Background and aim: 71% of all deaths worldwide are caused by non-communicable diseases such as cardiovascular diseases or type-2 diabetes. In modulation of those diseases, exercise and a healthy diet are preventive measures. Additionally, both components favorably impact the process of aging. Although some studies have investigated potential synergistic effects of exercise and dietary modifications, most research has focused on participants with health conditions (e.g., obesity, pre-diabetes, type-2 diabetes), leaving out the untrained but otherwise healthy older population. Therefore, it remains unknown to what extent healthy elderly individuals may benefit from exercise combined with dietary modifications. As a consequence, the aim of this thesis was to study the health benefits and synergistic effects of exercise combined with dietary measures (healthy diet vs. Calanus finmarchicus oil intake) in untrained healthy elderly individuals. Beyond positive health effects of exercise and a healthy diet, there are also scenarios in which exercise and (inadequate) dietary intake may negatively impact health. Therefore, a second aim was to assess the health risks associated with low energy availability, commonly observed in athletes in a performance-oriented environment.Methods: Altogether, 134 elderly participants (50-70 years, BMI 18-30 kg/m2, no sports within the last two years) were included in a 12-week randomized, controlled interventional trial. Using stratified randomization according to sex, BMI, and age, participants were allocated to one of four study groups: 1) control group with no intervention (CON); 2) 2x/week aerobic and resistance exercise only (EX); 3) exercise routine as in 2) combined with dietary counseling in accordance with the guidelines of the German Nutrition Society (EXDC); 4) exercise routine as in 2) combined with intake of 2 g/day Calanus finmarchicus oil (EXCO). At the beginning and end of the study body composition was analyzed and fasted blood samples were taken to evaluate metabolic markers of glucose and lipid metabolism, sirtuins and immune cells. Additionally, at the beginning, after six weeks, and at the end of the study, physical activity outside of the intervention using the Freiburger questionnaire for physical activity and dietary patterns using 3-day dietary food logs and food frequency questionnaires (FFQs) were recorded. Lastly, a literature review was performed to assess the health risks related to low energy availability.Results: Overall, exercise alone increased sirtuin activity but did not show any favorable changes in terms of body composition, glucose or lipid metabolism. In contrast, exercise combined with a healthy diet or Calanus finmarchicus oil intake led to a significant decrease in fat mass but did not affect markers of glucose metabolism or blood lipids. However, Calanus finmarchicus oil intake led to a significant increase of the omega-3 index. Regarding sirtuins, the effect of the exercise routine was further enhanced by the dietary modifications. Despite the impact of the intervention, an additional cross-sectional analysis of baseline data revealed that T-cell senescence may be a mediator of insulin sensitivity. Ultimately, despite all positive health benefits, exercise may impair health if exercise energy expenditure and dietary intake are not matched.Conclusion: Taken together, results from this thesis indicate that a combination of a low dose, moderate exercise training combined with a healthy diet or Calanus finmarchicus oil supplementation has a more favorable impact on health markers in elderly, untrained, overweight subjects when compared to exercise only. To further elucidate the additive effects of exercise combined with such dietary modifications, future studies should investigate potential effects of a healthy diet (rich in omega-3 polyunsaturated fatty acids) vs. Calanus finmarchicus oil supplementation. This would also be of interest in combination with a higher dose exercise program.