Dr. Douglas W. Stoddard
You have trained and left no stone unturned in your Ironman preparations. Assuming that you arrive on the starting line healthy, which of the following is most likely to stop your Ironman plans cold:
- Body breakdown (musculoskeletal injury)
- Mental breakdown (“My God, what am I doing here!”)
- Nutrition breakdown
- Mechanical breakdown (flat tire)
Those who answered “C” have done your homework. It is usually a nutrition breakdown that ruins many an Ironman.
Your number one concern, especially in the heat, is hydration. Optimal hydration supports every single function of your body, including maintenance of normal blood pressure, food/fluid digestion and absorption, efficient sweating and body core cooling, oxygen and nutrient delivery to working muscle cells and normal removal of waste products of metabolism.
An average sweat rate is one liter per hour. Obviously, there is wide variation dependent on genetics, gender, acclimation, ambient temperature and humidity. It can be difficult estimating the amount of fluid that you require, but one method is via weight loss. Since one kilogram (2.2 lbs) = one liter, for each kilogram lost, you need one liter of fluid. You must try to determine your average requirements before race day by assessing before and after weights in workouts.
Urinate before all weight assessments (both before and after your session).
Add the volume of fluid you drank during the session.
For example, if before a training run your body weight was 140 lbs (63.5 kilograms) and after the run ended it was 137 lbs (62 kilograms), and you drank one liter during the run, you would require: 63.5 kg (pre-weight) – 62 kg (post-weight) = 1.5 kilograms (or 1.5 liters) + 1 liter consumed during the run = 2.5 liters.
Let’s say it took you two hours to complete the run. If you divide the total liters by the number of hours you ran, this equals 2.5 liters per 2 hours = 1.25 liters per hour. You would have needed to consume this much fluid during the run to “break even,” which is the goal.
Energy is measured in calories. The number of calories you consume during the average Ironman is substantial. Assuming that Susan weighs 128 lbs (58 kg) and Michael weighs 200 lbs (90 kg), the following calculations give us some idea of the amount of calories required for each in the Ironman.
The average swim calorie consumption is four calories/kg/kilometer, regardless of speed. It’s interesting to note that swimming burns about four times the calories that running does for the same distance. Assuming you are swimming 3.8 km:
Susan: 4 calories x 58 kg x 3.8 km = 880 calories
Michael: 4 calories x 90 kg x 3.8 km = 1400 calories
For optimal performance, these are calories that you will have to “catch up” on, since one cannot eat or drink during the swim.
Assuming a distance of 180 km, and a time of six hours (36o minutes), Susan will burn about 10 calories per minute, and Michael will burn about 15.5
Susan: 10 calories/minute = 10 x 360 = 3600 calories.
Michael: 15.5 calories/minute = 15.5 x 360 = 5600 calories This means that Susan has to take in at least 6oo calories/hour (3600/6), and Michael 933 (5600/6).
If you add the calories used during the swim, this increases Susan’s requirement on the bike to 750 calories/hour and Michael’s to 1170 calories/hour. This has to be practiced because the human gut is not normally used to absorbing such quantities of calories during intense physical exertion.
The bike is the easiest time to take in nutrition, so plan on taking advantage of this.
Susan will burn about 9.5 calories/minute; Michael will burn about 15 calories/minute. Using an average of 4.5 hours (270 minutes) to complete the marathon:
Susan: 9.5 calories x 270 minutes = 2565 calories
Michael: 15 calories x 270 minutes = 4050 calories
Susan: 880 (swim) + 3600 (bike) + 2565 (run) = 7,045 calories.
Michael: 1400 (swim) + 5600 (bike) + 4050 (run) = 11,050 calories
To put it into real terms, the following are equivalent to Susanís need for 7000 calories: 63 PowerGels; or 30 Clif Bars; or 32 liters of e load; or 60 bananas. Michael would require 110 Clif Shots; or 45 PowerBars; or 88 bananas.
Yes, you would have to be a mobile snack bar to achieve these levels of energy intake! It is most important to maintain adequate levels of carbohydrate intake to prevent bonking, also known as hypoglycemia.
