choose lean cuts of beef, like sirloin or flank steak, for a high-protein source.
• Pork: Like beef, opt for lean cuts of pork, such as tenderloin or loin, for a flavorful protein source.

Eggs: A Nutrient-Packed Powerhouse

Eggs are a great source of protein, boasting a complete amino acid profile. They also provide essential nutrients like choline, which supports brain health and cognitive function.

Dairy Products: Calcium and Protein in One

Milk, yogurt, and cheese are excellent sources of protein, and they also provide calcium, which is essential for bone health. Choose low-fat or fat-free varieties to reduce your calorie intake. Greek yogurt is particularly popular among athletes for its high protein content and creamy texture.

Legumes: The Plant-Based Protein Powerhouses

Beans, lentils, and chickpeas are excellent plant-based sources of protein. They’re also rich in fiber, which can help you feel full and satisfied after a meal. Legumes are a versatile addition to soups, salads, and stews.

Nuts and Seeds: A Snacking Solution

Almonds, walnuts, chia seeds, and flax seeds are excellent sources of protein and healthy fats. They make a great snack option between meals or before or after workouts.

Protein Powders: A Convenient Boost

Protein powders can be a convenient way to boost your protein intake, especially if you struggle to get enough protein from whole foods. They are available in various flavors and forms, including whey protein, casein protein, plant-based protein powders, and protein blends. Choose powders that are low in sugar and additives.

When choosing protein powders, consider your dietary needs and preferences. Whey protein is a popular choice for athletes because it’s quickly absorbed and can help promote muscle growth, while casein protein is digested more slowly, providing a sustained release of amino acids. Plant-based protein powders, such as soy protein, pea protein, or brown rice protein, are great options for vegetarians and vegans.

Remember, protein powders are supplements, not replacements for whole food sources. A balanced diet that includes a variety of protein sources from whole foods is always the best approach to achieve optimal nutrition.

Now that you’ve learned about the top protein sources for athletes, it’s time to get creative and experiment with different options to find what works best for you. In the next section, we’ll explore the protein needs of specific sports and activities.

Protein for Different Sports and Activities

Now that you have a grasp of general protein guidelines, let’s look at how specific sports and activities influence protein needs. Understanding these variations can help you fine-tune your protein intake for optimal results in your chosen discipline.

Protein for Runners: Fueling Endurance

Runners, especially those participating in marathons or ultra-marathons, need sufficient protein to support muscle repair and recovery. While carbohydrates are the primary fuel source for endurance activities, protein plays a crucial role in preventing muscle breakdown and promoting adaptation to training.

A general recommendation for marathon runners is 1.2 to 1.4 grams of protein per kilogram of body weight per day (0.5 to 0.6 grams per pound of body weight). However, runners who train at a high intensity or for longer durations may benefit from consuming a higher protein intake.

Protein for Weightlifters: Building Strength and Muscle

Weightlifters and other strength athletes have a higher protein requirement compared to endurance athletes. Their training focuses on building muscle mass and strength, which demands a greater amount of protein for muscle synthesis and repair.

A general recommendation for weightlifters is 1.6 to 2.0 grams of protein per kilogram of body weight per day (0.7 to 0.9 grams per pound of body weight). This higher intake helps support the increased demands of strength training, ensuring adequate protein for muscle growth and recovery.

Protein for CrossFit Athletes: A Balanced Approach

CrossFit workouts combine elements of strength training, endurance, and metabolic conditioning, requiring a balanced approach to protein intake.

A general recommendation for CrossFit athletes is 1.6 to 1.8 grams of protein per kilogram of body weight per day (0.7 to 0.8 grams per pound of body weight). This range provides enough protein to support both muscle growth and recovery from the high-intensity workouts.

Protein for Team Sports: Supporting Performance

Athletes involved in team sports, such as basketball, soccer, football, and hockey, have varying protein needs depending on their specific position and training regimen.

For example, players in positions that require high levels of strength and power, like linemen in football, may benefit from a higher protein intake, while endurance-focused players, like midfielders in soccer, may need a slightly lower protein intake.

Protein for Other Activities: Staying Active

Protein is essential for anyone engaging in regular physical activity, even if it’s not at a competitive level. If you’re consistently exercising, aim for a daily protein intake of 0.8 to 1.2 grams per kilogram of body weight per day (0.4 to 0.5 grams per pound of body weight).

Remember, these are just general guidelines, and your individual protein needs may vary based on your training intensity, activity level, and other factors. Always consult with a healthcare professional or a registered dietitian to determine the best protein intake for your specific needs.

Next, we’ll delve into the world of protein supplements and explore whether they’re necessary for athletes.

Protein Supplements for Athletes: A Comprehensive Guide

Protein supplements are often a hot topic for athletes, but are they truly necessary? Let’s take a balanced look at the pros and cons of using protein supplements to support your athletic goals.

