Finding the energy balance for better bone health

by Karl Cooke

For some time now research has suggested that dancers may be at an increased risk of low Bone Mineral Density (BMD). BMD is measured using a low radiation x-ray (or DXA scan) and is a good indicator of overall bone strength. Strong bones are less likely to fracture but after the age of 35-40 our BMD and bone strength progressively decline and the rate of this decline is increased following the menopause in women. Osteoporosis is a disease characterised by fragile bones that are more susceptible to fracture, where the BMD is considerably below the normal range for young adults.

Dance and many other forms of exercise help to improve the density of our bones by providing an important strain stimulus for bone growth and therefore may help to minimise the effects of osteoporosis in later life. However, the current research suggests that many young dancers (and endurance athletes) may have a lower than average BMD for their age. The cause of this low BMD has been extremely difficult to establish but at least in female dancers and athletes it has been possible to determine that critical changes occur to the hormonal systems that regulate bone growth.

The balance between energy expenditure and energy intake is at the centre of the reason why we think that dancers have low BMD. When the female body is exposed to an energy deficit (where the expenditure is greater than the dietary intake) a number of important hormonal changes occur rather quickly. These hormonal changes can result in reduced bone formation and menstrual irregularities: oligomenorrhea (4-9 periods per year) or amenorrhea (0-3 periods per year). In some cases the negative effects of the energy deficit may not result in noticeable changes to menstrual regularity but it may still result in compromised bone growth.

Balancing the high energy demands of class, rehearsals and performances with a proper diet and rest is a significant challenge but one that needs to be met in order not to compromise health.

Energy deficit can have a number of negative side effects:

  • Negative moods
  • Fatigue
  • Increased susceptibility to illness
  • Low muscle stores of carbohydrate
  • Menstrual irregularity
  • Reduced bone formation

The size of the energy deficit required to cause a disruption to normal hormonal function is thought to be 14 kcal per kg of fat free mass per day or more. A simple body composition test can give you an estimate of your fat free mass by determining your percent body fat. For example, if you weight 60 Kg and have a percent body fat of 20%, then your fat mass is 12 Kg (60 x 20% = 12) and so your fat free mass is 48 Kg (60 – 12 Kg = 48 Kg). Therefore an energy deficit of 672 kcal per day (14 x 48 = 672 kcal) would lead to serious side effects such as those mentioned above.

If you consider that many athletes will have a total energy expenditure of 3,000 to 5,000 kcal per day it is easy to see how 600 to 700 kcal less than required might not be consumed. Unfortunately, there are no studies that have actually measured the total energy expenditure of dancers but it is likely to be in the order of 3,000 kcal per day. This suggests that the sort of energy deficit that is likely to have negative effects is in fact a relatively small proportion of the total daily expenditure. This is why it is essential to try to balance your dietary intake with your expenditure, the closer you can match intake with expenditure in your daily routine the less likely you are to have problems.

The following are signs that may indicate if you are at risk of low BMD:

  • Fewer than 10 menstrual cycles per year
  • Stress fractures
  • Body Mass Index (BMI) < 18.5
  • 16 or older when you had your first menstrual cycle
  • a history of irregular menstrual cycles

Our research has shown that dancers and athletes with low BMD normally have weaker and smaller muscles which will have an obvious impact on their movement and overall performance; dancers with low BMD tend to store more fat around the abdomen.

Overall, the research suggests that there are a large number of negative side effects to frequently imposing an energy deficit on the body and for current and long term health it is essential to try and ensure energy balance.

Originally published in Dance UK  magazine, Issue 60 – Spring 2006