Research Review By Dr. Jason Izraelski©

Date Posted:

September 2010

Review Title:

Supplements for Endurance Athletes

Publication Information:

Kersick C & Roberts M. Supplements for Endurance Athletes. Strength and Conditioning Journal 2010. 32(1); 55-64.


Endurance athletes have always strived to find nutritional supplements to give them an edge in grueling contests where tremendous amounts of endurance are required. In fact, one 2004 survey surveyed 207 collegiate endurance athletes and determined that only 11% of the respondents had never consumed a nutritional supplement. The reasons for needing supplements while completing endurance activities is self-evident; it is estimated that a typical male endurance athlete will expend approximately 1000 kcal and a female athlete 600-700 kcal when completing an hour of activity at approximately 70% of their maximal oxygen uptake (VO2max).

While there are many supplements on the market that are capable of increasing energy levels and decreasing rates of injury, there are many available supplements not verified by scientific research. This review will discuss those supplements that have adequate research for being beneficial.

Carbohydrate-Electrolyte Beverages

Carbohydrate-electrolyte supplements are considered to be the most important supplement for endurance athletes, and the concept has been around, albeit it in a primitive form, since the early 1900s when endurance athletes would ingest sugary candy and sweets before events. By the 1960s, the first carbohydrate-electrolyte drink became available. Since then, research has shown that these supplements can increase performance, attenuate fatigue, replace lost fluid and carbohydrates, prevent extraneous electrolyte loss and assist in thermoregulation during prolonged activity.

Fluid and Electrolyte Replacement
An average 70 kg athlete can lose up to 1.2-1.7 liters of bodily fluid per hour. This is significant as serious performance decreases have been shown after only a 2% reduction in body mass derived from fluid losses. In long-distance endurance events such as marathons where elite times consistently fall under 2 hours and 10 minutes, a reduction in performance would be seen after the first hour if fluid replacement was not undertaken. Simply replacing bodily fluid with water in itself would be beneficial, however, electrolyte losses must be considered as well. Supplementation with only water may lead to the development of hyponatremia (low blood sodium), a serious and potentially deadly complication.

Due to the variety of courses, temperature and individual physiology, it is impossible to give a specific recommendation on fluid intake. However, there are two practical pieces of advice to be considered: The first is for athletes to weigh themselves before and after exercise ensuring that weight loss is no more than 1% of total body mass when exercising for greater than 90 minutes. The second piece of advice is for athletes to consume a minimum of 100 mL of fluid for every 10 minutes for a total of 600 mL/hour, an amount shown to offset the magnitude of fluid loss seen when exercising for prolonged periods in hot and humid conditions. Table 2 shows how quickly body mass can be lost through fluid losses, and gives recommendations on fluid replacement.

Carbohydrate Replacement
Replacing carbohydrate stores is a primary concern for endurance athletes as it has been shown that stores become severely depleted after 60-90 minutes of prolonged exercise. This is equivalent to approximately 60 grams of carbohydrate per hour in an athlete exercising at a moderate intensity (65-75% VO2 max). Ingestion of 30-60 grams of carbohydrate per hour will significantly spare liver glycogen, sustain blood glucose levels and increase time to exhaustion. This can be accomplished via a number of commercially available supplements (Table 3) that deliver carbohydrate solutions at a concentration of 6-8% carbohydrate (i.e. 6-8 grams of carbohydrate for every 100 mL of fluid). At this concentration, consuming 0.5-1.5 cups (4-12 ounces) of fluid every 10-15 minutes will replace lost carbohydrates, fluids and electrolytes. One caveat to carbohydrate replacement is that fructose should not be used as the source carbohydrate. Fructose is known to decrease performance and glycogen re-synthesis as well as cause gastrointestinal distress due to different digestive kinetics and absorption mechanisms.


Caffeine is an alkaloid found in more than 60 plant species and reports of its use as a stimulant goes back several centuries. Caffeine enters the blood stream within 15-45 minutes of ingestion and has a half life of 2.5-7.5 hours. Ergogenic benefits are shown at 3-9 mg/kg and are derived from their ability to increase serum catecholamine (sympathetic ‘flight or flight’ hormones) and free fatty acid levels. This leads to a reduction in perceived effort as well as an increase in fat utilization and subsequent sparing of muscle glycogen.

Exercise test outcomes have been shown to improve by 9.1-15.4% when caffeine supplementation was provided, and even higher when combined with a carbohydrate-electrolyte solution.

