Physical activity is measured using waist-worn ActiGraph GT3X and GT3X+. Outcome measures will be counts/min, time spent in sedentary, light, moderate and vigorous activities. The participant are asked to note down non-wear periods in a logbook - mainly to increase compliance - and to note the amount of bicycling and if the child was sick within the measurement period. Questions regarding TV-watching, playing with videogames and physical activity, will help identify activity-pattern.

Sleep is measured using ActiGraph GT3X and GT3X+ with the monitor placed on the right hip for all students and on the non-dominant wrist for a sub-sample. Using a sleep diary, total sleep time and sleep quality will be derived from these actigraphy measurements. In addition, the abbreviated version of the Children's Sleep Habits Questionnaire (CSHQ) is used to identify sleep problems.

Cardio-respiratory fitness is measured using the Andersen test, which is a 10-min intermittent running test to estimate maximal oxygen uptake (mL*min-1*kg-1).

Dietary intake is measured as micro-and macronutrients as well as food groups (e.g. fish and fruit/vegetables) by means of a 7-day dietary survey, which is a web based recall record method developed especially for children. Families without internet access can use a paper version of a 7-day dietary survey. The meal time perception is measured as well.

Leptin and ghrelin. If resources are available at the end of the study period, ghrelin will be separated in acylated and deacylated ghrelin, and both the effect of the intervention on total, acylated, deacylated and the ratio between acylated:deacylated ghrelin will be assessed. If the resources are too tight at the end of the study, only total ghrelin will be used.

Serum concentrations of 25-hydroxyvitamin D [25(OH)D] will be used as marker of vitamin D status, related to food intake and time of season.

Bone Health will be evaluated by DXA scanning (total body and spine Bone Mineral Content (BMC), Bone Mineral Density (BMD), and Bone Area (BA). Osteocalcin and serum Parathyreoideahormon (PTH) are also included as markers of bone health.

Whole-blood hemoglobin and serum ferritin will be used as measures of iron status

The following five measures from the attention test D2 will be used:

TN=Total Number of items processed E%=Percentage of errors

Student wellness is assessed by means of the Danish WRS Wellness Rating Scale filled out by students. The scale contains five subscales and one combined scale (sum score) adding the five subscales. The subscales cover: Living environment, school, school mates, family, and self concept.

Student learning is assessed by means of the Danish LRS Learning Rating Scale filled out by students. The scale contains four subscales and one combined scale (sum score) adding the four subscales. The subscales cover: Academic learning, social situation in school, teaching style appropriateness, and school expectations.

Mathematics proficiency is assessed by means of the Danish MG test for 3rd And 4th grade. They each have one measure: Number of correct results within 45 minutes.

Reading proficiency is assessed by means of the Danish Sentence Reading test 2 having two measures: Percentage of correct sentences read within eight minutes. Reading speed - number of sentences reached.

Among those children with a diagnosis of Attention Deficit/Hyperactivity Disorder (ADHD) or Attention Deficit Disorder (ADD), the mean daily dose of methylphenidate (based on reported intake of drugs containing this substance) during the last two weeks will be evaluated.

Self-reported data collected by personal computer-assisted questionnaire interview with the parents. Outcomes are the number of days, during the last two weeks, with: asthma symptoms, use of asthma-medicine, hay-fever symptoms, use of hay-fever-medicine, atopic eczema, symptoms such as sore throat or fever, and the use of pain killers or antibiotics. The number of days of absence from school due to illness, the number of days of illness during weekends and holidays, and number of visits to the doctor will also be evaluated.

Plasma concentrations of the cytokines Interleucin 6 (IL-6) and Tumor Necrosis factor (TNF-alfa), the adipokine adiponectin and the acute-phase protein C-reative Proetin (CRP)(measured as high-sensitive) in plasma will be used as markers of subclinical inflammation and related to the other early metabolic syndrome and cardiovascular risk markers.

Self-reported data collected by personal computer-assisted questionnaire interview with the parents. Data are computed on household level. For education and occupation household level = level for the parent or cohabiting partner with the highest value. Education is computed as years of education and as education type. The occupation categorisation is based on the system SOCIO used by Statistics Denmark.

Systolic and diastolic blood pressure, serum blood cholesterol (total, LDL, VLDL and HDL) and triglyceride, plasma glucose, serum insulin and the HOMA index.

Pubertal status is assessed by menstruation status (menstruation started or not) and breast development in girls (Tanner stage 1-5) and by development of pubic hair in boys (Tanner stage 1-5).