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Why osmosis make sports drinks a simpler choice than gels

Illustration by Fred Rix

Nearly all the biochemical reactions that occur in body cells depend on water and electrolyte (sodium, potassium, calcium, chloride, phosphorous, magnesium, etc.) balance. These balances are not only vital to maintaining life, but also affect physical and mental performance.

Water is the most abundant component of the body (more than 60 per cent by weight). I believe it was Mike Colgan of the Colgan Institute who referred to the body as a "hairy protein bag full of water." This bag of water has many holes that allow for leakage. These holes include skin pores that allow for perspiration (skin leakage); the kidney/bladder system that expels wastes carried by water; and the respiratory system that must be moist, or breathing would be very dry and painful. Adequate hydration is very important in the maintenance of body temperature. When muscles contract, they generate heat, which must be dissipated from the core to the body surface and adequate water to maintain adequate blood volume is vital.
Sports drinks are widely used during exercise to avoid the onset of dehydration and delay the depletion of the body's carbohydrate stores. It is this popularity and the many mistakes I see many athletes practising and marketers promoting that prompted me to write this explanation of sport drinks.
It is well established that sports performance can be impaired by many causes, but the dominant factors leading to premature fatigue are:

• Depletion of the body's carbohydrate stores;
• Onset of dehydration resulting from the loss of water; and
• Loss of electrolytes through perspiration.

The main purpose of sports drinks is to prevent dehydration, to supply energy and to replace electrolytes.

Sports drinks are said to be some of the best-researched food items in the market place today. In the sport science community and in published research, there is a consensus about the optimal composition of such drinks - sports drinks should contain water, carbohydrates as an energy source, and the electrolytes listed above.

All of this must be in a defined osmolality consistent with optimal gastric emptying. In other words, the water, sugars and the electrolytes must be in a concentration that allows for the free flow of the drink from the gut to the blood.

Please be aware that "recovery drinks" serve a different purpose than "sports drinks" and are based on the science of glycogen storage and amino acid replacement. Also be aware that "energy drinks" are not sport drinks and were never designed as such. They are fundamentally liquid candy that is caffeinated, and sometimes fortified with a few vitamins.

Some of the most thorough research on this topic was reported by physiologist Ethan Nadel, PhD, of Yale University in American Scientist in 1988 when he was lead physiologist for the Daedalus Project (man-powered flight from the Greek Island of Crete to the island of Santorini across the South Aegean Sea). This work was followed by physiologist John Greenleaf, PhD, at the NASA Ames Research Centre in California and reported in several technical bulletins from NASA in 1992.

It's all about osmolality
The effectiveness of sports drinks is dependent on the osmolality of the product. Sports drinks come in various forms - pre-mixed commercial sport drinks, powdered ingredients that must be mixed in water, the convenience packed gel products and "homemade sport drinks." Ignoring osmolality is where most of the mistakes are made. Imbalances will inhibit gastric emptying, intestinal fluid absorption, blood flow and induce side-effects such as fever and cramping.

This low osmolality of the gastric fluids would be defined as hypotonic (less concentrated than the blood). On the other hand, if the gastric fluid were more concentrated (meaning more particles and a high osmolality) than the blood, there would be net flow of water from the blood to the gut, and this would be known as a hypertonic solution.

The osmolality of blood hovers around 280 to 290 mmoles/kg (that's how osmolality is measured), so the best sport drinks have osmolality measures near or below these values. Ronald Maughan reported in the Canadian Journal of Applied Physiology in 1999, after a thorough review of the science, that the optimal osmolality for sports drink to be defined as in "the slightly hypotonic range between 200 and 250 mmoles/kg."

The carbohydrate content of the drink seems to exercise the greatest influence on osmolality (electrolyte content also contributes), and that is why you see most physiologists recommending six- to eight-per cent carbohydrate solutions (see the most popular grocery store sport drinks). This usually results in a solution of 12 to 16 grams of carbohydrate (50 to 60 calories). However, there are a few drinks that are 10 to 12 per cent carbohydrate and contain up to 25 grams of carbohydrate (100 calories).

This can make a big difference in the glycogen-sparing action during an Ironman or ultra-marathon race. Even mild dehydration - one per cent of body weight, which would represent approximately 0.75 to 1 litre of water (one per cent of 75 kg equals 750 ml) - can reduce muscle performance and start to show dehydration symptoms. If the dehydration is two to three per cent, serious performance inhibition occurs. Exercise physiologist David Costill, PhD, of Ball State University, demonstrated that at these low levels of dehydration, one to three per cent, even the time for a 1,500-metre run was inhibited. The time for a competitive 10K was reduced by 2.5 minutes, which is serious in a 30-minute 10K.
So, how can there be such a variation in carbohydrate content if osmolality is to be maintained? The answer is - not all sugars are created equal.

Not all sugars are equal
Osmolality is influenced by the concentration of particles in solutions - the number of particles and not the size of the particles. Glucose (also known as dextrose) is a monosaccharide or single-molecule sugar, sucrose is a disaccharide or two-molecule sugar, and maltodextrin is a glucose polymer or polysaccharide or multiple-molecule sugar. Therefore, a single molecule of glucose or sucrose or maltodextrin will have the same osmotic effect, but these single units are of different sizes and therefore supply different amounts of calories from carbohydrate. So the grocery store sport drinks are primarily sucrose and maybe glucose, and therefore can deliver fewer grams of carbohydrate than the more sophisticated drinks made from maltodextrin. Maltodextrin is defined as a chain of glucose (usually eight to 10, but could be more) molecules "loosely" linked and therefore easy to digest. It is known as complex carbohydrate but still has a very high glycemic index that allows for rapid consumption. Hence, the better sport drinks can deliver more energy without upsetting the osmolality.

Sam Mettler, PhD, of ETH Zurich, reviewed the osmolality and pH of 35 sports drinks and 53 other drinks commonly used by athletes and reported the results in a Swiss Journal of Physiology in 2006. Among commercial sport drinks, the osmolality varied from 210 to 391 mmoles/kg and the carbohydrate grams varied from 4.1 to 15.1 grams. Only three were considered in the ideal range. Fruit drinks and sodas all tended to have very high (hypertonic) osmolality.

This review also included several "gel" products that create some unique problems. I always tell my clients that using a gel or a syrup concentrate while performing requires you to "play biochemist" on the run. An average gel provides 20 to 30 grams of carbohydrate (and few, if any, are made with maltodextrin), and that means a very specific amount of water must accompany a gel in order to avoid upsetting the osmolality. In addition, they may contain milk products, gycerin, carnuba wax or gelatin, all of which will affect the osmolality and the glycemic index. If the average gel contains 25 grams of carbohydrate, you would require 1.7 cups of water for a six-per cent solution, and 1.2 cups for an eight-per cent solution. When someone says they carry gels "just in case, because they would be better than nothing," they don't understand how sport drinks work. They may not be better than water alone if you don't drink enough water at the exact same time as you ingest the gel.

So for optimal performance and optimal protection from dehydration, I recommend you use a reputable, scientifically based product (mix according to directions) and avoid playing biochemist on the run. Long endurance challenges require more than just water. The delivery of energy and electrolytes is also critical to avoid fatigue and health problems.

L. Lee Coyne, PhD, is a Calgary-based nutritional consultant, lecturer and author of Fat Won't Make you Fat, The Little Book of Nutritional Nuggets, and the Lean Seekers coaching program.

"Smart Sipping: Sports Drink Science" first appeared in the 2010 May/June Multisport Issue of IMPACT Magazine.