The brain is the most energy intensive organ in the human body, so it is to be expected that weaknesses in brain energy metabolism could be a potential factor in neurodegenerative conditions. This is the starting point for a systems biology study of how known Parkinson's disease (PD) risks can weaken brain energy metabolism and contribute to preconditions for disease. We begin by describing PD as a multi-factorial condition in which energy deficits form a common denominator for known risk factors. This is followed by a description of a mathematical model of brain energy metabolism, and its structural and dynamic properties. Simulations of the model are then used to illustrate how external risk factors, plus structural and dynamic weaknesses in neural energy supplies, particularly affect the neurons most vulnerable to PD damage. Taken together these issues form the basis of an energy-deficit theory for how the preconditions for PD are formed.