To carbo-load, or just hit the road? What is an Ironman to do?
For years now athletes have been experimenting with carbohydrate loading to enhance
performance in endurance events. But how much can it help for the endurance sport like triathlon or
ultra-endurance sports like Ironman? Most of the research for carbo-loading is done with cycling tests
or 10 km running. The method for carbo-loading used to be over 5 days but now is focused on the 3
days prior to the event. It is typical to taper exercise for about 3 days while consuming about 1.5 times
the amount of carbohydrates over your normal amount; for those who like numbers, it is typical for
athletes to consume 6 grams of carbohydrate per kg body weight and to elevate this to 9 grams during
carbo-loading. The amount of glycogen stored in muscle is typically elevated by about 20% by doing
this. We know that typical stores of the body’s carbohydrate, glycogen, are depleted easily within 2
hours of exercise and that 20% more might confer an additional 40 minutes if the math works out. This
might be of benefit, though, for the shorter distances where opportunity for eating and drinking is a bit
limited, but is a far reach from the 8 to 17 hours for an Ironman event. Let’s consider some facts from
the scientific literature. There are few carbo-loading studies conducted in triathletes, and none that I
can find conducted under actual race conditions for triathletes. So keep this in mind when reading on.
None of this has been tested for the correct sequence of events, or for all triathlon distances.
Very little work has been reported for triathlons of Sprint or Olympic course distance, and for
the most part, we do not know if carbo-loading helps Ironmen during an event. There is some
information on similar distances for each component of the Ironman race. Let’s begin with the swim.
I could not find much in the scientific literature on swimming and carbo-loading. I did find one study
about sports drinks the morning prior to the event. This study used male triathletes and tested a drink
containing 10% glucose taken 5 or 35 minutes before a 4 km indoor swimming event or a placebo
drink (1). The volume of drink was standardized by weight such that for each kg of weight, 5 ml was
consumed; that would be 350 ml for a 70 kg person and about 120 to 140 Cal. The glucose drink
seemed to reduce times by 24 seconds to 5 minutes among the athletes, but the difference was not
different from that due to chance alone. What is important though, is that the drink did not limit the
performance, and might have helped keep some glycogen stored for the next event.
For Ironmen, the cycle distance of 180 km is a great opportunity to get some nourishment in.
But can carbo-loading help those with race nerves who can’t seem to get that sports drink or bar down?
Most studies of carbo-loading focus on shorter trials like 20 or 40 km. One study in male triathletes
and cyclists did simulate a 100 km road race including 1 and 4 minute intervals and used carbo-loading
or a placebo regimen for 3 days prior to the trial (2). The athletes in both groups drank a sports drink
during the event since this is typical to the sport. What they observed was no difference in time to
complete the 100 km trial or feeling of exertion. This was observed despite higher muscle glycogen
with the carbo-loading. So why not? The most likely answer is that the sports drink provided enough
energy during the event that the muscle glycogen was not fully depleted in either group. In other
words, they had not used up all of their energy or hit the wall yet. Thus drinking sports drinks that
provide about 1 g of carbohydrate per kg of body weight per hour of the event might be just as
important as a pre-race carbo-loading regimen; for a 70 kg person this translates into 70 g/hour or just
over 1 litre of Gatorade. But cycling is just one of the three events and in the study only represented
about 2.5 hours of exercise. Although those in the placebo group did have slightly lower stores of
glycogen left after cycling 100 km, both groups were getting close to rock bottom. Post-race repletion
would be important for both groups if they were training and if the cycling was to followed by a ½
marathon run for the long course or another 80 km plus a full marathon as in Ironman, eating on the go
would be critical.
By now the men reading this article have not noticed and the women have noticed that the
research is mostly in men. Well, one study in women runners tested the benefits of carbo-loading on
24 km treadmill run (3). There are other studies in women too, but this one was a good distance.
These scientists tested a carbo-loading regimen and also drinking carbohydrate beverage during the
event compared to a placebo. Over three separate treadmill runs, the women were tested for time to
complete the run, fatigue and blood sugar. The carbo-loading and carbohydrate drink did not enhance
time to complete the run. However, blood sugar was higher and suggests that this is very important for
those training runs because you will be right back at the training in some way the next day or so, and
you will need more energy.
From my stand-point, carbo-loading is important for training events and also for the day of the
event, but obviously Ironmen need to focus on eating and drinking throughout the event to keep them
moving at their expected pace. This is demonstrated more so in men based on a study of 10 male and 8
female Ironmen during a real event (4). In women, eating and drinking more during the cycle was
linked to improved performance, in contrast to the men where higher carbohydrate intake in the run
was linked to improved performance. This report however was not able to determine the best
approach. It simply described eating and drinking behavior and related it to performance. It is
possible that the best-trained athletes know exactly what to eat and are successful at it. You all know
you need to experiment with what goes down easy when exercising. Even if carbo-loading and eating
on the go does not enhance your performance, if you feel less fatigued you will enjoy your event more.
