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Quick googles on Rett and growth & nutrition. It seems that nutrition is a problem.

Excerpt 2012 study:

Growth failure is a prominent feature in Rett syndrome (RTT); however, no RTT-specific growth charts exist. Many comorbid disorders have an impact on growth in RTT, such as oropharyngeal and gastrointestinal dysfunction, scoliosis, seizures, and osteopenia. The pattern of growth in female patients with RTT trends well below the lowest centile on standard growth references,1 which fail to differentiate a normal RTT growth pattern from one caused by malnutrition or illness.

Disease-specific standards screen for disease2–13 and measure the effect of therapeutic interventions designed to improve nutrition and neurologic function.14,15 With more than 200 mutation sites identified in the methyl-CpG-binding protein 2 gene (MECP2), the clinical severity in RTT varies widely.16 Associations exist among specific mutations and functional outcomes such as ambulation, hand use, and language.17 No study has generated accurate RTT growth references or adequately examined the associations between the degree of growth failure and genotype or clinical severity.

The aim of this study was to develop RTT-specific growth references for clinical and research use. The secondary objectives were to 1) compare the patterns of growth between patients with RTT and the normative population and 2) examine the effects of secular changes, disease severity, and MECP2 mutation on growth.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3468773/

Doctoral thesis:

PREVALENCE OF OVERWEIGHT AND OBESITY IN CLASSIC RETT
SYNDROME

...

Introduction: Rett syndrome (RTT) is a genetic, neurodevelopmental disorder
that displays high incidence of growth failure, variations in body composition,
feeding/gastrointestinal complications, and undernutrition. The ongoing Rare Diseases
Clinical Research Network (RDCRN) Rett Syndrome Natural History Research Project
has yielded observations regarding the less recognized occurrence of overnutrition in
adult females with classic Rett syndrome. The purpose of this study was the determine
the prevalence of overweight/obesity in children and young adults with classic Rett
syndrome, the percent overweight and obese among adults with classic Rett syndrome,
and to determine if the prevalence of overnutrition increases after the pubertal years and
in to adulthood.

...

Results: No statistical significance was found in the frequency of overweight and
obese between children and young adults with RTT. Statistically significant differences
were found within the pairwise comparisons of BMI z-scores between the ages of 2.00-
3.99 and 10.00 -11.99, between 2.00-3.99 and 14.00-15.99, and between 4.00-5.99 and
14.00-15.99. Among RTT women, 13.64% were overweight and 0% were obese.
Conclusion: This analysis of data concluded that there is no difference in
prevalence of overweight and obesity among children and young adults with classic RTT
and no identifiable pattern in changes of BMI z-scores was found as age progressed
across these groups.

http://www.mhsl.uab.edu/dt/2013/Grigsby_uab_0005M_11018.pdf

Abstract
Objectives:
Feeding difficulties in Rett syndrome are complex and multifactorial. In this study, we describe the feeding experiences in Rett syndrome and examine the factors affecting growth.
Materials and Methods:
Using questionnaire data related to a population-based cohort, ages 2 to 29 years (n = 201), we measured the feeding experiences, growth, and factors affecting growth (enteral nutritional support, mutations, mobility, breath-holding, hyperventiliation) in subjects with Rett syndrome.
Results:
The mean weight, height, and body mass index z scores in subjects with Rett syndrome were below that of their age group and decreased steadily with age. Twenty percent of subjects had enteral nutrition support, and it was more common in the older age group. Those with truncating mutations had significantly less enteral nutrition support than the other mutation groups. Furthermore, those with low mobility had lower mean body mass index z scores than those with higher mobility, and increased frequency of breath-holding and hyperventilation also was associated with lower body mass index z scores.
Conclusions:
Routine monitoring of growth should continue to determine the severity of nutritional problems in Rett syndrome. Active nutritional management is recommended to ensure females affected with Rett syndrome have the best opportunity to reach their growth potential.

https://insights.ovid.com/crossref?an=00005176-200711000-00011