The Pros of Protein Supplements

• Convenience: Protein supplements provide a convenient and quick way to boost your protein intake, especially if you’re short on time or have trouble getting enough protein from whole foods.
• Portability: They’re easy to pack and take with you, making it simple to get a protein boost on the go, whether you’re heading to the gym, traveling, or just need a quick snack.
• Variety: Protein supplements come in a wide array of flavors, making it easier to incorporate them into your diet without feeling bored.
• Cost-Effective: Depending on the brand and type, protein supplements can be a cost-effective way to meet your protein needs, especially compared to consistently buying large quantities of protein-rich foods.

Types of Protein Supplements

• Whey Protein: A popular choice for athletes, whey protein is a fast-digesting protein source derived from milk. It’s known for its ability to stimulate muscle protein synthesis and promote muscle growth.
• Casein Protein: Casein protein is a slow-digesting protein source also derived from milk. It provides a sustained release of amino acids, making it a good option for nighttime consumption to support muscle recovery overnight.
• Plant-Based Protein: Soy protein, pea protein, and brown rice protein are great alternatives for vegetarians and vegans. They provide a complete amino acid profile and can be just as effective as whey protein for supporting muscle growth.
• Protein Blends: Some protein supplements combine different types of protein, offering a blend of fast-digesting and slow-digesting proteins, potentially providing a more balanced amino acid delivery over time.

Common Concerns about Protein Supplements

• Safety: Most protein supplements are safe when consumed as directed, but it’s important to choose reputable brands and follow the recommended dosage.
• Effectiveness: While protein supplements can be helpful, they are not a magic bullet for muscle growth. They are most effective when combined with a balanced diet, regular exercise, and adequate sleep.
• Potential Side Effects: Some people may experience digestive discomfort or bloating when consuming protein supplements, particularly when starting with a high dosage. It’s essential to start slowly and gradually increase your intake to assess your tolerance.

Remember, protein supplements are not a substitute for whole food sources. They should be considered a convenient way to supplement your diet, not replace a balanced and varied intake of protein-rich foods.

In the next section, we’ll focus on the specific protein needs of vegetarian athletes, offering guidance on getting enough protein from plant-based sources.

Protein and Vegetarian Athletes

You might be wondering, “Can I still reach my athletic goals on a vegetarian diet?” The answer is a resounding YES! While it’s true that animal products are often considered the primary sources of protein, there are plenty of plant-based options that can effectively support your athletic performance.

Key Considerations for Vegetarian Athletes

• Complete Protein: Animal products typically provide complete protein, meaning they contain all nine essential amino acids that your body can’t produce on its own. Plant-based protein sources often lack one or more of these essential amino acids. However, you can get all the essential amino acids by combining different plant-based protein sources throughout the day.
• Iron: Vegetarian athletes need to pay special attention to their iron intake, as iron is crucial for red blood cell production and oxygen transport, which is vital for athletic performance. Plant-based iron sources are not as readily absorbed by the body, so vegetarians may need to consume more iron-rich foods and consider supplementing with iron.
• Vitamin B12: Vitamin B12 is only found naturally in animal products. Vegetarians need to ensure they’re getting enough vitamin B12 through fortified foods or supplements, as deficiency can lead to fatigue, anemia, and other health issues.
• Calcium: While milk and dairy products are good sources of calcium, there are also plant-based sources, such as leafy green vegetables, fortified plant milks, and tofu.

Plant-Based Protein Sources for Athletes

• Legumes: Beans, lentils, chickpeas, and peas are packed with protein, fiber, and other essential nutrients.
• Tofu and Tempeh: These soy-based products are excellent sources of protein and can be used in a variety of dishes.
• Nuts and Seeds: Almonds, walnuts, chia seeds, hemp seeds, and pumpkin seeds offer a combination of protein and healthy fats.
• Quinoa: This ancient grain is a complete protein source, meaning it contains all nine essential amino acids.
• Edamame: These immature soybeans are a good source of protein and fiber.
• Seitan: A wheat-based protein source that can be used as a meat substitute in various dishes.

Remember, a balanced vegetarian diet that includes a variety of plant-based protein sources can easily meet your protein needs as an athlete. Consulting with a registered dietitian or sports nutritionist can provide personalized guidance and ensure you’re getting all the essential nutrients for optimal health and performance.

Next, we’ll address some frequently asked questions about protein for athletes.

FAQs: Answering Your Protein Questions

You’ve learned a lot about protein and its importance for athletes, but you may still have some questions. Let’s address some commonly asked questions about protein intake for athletes.

What are the Signs of Protein Deficiency in Athletes?

Protein deficiency is less common in athletes who consume a balanced diet, but it can occur if you’re not meeting your protein needs. Here are some signs to watch out for:

• Muscle Weakness and Fatigue: Protein is crucial for muscle repair and growth. A deficiency can lead to muscle weakness and fatigue, making it harder to perform workouts and recover effectively.
• Slowed Recovery Time: Protein is essential for muscle repair and rebuilding. A deficiency can slow down your recovery time after workouts, leading to prolonged muscle soreness and fatigue.
• Increased Risk of Injury: Muscle weakness and slowed recovery can increase your risk of injuries.
• Immune System Weakness: Protein is vital for immune function. A deficiency can weaken your immune system, making you more susceptible to illness.
• Hair Loss and Brittle Nails: Protein is also essential for hair and nail health. A deficiency can lead to hair loss and brittle nails.

