All legionnaires of this rapid reaction force -- more than 3,000 soldiers -- underwent a medical exam in September that measured their body mass index (BMI), a measure of body fat.
The result: "Six percent of the members, or 180 legionnaires, had a BMI of over 30 (the obesity limit) and the decision was taken to launch a plan to help them," a military source, who asked not to be named, told AFP.
"Those who join the Legion's brigade are very competitive, everyone wants to take part in everything, but those with a high BMI won't participate in military parades or activities that affect the image of the Legion."
Or as an internal document quoted by the El Pais daily said, "without heightened vigilance, we would be faced with loss of prestige."
The weight-loss programme includes a nutrition plan and sport, as well as medical tests, conferences and psychological monitoring.
The Spanish Legion is not the only army unit to develop this type of programme.
But according to the internal document quoted by El Pais, the weight issues stem from the relatively high average age of members of the Legion as well as a lack of sports equipment that would give them "varied and motivating" training.