And this is what it is all about. Crossing that finish line and feeling great!
We also know that many athletes do not eat the ~3000 to 4000 kcal per day, depending on the
triathlon distance, that is required during training. A couple of scientists in Ohio helped ½ Ironman
distance elite triathletes to increase their energy intake by adding high carbohydrate foods to meals
plus to the pre- and post-training snacks (5). Typically high carbohydrate snacks were fruits, bagels,
sport drinks and sport bars. By doing so, energy intake for the 11-hour work-out week was more in line
with 4000 kcal per day. After 4 weeks of eating better, but training the same amount and intensity, the
athletes took 25 minutes off their time on a similar course; 5:25 before the nutritional advice and 5:00
afterwards. Very impressive and a good reason to get eating. By the way, they did not gain weight or
change body composition!
For the Olympic and sprint triathlon, there are other benefits of carbohydrates too. For
example, ingestion of carbohydrate beverages improved perception of exertion that might help you
with that competitive edge. Utter and their colleagues (6) demonstrated this in triathletes over cycle or
running tests where they consumed a carbohydrate drink (Gatorade) or a similar looking sweetened
drink with no energy. After 2.5 hours of biking or running those who consumed the Gatorade drink
felt less exerted, but this was mostly apparent in the cycling test. The problem with this work is the
bike was not followed by the run so it is not really like a triathlon. Another research group looked at
cyclists and triathletes consuming a carbohydrate drink or a placebo during cycling for 90 minutes to
see if sprinting ability can be enhanced (7). They found that ingestion of the drink halfway through the
event helped the athletes to sprint better at the end; they had more power. So maybe if you find
yourself beat-out by your peers in a sprint to the finish leg of the cycle, you might consider if you are
getting in enough carbohydrates during the event.
There are other studies too. One shows that performance in high-intensity exercise like cycling
is not different in cyclists and triathletes who carbohydrate load, or at least not as long as their total
energy intake is adequate (8). One study in women cycling to exhaustion showed that carbo-loading
enhanced performance in only 2 of 8 women (4). For men, both 25 and 30 km treadmill running is
enhanced by carbo-loading (9, 10). In both of these studies the last 5 km was run faster. Thus there
appears to be individual variability and some benefit depending on how performance is assessed. It
seems that there is much more work to be done to help triathletes know exactly when or not to carbo-
load. At this point, eating enough during training and the days leading up to an event seem practical
and feasible. Same with drinking sports beverages during the event.
Take a look at what you are eating and compare to the example provided next. The example
has been designed with your limited time and energy in mind. Most triathletes hold full-time
employment or are in educational programs so time is limited, and after the hard workout, you might
not feel like cooking. At least during the taper, you are not exercising as much. The foods in the
example are easy to prepare such as breakfast cereals, fruits, bagels, juices, milk, sandwiches etc. You
might also consider seeking nutritional advice. If so, go to www.dietitians.ca and click on Find a
Nutrition Professional on the left-hand menu bar.
This article was written by Dr. Hope Weiler, RD, PhD, Associate Professor, Department of Human
Nutritional Sciences, University of Manitoba.
References if you wish to read the original research reports.
1. Smith GJ, Rhodes EC, Langill RH. The effect of pre-exercise glucose ingestion on performance during prolonged
swimming. Int J Sport Nutr Exerc Metab 2002;12:136-44.
2. Burke LM, Hawley JA, Schabort EJ, St Clair Gibson A, Mujika I, Noakes TD. Carbohydrate loading failed to
improve 100-km cycling performance in a placebo-controlled trial. J Appl Physiol 2000;88:1284-90.
3. Andrews JL, Sedlock DA, Flynn MG, Navalta JW, Ji H. Carbohydrate loading and supplementation in endurance-
trained women runners. J Appl Physiol 2003;95:584-90.
4. Kimber NE, Ross JJ, Mason SL, Speedy DB. Energy balance during an ironman triathlon in male and female
triathletes. Int J Sport Nutr Exerc Metab 2002;12:47-62.
5. Frentsos J, Baer J. Increased energy and nutrient intake during training and competition improves elite triathletes'
endurance performance. International Journal of Sport Nutrition 1997;7:61-71.