Your total carbohydrate usage is going to be about 55 to 70 percent of total energy required, depending on how fast you are going. Therefore, Susan requires at least 3880 calories of carbohydrate; Michael requires at least 6050 calories of carbohydrate. There are four calories in each gram of carbohydrate, which means Susan needs at least 970 grams of carbohydrate (3880/4), and Michael needs 1510 grams of carbohydrate (6050/4).
The rest of the calories can come from your fat stores even the leanest athlete has enough fat to sustain an Ironman. And, yes, to be complete, some of the calories will also come from your protein stores (muscle is the only expendable source of protein your body has), anywhere from five to fifteen percent.
Electrolytes (sodium, potassium, calcium, magnesium, zinc and chloride) are incredibly important for performance in the heat. Electrolytes are responsible for maintaining many functions in the human body, including normal muscle contraction, blood pressure, nerve conduction, heart rate and gastrointestinal motility. They also play an important role in energy metabolism. Allowing your electrolyte concentrations to fall below normal can contribute to many problems experienced by Ironman participants, including muscle cramping, stomach bloating, nausea and dizziness.
Usually, most athletes rely on their sports drink to supply them with electrolytes. However, most traditional sports drinks have minimal concentrations of electrolyte and do little to counter the electrolyte-depleting effects of sweating over prolonged periods of time. Failure to replace these electrolytes can result in hyponatremia (reduced blood sodium levels) and hypokalemia (reduced blood potassium levels).
The principal electrolytes in sweat include sodium, chloride and potassium. Most human sweat contains between 700 and 1200 milligrams of sodium per liter (33 fluid ounces). Most sports drinks, which typically have between 120-430 milligrams of sodium per liter, fall far short of the ideal. The optimal solution would be to increase the amount of sodium in your drink to match sweat losses.
Also, consider that potassium in most human sweat is at least 170 milligrams per liter and can be as high as 320 milligrams. Sports drinks typically have between 80-115 milligrams of potassium per liter. In addition, the sweat sodium-to-potassium ratio is between 3:1 and 5:1. Additionally, a higher electrolyte drink provides the following benefits:
Stimulation of thirst, so that you keep drinking. The lower the sodium concentration in your drink, the less the thirst stimulation
Reduced urination. Drinks with low or no sodium are potent stimulators of urination because they perpetuate low sodium levels in the blood. This in turn provides a signal to your kidneys to make more urine, contributing to dehydration.
Stimulation of carbohydrate absorption in the gut. The higher the sodium content in the gut, the more carbohydrate (glucose) that is absorbed. Conversely, the lower the sodium content in the gut, the less glucose absorbed.
Several other important electrolytes are lost in sweat, including zinc, which may have a positive effect on energy metabolism, endurance and tissue regeneration; calcium, which may help with muscle contraction and nerve conduction; and magnesium, which may have a preventative effect on muscle cramping.
The above information is useful only if you can actually absorb what you are putting into your stomach, and this often isn’t the case, especially in prolonged events.
Why does your gut shut down, giving you that bloated feeling?
Ingestion of fructose, or fruit sugar, which is known to cause gastrointestinal irritation. This sugar is used in many sports drinks, gels and bars and is also found in fruits such as bananas. Try to minimize ingestion of this sugar.
Ingestion of high concentrations of carbohydrates, as found in many gels. This can be very irritating to your gastrointestinal tract. Diluting these substances by drinking may help.
Not maintaining a partially-filled stomach at all times.
Ingesting caffeine-containing substances. Caffeine helps induce relaxation of your gas- esophageal sphincter (the valve between your esophagus and stomach), which can contribute to heartburn and stomach pain.
Failure to maintain adequate hydration, electrolyte and blood sugar levels.
Pushing yourself too hard. Your gut is challenged during exercise because blood flow is shunted away from your gut and into your muscles (for performance) and skin (to help with cooling and sweating). Pronounced bloating may require you to slow down for a few minutes.
If in spite of these suggestions you still are having problems, you may need to see a sports medicine doctor regarding tests to rule out medical causes of gut pain.
Do your best to prevent this syndrome from even starting. Once it starts, it is often difficult to stop it during a race.
Dr. Stoddard (MD, M Sport Ned, Dip Sport Med, ES) lives in Toronto, Canada, and is the medical director for the Subaru Triathlon Series.
Dr. Douglas W. Stoddard