New research has shown that after completely depleting muscle glycogen stores, a high-carbohydrate meal combined with caffeine significantly increased the rate of glycogen re-synthesis (66% greater) as compared to a high-carbohydrate only control group. This suggests that caffeine may also play a role in the recovery process; however, more research is required before definitive conclusions can be drawn.

Athletes participating in National Collegiate Athletic Association (NCAA) or International Olympic Committee (IOC) sanctioned events should exercise caution when using caffeine as it is a banned stimulant at urinary levels of 12 mg/mL. However, ergogenic effects are still demonstrated in urinary caffeine levels below the banned levels. Numerous studies contend that caffeine should be used with caution due to its diuretic effects that when combined with fluid and electrolyte losses associated with prolonged exercise in hot and humid environments, could be a health concern.

These findings have been combated by many clinical trials showing that the diuretic effect of caffeine is similar to that of water, and that a caffeine-carbohydrate solution has no negative impact related to hydration, electrolytes and other relevant markers.

Addition of Protein to Carbohydrate

Regardless of its form, making carbohydrates available allows endurance athletes to restore muscle glycogen. Ensuring proper timing of carbohydrate intake is important as much of the muscle’s ability to recover is lost after the first 2 hours post-exercise. If the athlete does not need to recovery quickly from activity, maximal glycogen restoration can be accomplished by ingesting 1.2 grams of carbohydrate per kilogram every 15-30 minutes for several hours (approximately 84-120 grams of carbohydrate per hour for 70-100 kg athletes).

Another way to restore maximal glycogen restoration is to ingest 8-10 grams of carbohydrate per kilogram per day (560-1000 grams of carbohydrate per day for 70-100 kg athletes). Many scientists have begun adding small amounts of protein to existing carbohydrate-electrolyte solutions, and have found a corresponding increase in performance and muscle glycogen recovery as well as a decrease in muscle damage.

Performance Benefits
Adding protein to carbohydrate at a rate of either 3:1 or 4:1 (carbohydrate to protein) is now thought to be beneficial for athletes. Studies using cyclists have shown that ingesting a 4:1 carbohydrate:protein solution in 200 mL doses every 15 minutes during exercise can increase the time to exhaustion as compared to the carbohydrate only group. Additionally, when a carbohydrate plus protein solution was ingested immediately following an exhaustive cycling trial, the group showed an increased power production when an additional exhaustive bout was undertaken 6 hours later.

Glycogen Re-synthesis
There are mixed reports that adding protein to carbohydrates may help promote recovery of lost muscle glycogen. Using cycling trials to deplete muscle glycogen, recovery was then measured using a variety of carbohydrate, protein and fat combinations. The authors of this particular study concluded that muscle glycogen was significantly higher in the carbohydrate plus protein plus fat group 4 hours after ingestion.

Further work has suggested that high carbohydrate intake of 84-120 grams of carbohydrate per hour for 70-100 kg athletes may be all that is required to promote maximal glycogen recovery, but added that protein may support muscle protein synthesis and net protein balance after exercise.

Muscle Damage
There is emerging evidence that a carbohydrate plus protein solution or gel ingested during and after prolonged endurance exercise can prevent the muscle damage associated with this type of activity. One study measured the serum levels of creatine kinase, a marker of muscle damage, during prolonged cycling.

The results of the study indicated that both the carbohydrate and carbohydrate plus protein solutions and gels significantly reduced creatine kinase levels. A further study of endurance athletes showed that net protein oxidation rates, a marker of muscle breakdown, were not different between carbohydrate and carbohydrate plus protein groups. Evidence for carbohydrate plus protein solutions reducing muscle damage comes from its ability to improve overall net protein balance.

While most studies do show a negative protein balance following supplementation with a carbohydrate plus protein solution, implying that muscle breakdown rather than synthesis is occurring, these findings suggests that such a solution did aid in preventing muscle breakdown and helped improve recovery (i.e. more breakdown would have occurred without the supplementation).


Endurance athletes place great demands on their bodies and require supplementation while training and competing for the basic need of homeostasis. In addition to this, many athletes supplement in order to improve performance and promote recovery while decreasing time to exhaustion and muscle damage. There are many supplements available commercially to endurance athletes.

Unfortunately many of these supplements lack scientific evidence and as such cannot be recommended. The three supplements reviewed here can safely be recommended to endurance athletes. Carbohydrate-electrolyte solutions, caffeine and carbohydrate-protein solutions are natural and legal complements to an athlete’s training regimen and can be used as ergogenic aids to boost performance.