There are plenty of studies. If You are constructing dietary plan and exercise program You should be careful because of all the changes in physiology in individuals with down syndrome. You should be aware of lower relative peakVO2, reduced catecholamine response to exercise, chronotropic incompetence and limited cardiac output at peak exercise intensities and other factors that are demanding different exercise intervention. (e.g. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3012449/  ,

http://europepmc.org/abstract/med/8776225 )

Of course the same goes for nutritional intervention.

Some literature You might find helpful:

http://www.ncbi.nlm.nih.gov/pubmed/23307488

http://forum.annalsoftransplantation.com/abstract/index/idArt/459195/act/3

http://www.sciencedirect.com/science/article/pii/S0891422297000085

http://www.tandfonline.com/doi/abs/10.1300/J083v38n01_03#.VSlGgPmsX50

http://www.sciencedirect.com/science/article/pii/S0002822396003306

http://cpj.sagepub.com/content/31/6/341.short

http://ptjournal.apta.org/content/87/10/1399.short

http://www.aaiddjournals.org/doi/abs/10.1352/0895-8017%282004%29109%3C165%3AIIPFIA%3E2.0.CO%3B2

http://onlinelibrary.wiley.com/doi/10.1002/14651858.CD005176.pub4/abstract;jsessionid=FAFA08426641138949A2352C922D73C8.f04t04?deniedAccessCustomisedMessage=&userIsAuthenticated=false

http://journals.lww.com/pedpt/Abstract/2005/01710/Effects_of_Aerobic_Conditioning_and_Strength.5.aspx

http://journals.lww.com/co-psychiatry/Abstract/2005/03000/Exercise_and_well_being__a_review_of_mental_and.13.aspx

http://onlinelibrary.wiley.com/doi/10.1111/j.1600-0838.2010.01120.x/abstract?deniedAccessCustomisedMessage=&userIsAuthenticated=false

Kind regards,

Jan Homolak

Well, I can suggest one of my papers, which shows calculations to obtain Status (pertaining-to-mass), as percentage. This could be taken as measure of obesity, but you have to use growth charts for down-syndrome children of your region, not the normal charts.

Growth-and-Obesity Profiles of Children of Karachi using Box-Interpolation Method

http://www.ngds-ku.org/Papers/J29.pdf

You may like to visit our project website http://ngds.uok.edu.pk for additional resources.