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Bone structure in Sports Nutrition: why bother?
Francis Holway
Departamento de Medicina Aplicada a los Deportes, Club Atlético River Plate,
Buenos Aires, Argentina.
Sports Dietitians are mainly concerned with an athlete’s fatness. This is logical since
excess fatness can hamper performance in many activities where the force of gravity is a
concern. A second concern is whether an athlete has the right amount of muscle mass,
since muscle contractions are what generate movement, and thereby, sport. These two
tissues, adipose and muscle, can be modified in weeks and months by the appropriate
amalgam of nutrition and exercise. However skeletal tissue is largely ignored, except for
DEXA evaluations when low-bone mass is suspected as a consequence of, perhaps,
eating disorders. Nevertheless an evaluation of bone structure can aid the Sports
Dietitian and the whole Sports Medicine team in understanding performance and
reaching the right decision regarding dietary planning.
Modern anthropometry technique evolved from the time when anthropologists were
using engineering calipers to measure skeletal remains. Fossil bone breadths and lengths
were studied to estimate the size of individuals, and thereby infer the nutrition to grow
to such a dimension and the physical activity performed to find that quantity of food.
The skeletal remains told the tale of our ancestors’ sports nutrition. Living-tissue
measuring tools like skinfold calipers and limb girth tapes came later, but currently
dominate anthropometric assessments. The old art and science of measuring the
skeleton lost favor to the over-emphasized skinfolds or was delegated to DEXA
machines, provided you fit into them. Unfortunately this turn of affairs has diminished
our capacity for analysis, to the point where sometimes an incorrect dietary prescription
can hamper performance. Take, for example, a long-distance runner who wants to lose
weight to match the mass-to-height relation of East African marathoners. This runner
has the same height as his record-performing counterparts, but is about seven kilograms
heavier, and asks the dietitian for a diet to lose this weight. When the dietitian runs some
skinfolds on the athlete, we find his fat to be at rock-bottom levels, so any weight loss
will diminish muscle mass, thereby impoverishing his muscle-to-bone ratio, decreasing
performance instead of improving it.
So why does our runner weigh so much more that his elite competitors? Height and
skinfolds are similar to the winners’, but his skeleton is about two kilograms heavier.
How can only 2 kg affect weight and performance in such a way? I like to call it “the
bookcase effect”. Imagine two empty wooden bookcases of equal height and depth,
about 170 cm tall and 30 cm deep, each with six shelves. However one is only 10 cm
wider and weights 2 kg more. Now fill the shelves with good sports nutrition books, and
the wider bookcase will weigh about 20 kg more. Likewise with having skeletal
breadths that are only a few centimeters wider, especially trunk breadths: the mass of
the skeleton may not be that different, but when filled with lean tissue the difference is
large. One particular trunk breadth that notably affects body mass is the hip-breadth or
biiliocristal; in fact, we find that in groups of athletes this bone breadth explains
differences in body mass better than height! In 302 male athletes from gymnastics to
rowing measured at the Montreal Olympic Games in 1976, biiliocristal breadth had a
correlation of 0.790 (p < 0.001) with body mass, better than height (r = 0.757; p <
0.001). Moreover, this breadth is the one chosen to plug into regression equations to
estimate the total body mass of our extant hominid relatives in anthropology.
On the other side of the scale many athletes attempt to gain muscle and weight to help
them in their sport, like gridiron football, rugby, baseball, or throws, to name a few. We
have often come across difficulties in athletes trying to gain weight with biiliocristale
breadths under 26 and 25 cm for adult males and females respectively. In these “hard-
gainers”, most or all their bone breadths may correspond to one standard deviation or
less below the reference norm. This may mean that the bone area available for muscle
tendon attachments may be a limiting factor in muscle hypertrophy potential, so by
assessing bone breadths we may be able to estimate maximal non-pharmacological
muscle hypertrophy potential. Several years of field experience have shown that male
athletes attempting to maximize muscle mass usually plateau at about a muscle-to-bone
ratio of 5.0, while this value is 4.2 for females. This means that if you are a male with a
9.0 kg skeleton, you will find it very hard to train and feed your way past 45.0 kg of
muscle, although male body-builders in doping-free events reach the 6.0 mark.
Assessing bone structure can help the Sports Dietitian infer if the athlete matches the
skeletal proportions found in the elite, and aid in establishing “floors and ceilings”,
lower and upper limits of body mass modification.