This study was designed to test whether a ketogenic diet can inhibit the growth of tumours of the human gastric adenocarcinoma cell line 23132/87 in a xenograft model. The tumour cells demonstrated increased glucose consumption and lactate production in vitro
. They were positive for TKTL1, a marker enzyme of aerobic glycolysis [36
], whose expression has been shown to correlate with a poor prognosis in a variety of carcinomas [37
]. The ketogenic diet used here provides average protein and is low in carbohydrates and high in fat enriched with omega-3 fatty acids and MCT. Compared to the applied standard diet, the unrestricted ketogenic diet had a retarding effect on tumour growth and resulted in larger necrotic areas within the tumours. Blood glucose levels in the KD group were unaltered, but its ketone body levels were significantly elevated compared to those of the SD group. Since our study does not allow to decide whether the effects of the diet are due primarily to omega-3 fatty acids and MCT, or to a combination, further studies are needed to address this issue.
The observation that unrestricted access to the ketogenic diet retarded tumour growth contrasts with data on another ketogenic diet, KetoCal, a commercially available diet for children with epilepsy [16
]. The therapeutic effect of KetoCal on tumour growth was apparent in adult mice only when their caloric intake was restricted, which resulted in a 20% – 23% loss in body weight within eight days after start of feeding. KetoCal provided to animals ad libitum
produced no marked loss in body weight but also had no influence on tumour growth [16
]. In contrast to the calorically restricted KetoCal diet, we observed neither significant weight loss nor reduced blood glucose levels in our animals, although the tumour suppressive effects of the diets were comparable. Our data therefore suggest that an effective metabolic tumour therapy is not necessarily accompanied by reduced blood glucose levels. A possible cause of the observed delaying effect of the ketogenic diet on tumour growth is the high levels of omega-3 fatty acids and MCT in the diet. An antitumour effect has been demonstrated for both omega-3 fatty acids and MCT in patients and experimental models [17
Cancer patients with advanced incurable cancer are typically threatened by cancer cachexia, characterised by progressive weight loss, mainly due to loss of fat and skeletal muscle, and anorexia [41
]. Although cancer cachexia accounts for about 20% of cancer deaths, its underlying mechanisms are not known in detail [42
]. To improve the quality of life and survival time of incurable patients, it is important to avert the onset of cachexia. Calorically restricted diets are therefore not suitable as treatment for these patients. Ketogenic diets, however, with high fat, adequate protein and low carbohydrates, have been shown to prevent or limit the protein catabolism in skeletal muscle [43
]. In 1995 Nebeling et al. proposed a ketogenic diet rich in MCT as a successful therapeutic option in pediatric cancer patients [12
]. Barber et al. later reported that the combination of fish oil and an energy-dense nutritional supplement increased body weight in cachectic cancer patients [44
]. A non-restricted ketogenic diet may thus indeed be capable of benefiting cachectic cancer patients when supplemented with adequate lipids. The ketogenic diet described in this study induced both a slight increase in body weight and a slower growth rate of human tumour cells in nude mice.
Tumours of the KD group were characterised by significantly larger necrotic areas than those of the SD group. This finding may be explained by the restricted glucose supply in the KD group. However, we did not find significant differences in blood glucose levels of KD and SD animals. This observation indicates that glucose is synthesised from noncarbohydrate precursors by a process called gluconeogenesis. Fearon et al. even found higher blood glucose levels in ketotic, tumour bearing rats than in ketotic, non-tumour bearing rats. The authors considered that the inability of the ketogenic diet to reduce tumour growth was due to persistently high glucose levels [45
]. In contrast, different feeding studies with carbohydrate-free diets showed significantly lower levels of circulating glucose compared to carbohydrate-enriched diets [46
]. Nebeling et al. described that within 7 days of initiating ketogenic diet, blood glucose levels declined to low levels [12
]. In addition, the authors calculated from results of PET scans a 21.8% average decrease in glucose uptake at the tumour site. One possible explanation for the significantly delayed tumour growth despite constant blood glucose levels in mice of the KD group is the ability of ketogenic diets to significantly reduce blood insulin levels [47
]. It is widely accepted that frequently elevated levels of insulin can stimulate tumour growth [48
]. We found slightly reduced insulin levels in KD animals, but the difference to insulin levels of SD animals was not significant.
Another possible explanation for the antitumour effect of the ketogenic diet is its ability to delay tumour take. Following tumour cell injection the animals of the KD group were fed with the ketogenic diet. The comparison of the individual tumour volumes of KD animals reveals that the unrestricted ketogenic diet delayed tumour growth strongly in the first 20 days after tumour cell inoculation. Five tumours in the KD group did not grow, 3 tumours grew slightly, whereas only 4 tumours grew as fast as the tumours of the SD group. The observation that the ketogenic diet delays the tumour cell take could be clinically significant for prevention of metastatic tumour cell take. However, further studies are required to accurately discriminate the effects of the ketogenic diet.
The significantly larger necrotic areas in the centre of tumours grown in the KD mice correlate well with the reduced microvessel density in these tumours. The suppression of neovascularization may be provoked by the anti-angiogenic effect of omega-3 fatty acids [28
], as well as by reduced levels of lactate/pyruvate in glucose-starved tumour cells, which are able to stimulate angiogenesis via HIF-1-mediated transactivation of VEGF [49
]. Suppressed neovascularisation may further inhibit an adequate supply of glucose to the centre of the tumours. In aggressive tumour cells such a severe limitation of substrate produces a state termed 'metabolic catastrophe', which enhances necrosis. The therapeutic induction of metabolic catastrophe was recently proposed as an approach to killing "unkillable" tumour cells [50
]. Since glucose-fermenting tumour cells have been shown to have substantially enhanced resistance to several anticancer drugs [51
], the combined application of conventional chemotherapy and metabolic tumour therapy may represent an effective approach for targeting both fermentative and respiratory cell populations.
Transplantation of human cancer cells or tumour biopsies into immunodeficient mice is a commonly used xenograft model [53
]. Since this model precludes T cell-mediated cellular immunity, the results do not reflect any immunosuppressive effects of lactate as a by-product of glucose fermentation. Due to lactate's suppressive effect on the cellular immune response [9
], it is conceivable that the ketogenic diet we applied would have more profound effects in a model that allows a T-cell response directed against tumour-associated antigens. Additional studies using other models are necessary to further explore the true potential of metabolic tumour therapy in a functionally active immune system.