What Happens If You Eat Too Much Protein as an Athlete?

While getting enough protein is essential for athletes, it’s also possible to overdo it. Consuming excessive amounts of protein can lead to various issues, including:

• Digestive Problems: Eating too much protein can strain your digestive system, leading to bloating, gas, and constipation.
• Kidney Strain: Your kidneys are responsible for filtering waste products, including nitrogen from protein breakdown. While your kidneys are designed to handle a normal protein intake, excessive protein can put extra strain on them, especially if you have pre-existing kidney conditions.
• Dehydration: Protein requires water for proper digestion and metabolism. If you’re not drinking enough water, consuming too much protein can lead to dehydration.
• Increased Risk of Bone Loss: Eating too much protein can increase the acidity of your urine, potentially leading to calcium loss from bones.
• Possible Increased Risk of Certain Cancers: Some studies suggest that high protein intake may be linked to an increased risk of certain types of cancer, but more research is needed.

Is Protein Powder Necessary for Athletes?

Protein powder is a convenient way to supplement your protein intake, but it’s not essential for everyone. If you can consistently meet your protein needs through a balanced diet of whole foods, protein powder may not be necessary. However, protein powder can be helpful for athletes who struggle to get enough protein from their diet, especially those who are very active or have higher protein requirements.

Can You Get Enough Protein From Whole Foods?

Absolutely! Many whole foods are excellent sources of protein, and you can easily meet your protein needs without relying on supplements. Focus on incorporating a variety of protein-rich foods into your diet, including:

• Lean meats: Chicken, turkey, fish, beef, pork
• Eggs
• Dairy products: Milk, yogurt, cheese
• Legumes: Beans, lentils, chickpeas
• Nuts and seeds: Almonds, walnuts, chia seeds, flax seeds
• Tofu and tempeh
• Quinoa
• Edamame
• Seitan

By focusing on whole food sources, you’ll also be getting a wealth of other essential nutrients that support your overall health and performance.

In the next section, we’ll provide a comprehensive guide to protein sources and offer some tips for maximizing protein intake.

Conclusion: Fuel Your Performance with the Right Protein

You’ve now gained a comprehensive understanding of how protein fuels your athletic journey. From the science behind muscle growth and recovery to the best protein sources and timing strategies, you’re equipped to make informed decisions about your protein intake for athletes.

Key Takeaways:

• Individual Needs: Remember, there’s no one-size-fits-all answer. Your individual protein needs are influenced by your sport, training intensity, body composition, and energy needs.
• Balance is Key: Focus on a balanced diet rich in protein sources from both animal and plant-based options.
• Quality Counts: Choose high-quality protein sources, whether from whole foods or supplements, to ensure you’re getting the full range of essential amino acids.
• Timing Matters: Optimize protein intake before and after workouts, and spread your protein intake evenly throughout the day.
• Seek Professional Guidance: Consult with a registered dietitian or sports nutritionist for personalized advice based on your specific goals and needs.

By incorporating these key takeaways into your nutrition plan, you can optimize your protein intake for maximum athletic performance. Remember, the right amount of protein can fuel your muscle growth, enhance your recovery, and boost your overall athletic potential.

So, embrace the power of protein and fuel your journey to athletic greatness!

Resources

Ready to optimize your protein intake? Explore these resources for personalized guidance and in-depth information:

• Optimal Protein Consumption for Active Individuals: This article from Michigan State University Extension addresses common misconceptions about protein needs for athletes and provides evidence-based recommendations. It suggests that active individuals should consume 1.2 to 2.0 grams of protein per kilogram of body weight, with specific guidance on timing and distribution to enhance muscle synthesis and adaptation. Discover insights on protein intake for athletes and optimize your nutrition strategy.
• Optimal Protein Intake Post-Exercise: This article from Mass General Brigham emphasizes the importance of consuming at least 30 grams of protein in your first meal of the day to support muscle growth, particularly after workouts. Discover insights on protein intake for muscle development.
• Protein for Exercise and Sports Performance: Explore the science behind protein’s role in athletic performance and recovery in this comprehensive guide from the National Library of Medicine. https://medlineplus.gov/ency/article/002467.htm
• Impact of Vegetarian Diets on Athletic Performance: This systematic review examines the effects of vegetarian diets on sports performance, comparing omnivorous and vegetarian athletes in terms of physical performance, nutritional intake, and health. The study found no significant differences in performance metrics but noted higher carbohydrate and lower protein intake among vegetarian athletes. Discover insights into how diet influences athletic performance.
• USDA Food Composition Database: Use this comprehensive database to analyze the protein content of various foods, helping you make informed choices to meet your dietary needs. https://fdc.nal.usda.gov/