The aim of the study is to analyze if a ketogenic diet (KD) compared to either a "low glycämic and insulinemic" (LOGI) diet or to a standard diet (SD) is feasible, safe and tolerable and will improve quality of life and physical performance in patients with Breast Cancer during the rehabilitation phase.

It will be an open-label trial of nutritional intervention for 20 weeks spanning three phases: 3 weeks of stationary intervention, 16 weeks of outhouse phase and one final week of stationary intervention.

Cancer patient's whole body metabolism is characterized by an increased fat oxidation rate and a decrease ability to metabolize glucose for energy demands (insulin resistance). In preliminary studies it was shown, that a fat rich diet allows to spare muscle mass from being reduced due to catabolic mechanisms. The resulting better body composition relates to less fatigue and higher quality of life, the latter was shown in advanced cancer patients for a ketogenic diet.

The aim of the KOLIBRI-study is to analyze if a very fat rich ketogenic diet (KD) or a moderate fat rich "low glycämic and insulinemic" (LOGI) diet compared to a standard low fat diet (SD) are feasible, safe and tolerable and will improve quality of life and physical performance in patients with Breast Cancer during the rehabilitation phase.

The most stringent nutritional regime high in fat and low in carbohydrates is the ketogenic diet (KD) comprised of at least 75% of daily calories via fat (derived from several plant oils, nuts, avocado, butter, cream, cheese, eggs, fatty fish and fat rich meat), balanced in protein (1.4 g/kg body weight/day) and very low in carbohydrates (approx. 20-30 grams per day, derived from salad, vegetable, some berries or fruits, and milk products like yoghurt).

A less strict but even lower in carbohydrates and richer in fat and protein than a standard diet is the LOGI regimen. It allows up to 120 grams of carbohydrates (inclusion of more vegetable and fruit and rare grains and potatoes compared to KD), is high in protein (1.7 g/kg body weight/day) and rich in fat (remaining calories) The control nutritional regime follows the standard recommendations of the German society for nutrition (DGE) and is comprised of at least 50% of the daily energy derived from carbohydrates, 0.8 grams/kg of body weight protein and 30% of daily energy expenditure from fat.

It will be an open-label trial of nutritional intervention for 20 weeks spanning three phases:

The first phase spans 3 weeks of stationary intervention in a hospital (location: Bad Kissingen, Bavaria, Germany) with the initial examination (medical, body composition, food preferences, quality of life, blood samples, physical performance) followed by implementation of the allocated diet and respective training of the patients in diet calculation, cooking and realization of the diet regimen in routine daily life.

The second phase encompasses16 weeks of an outhouse phase with continuing the selected nutritional regime under close contact and supervision of the study team accompanied by urine measurements (KD group), food diaries, questionnaires covering the quality of life and one blood sample) The third phase of the study represents one week of stationary intervention at the hospital with the final examination and further teaching in eiterh sticking to the selected diet or in changing the diet to the individually preferred one.

Inclusion Criteria:

1. Consenting women aged 18 - 70 years.
2. Primary or recurrent breast cancer after primary standard therapy during the standard rehabilitation phase
3. Karnofsky Index >70.
4. Willing and being able to follow the allocated dietary regime for 20 weeks

Exclusion Criteria:

1. Patients with additional malignant tumors at the time of the recruitment
2. Participation at other trials
3. Dementia or other clinically relevant alterations of the mental status which could impair the ability of the patient to apply to the diet or understand the informed consent of the study
4. Not able to follow the teaching due to deficits in teaching language (German)
5. Metabolic aberration banning fat rich nutrition or were a KD is contraindicated
6. Expected life span < 12 month
7. Insulin dependent Diabetes
8. Decompensated heart failure (NYHA > 2)
9. Myocardial infarction within the last 6 months, symptomatic atrial fibrillation
10. Severe acute infection
11. Pregnancy
12. Pancreatic insufficiency