Get access to everything we publish when you sign up for Outside+.
Your body is full of signals that are saying something—whether you’re training too hard, not eating enough, needing more sleep. The problem is you don’t always recognize the signals your body is giving you (or we don’t listen to them). These signals don’t come in words; they come in the form of organic molecules and complex interplays between them. Some of those interplays give off odd sensations and experiences when things go askew in training, and it’s those sensations that you should listen to.
Here are some common body signals you should pay attention to:
1. Ammonia Smell
Smelling ammonia usually happens towards the end of a long or challenging workout and is a strong indicator that you have been burning protein as fuel. The reason you smell ammonia is because the protein breakdown product urea is being produced faster than it can be excreted by your kidneys, and is subsequently leached into your sweat as ammonia.
What your body is telling you is your muscles are being directly broken down, metabolized, and used for energy during your exercise. Not ideal! The way to prevent this is to fuel with a higher carbohydrate intake before and during exercise.
2. Metallic Taste
“Tasting the penny” is a badge of honor for old-school track and cross-country folks, but it probably just means you are rupturing blood cells because of your extremely high-intensity effort. Running an all-out 400 meters or doing a hard 90-second anaerobic effort on a bike might get you there, because of the high intensity.
While probably not harmful overall, what you are really tasting is the contents of your damaged blood cells—so definitely not something to strive for.
3. Craving Solid Food in Training
If you are hungry in training, you are way behind in your intra-workout fueling and, as far as your performance is concerned, it’s an emergency. Here’s why:
Imagine being in a haunted house and getting scared so much you jumped and your skin tingled. I doubt your next thought was, “I’m hungry.” Hunger was the last thing on your mind, because the body has advanced cellular and hormone signaling that is designed to strongly inhibit hunger during times of fight or flight. The strongest of those hormones is epinephrine.
Epinephrine is also released during exercise, because it’s responsible for many of the cellular cascades that result in energy availability.
If you sense hunger during prolonged exercise, it means your hunger signals are so overwhelmingly strong that they have overpowered the extraordinarily strong hunger-inhibiting effect of epinephrine. This only happens when you are lightyears behind on intra-workout fueling.
Craving solid food is such a common report among endurance folks, there’s a quantitative guide to help athletes fuel their training and racing more appropriately.
4. Weakness, Faintness, Irritability, Profuse or Unusual Sweating
These are signs of hypoglycemia. You know that time your niece or nephew ate cake and ice cream, was energetic for 45 minutes, but then started screaming and sobbing because they dropped their crayon and you didn’t pick it up fast enough? That was hypoglycemia. It’s the same sugar crash parents try to avoid in their children. The most common times we experience this as adults is soon after the onset of exercise, if we mistimed our prior meal or delayed our intra-workout nutrition.
How to avoid hypoglycemia: Do not eat in the 30 to 75 minutes pre-exercise. Limit yourself to high-glycemic index foods in the 20-minute window pre-exercise, or stick to a healthy balanced meal when eating 2 to 2.5 hours pre-exercise. Start your intra-workout fueling strategy at the onset of training.
5. Insatiable Thirst, No Matter How Much You Drink
You may be perfectly well-hydrated if this is something you feel. Chances are you are a health-conscious person who generally chooses low-sodium foods and drinks plenty of water. Turns out, if you are thirsty during exercise and you have been drinking plenty, you probably need more sodium, not water!
Hyponatremia has grave clinical consequences when severe, but when mild it makes you thirstier than your hydration status should lead you to be. High-electrolyte beverages or adding table salt to your carbohydrate beverage will help.
6. Queasiness or Urge to Vomit
Usually the urge to vomit, in the absence of gut cramping or pain, is the result of acid dumping into your stomach from very high-intensity activity. Think: a sprint finish. This effort causes rapid acid production in muscle and it gets shuttled into your stomach as your body tries to get rid of it.
7. Painful or Cramping Stomach
Stomach pain and upper gut cramping is usually the result of over-concentration or excess energy density of stomach contents. It might seem like not too big of a deal to take a couple gels since they’re so low-volume, but that concentration of carbohydrate and pure molecular concentration in the solution of your stomach is the perfect recipe for cramping and some severe stomach pain. The key is to drink water with your gels, especially if on an empty stomach.
8. Upset Lower Intestines
Best way to upset your lower GI tract in a hurry during a race or training? Consume a hyperosmolar solution and do not chase it with water. Usually this is the result of consuming too many gels with too little fluid or something more than 2000mg sodium per liter of water. Keep the carb solutions between 60-140g carbs per liter, and the sodium between 300-1500mg per liter and you should avoid this, no problem.
9. Sweet Taste- or Flavor-Avoidance
A common report among those of us who go long in training: “X product is too sweet for me.” And you are probably right. Many products simply overdo the flavor because they are not designed for more than three hours of exercise, let alone long-distance triathlons. If you’re not consuming one of these high-flavor or super-sweet beverages, and you’re still finding that you just want something savory, rather than sweet, you’re probably way behind on your sodium consumption needs, and would benefit from increasing your current sodium consumption rates to 1000-1500 mg per liter.
When your body talks and gives you signals, listen to it—even if it takes some paying attention to figure out what it’s trying to say.
Dr. Alex Harrison, a certified USA Triathlon coach, holds a PhD in Sport Physiology and Performance. He is the author of The RP Diet for Endurance, creator of the RP Endurance Macro Calculator, and has authored and contributed to dozens of articles. When he isn’t pumping out training and nutrition plans in his RV-garage-turned-mobile-office, he can be found on his bike, clinging for dear life to his wife’s wheel.