Brief summary: This study is conducted in the context of a 60 days strict bed-rest study with 6° head-down-tilt (HDT) body position. Food intake can be challenging during bed-rest due to loss of appetite and constantly feeling full. The study aims to assess whether or not energy intake varies during a meal when the energy density of a meal is increased by the fat content. This experiment follows a crossover design with repeated measures within subjects. The order of experimental conditions is balanced among participants and sex, and the assignment of orders to participants is randomly. On each test day, participants are provided with breakfast, lunch, and dinner as usual. Across test days, the ED of breakfast (24 participants) or lunch (24 participants) is modified to two levels: the standard level and an increased level (+ 0.42 kcal/g compared to the standard). The meals are adjusted in energy density without noticeable changes in appearance or taste. These meals have identical ingredients, but the macronutrient proportions are modified. Entrées with increased fat content are prepared by incorporating neutral-tasting vegetable oil into the standard recipes. Participants receive their meals and are given the flexibility to eat ad libitum for 45 minutes. To comply with the strict bed rest rules, the next meal will be used for counterbalancing total energy intake over the day if necessary. In case of problems, a nutrient drink could be used to assure the requested total energy intake in order to maintain body weight.The experiments are conducted during 4 campaigns, each campaign consisting of 12 participants.

Inclusion Criteria:

• Physically and mentally healthy test subjects that declare their willingness to participate in the entire study and successfully pass the psychological and medical screening
• Aged between 24 and 55 years
• Body mass index between 18 to 28 kg/m2
• Body height between 153 to 190 cm (+/- 2 cm)
• Signed informed consent
• Demonstrable medical insurance and official certificate of absence of criminal record
• No less than 2 SD below the average normal bone mineral density (measured by dual-energy x-ray absorptiometry) for hip and lumbar spine (i.e., mean peak bone mineral density of Caucasian, sex-matched young adults) denoted by T-score (T-score > -2)
• Demonstrable dentist certificate
• Hormonal contraception for female participants is allowed, but details (manufacturer, doses, dates) must be reported

Exclusion Criteria:

Candidate subject exclusion criteria for the study will include the following:

• Any infectious, genetic, autoimmune, endocrinological, hematological, cardiovascular, lung, gastroenterological, kidney, rheumatic, muscle, bone, orthopedic, dermatological, gynecological, urological, ophthalmological, otorhinolaryngological, neurological or psychiatric disease (see at the end of document; a) that precludes a safe participation in the study or might significantly impact scientific results of the study according to the study physician
• Drug, medication or alcohol abuse (regular consumption of more than 30 g alcohol/day)
• Smoker within 6 months prior to enrollment
• Any long-term medication use that might significantly impact scientific results of the study (for instance bisphosphonate therapy, anti-hypertensives, gastro-esophageal reflux).

Special dietary requests, that cannot be provided by the project team (e.g. vegan, vegetarian or other ineligible diet)

• Claustrophobia
• Increased intraocular pressure
• Hyperopia / Myopia > +/- 5.0 Diopters
• Astigmatism > 3 Diopters
• History of laser surgery of the eye, glaucoma and retinal surgery
• Any ocular condition that would significantly impact visual function
• Significant chronic gastro-esophageal reflux precluding a safe participation in the study
• Clinically relevant allergy (e.g. requirement to carry a stand-by emergency injection), that precludes a safe participation in the study
• History of chronic back complaints, that precludes a safe participation in the study
• History of kidney stones
• Anemia: Hemoglobin under normal values (see at the end of document; b). (Normal values of Hemoglobin for men: 13.0-17.5 g/dl; women 12.0-16.0 g/dl)
• Elevated risk of thrombosis (see at the end of document; c)
• Inability to successfully complete a 10-minute upright posture test without clinically relevant orthostatic symptoms
• Current or history of hemorrhagic diathesis or coagulations disorders
• History of spinal cord disease, including radiculopathy, myelopathy, or neuropathy
• History of disorders affecting CSF circulation (i.e. hydrocephalus, idiopathic intracranial hypertension) History of more than one depressive episode and/or history of a severe depressive episode, such as requiring inpatient or intensive outpatient treatment
• Known chronic tinnitus
• Hearing loss (bi-lateral or one-sided) precluding successful participation in the study
• Female candidate is pregnant or breast-feeding
• Female candidate is in menopause or post-menopause, unless on hormone replacement therapy (but not treated with bisphosphonates or anti-resorptives)
• Female candidate stopped hormonal contraception medication up to 3 months prior to study start
• Metal implants (or objects like metallic slivers in the eyes, or bullets or shrapnel in the body) or other kinds of bone synthesis materials, implanted medical devices, tattoos or permanent make-up incompatible with MRI
• Participation in a (clinical) study within the last 3 months before start of this study that significantly confounds participation in the study Any other condition which makes the test subject unsuitable for study inclusion in the opinion of the project team

Explanations to exclusion criteria (a-c)

1. History of single episode of prolonged grief or mild depression with no symptoms in the 12 months prior to study start is acceptable
2. Normal values from: https://www.lab-quade.de/leistungen/?idLoc=1
3. A thrombosis risk stratification will be conducted according to clinical standards: in the anamnesis, the subject will be extensively questioned for thrombosis risk factors and assessed by means of specific analytical laboratory methods for assessing thrombosis risk using thrombophilia screening

The meals are adjusted in energy density without noticeable changes in appearance or taste. These meals have identical ingredients, but the macronutrient proportions are modified. It can be not ruled out that some participants may recognize differences of the meals.