6. Utter AC, Kang J, Nieman DC, et al. Effect of carbohydrate ingestion and hormonal responses on ratings of
perceived exertion during prolonged cycling and running. Eur J Appl Physiol Occup Physiol 1999;80:92-9.
7. Sugiura K, Kobayashi K. Effect of carbohydrate ingestion on sprint performance following continuous and
intermittent exercise. Med Sci Sports Exerc 1998;30:1624-30.
8. Pitsiladis YP, Maughan RJ. The effects of alterations in dietary carbohydrate intake on the performance of high-
intensity exercise in trained individuals. Eur J Appl Physiol Occup Physiol 1999;79:433-42.
9. Williams C, Brewer J, Walker M. The effect of a high carbohydrate diet on running performance during a 30-km
treadmill time trial. Eur J Appl Physiol Occup Physiol 1992;65:18-24.
10. Sullo A, Monda M, Brizzi G, et al. The effect of a carbohydrate loading on running performance during a 25-km
treadmill time trial by level of aerobic capacity in athletes. Eur Rev Med Pharmacol Sci 1998;2:195-202.
How to carbohydrate load
There are many plans for carbohydrate loading. The most common is during the three days before the
event. Taper your exercise and increase your carbohydrate intake from the typical 6 to 9 g per kg of
body weight. When choosing foods try to pick those with little bulk like pastas and rice. For fruits
pick starchy choices like bananas or juices and for vegetables pick starchy items like potatoes, carrots
and peas. For the sports drink, pick your favorite one and check how much carbohydrate is in each
litre. The best thing is to try these foods during training events. You might wish to use sports bars
when carbo-loading but establish if the number you are going to eat is tolerable. Some bars have
additional milk solids that might bring along lactose and if you are not used to the higher intake you
might feel bloated. An example of carbo-loading needs is below.
Typical carbohydrate intake per kg for individuals based on weights 50 or 70 kg.
Grams Carbohydrate Weight Goal of carbohydrate intake in grams
per kg weight to target: (kg)
6 50 300
9 50 450
Example servings are designed to help count the carbohydrates and do not necessarily represent
the amount eaten at one meal:
Food groupings (typical g carbohydrate) Example servings
Breads and cereals (~ 15 g) 1 slice of standard white or whole wheat
½ to 1/3 of a regular sized bagel (the carb
is more densely packed than regular bread)
½ cup of rice, pasta or cereal
Fruits and vegetables (~10 g) ½ of a medium banana
1/3 cup apple juice
½ cup green beans,
¼ cup of baked potato,
½ cup peas or carrots
Milk and milk products (~12 g) 1 cup milk, all types
¾ cup plain yogurt (fruit bottom will have
double the carbohydrates)
(Note; cheese has much less carbohydrate
than milk or yogurt)
Sports drinks (15 g) 1 cup for generic estimate
Gatorade has 66 g/litre or 16.5/250 ml
Sports bars (30-45 g) Clif Bar 44 g, about 3 servings
PowerBar 41 g, about 3 servings
Body Smarts 30 g, 2 servings
Example for 70 kg person when carbohydrate loading: ~ 631 g.
Meal: total g Carbohydrate Food item
carbohydrate g for food
Breakfast: 152 g 12 g 1 c milk (1/2 cup on cereal, ½ cup drink)
30 g 1 c breakfast cereal
30 g 1 cup apple juice
60 g 1 large bagel
20 g 2 tbsp fruit preserve/jam (generous)
Snack, morning: 30 g 500 ml sports drink
74 g 44 g Clif Bar
Lunch: 146 g 32 g 1 cup fruit bottom yogurt
30 g 1 cup pasta salad with vegetables (prepare on
weekend as part of meal but make
enough for lunches)
24-30 g 1 medium to large banana
30 g 500 ml sports drink
30 g 2 slices bread for sandwich (filling by choice,
peanut butter is easy)
Snack, afternoon: 20 g 1 cup carrots, raw
88 g 12 g ½ cup or about 15 grapes
15 g 250 ml sports drink
41 g 1 Power bar
Dinner: 117 g -- Bar-B-Q chicken (if too tired just add spice to
the chicken then bake with potato and
carrots vs peas in roasting pan for 45 –60
minutes at 350 F while resting)
45 g 1 large baked potato
12 g ½ cup green peas
30 g 1 large slice French bread
-- Olive oil
30 g 1 cup cranberry juice
Snack, evening: 54 g 12 g 1 cup tomato juice
36 g 30 tortilla snack chips
6g ½ cup salsa
Note: foods in italics are added for carbohydrate loading to increase from 420 to 630. The 420 g plan
(without foods in italics) is also close to that needed for a 50 kg person carbohydrate loading; just add
500 ml of sports drink to get 450 g.