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This work was conducted in the framework of the French network for Nutrition And Cancer Research (NACRe network), www.inra.fr/nacre.; International audience; Circadian disruption has been classified as probably carcinogenic to humans by the World Health Organization's International Agency for Research on Cancer. The circadian clock is subject to environmental factors, particularly light exposure and food intake rhythms. However, the association between nutritional circadian behaviours and cancer is not well understood. We investigated the longitudinal associations between number of eating episodes, night-time fasting duration, time of first and last eating episodes, as well as nutritional quality of last eating episode, respectively, with breast and prostate cancer risks, the two main cancer locations in women and men respectively. This prospective study included 41,389 day-working adults in the French NutriNet-Santé cohort (2009-2016) who completed at least three 24 h dietary records during the first 2 years of follow-up. Multivariable Cox models were computed. 1,732 first primary incident cancer cases were diagnosed during the follow-up, among which 428 breast and 179 prostate cancers. After adjustment for covariates including sleep duration, late eaters (last eating episode after 9:30 pm) had an increased risk of breast (Hazard ratio [HR] = 1.48 [1.02-2.17], p = 0.03) and prostate (HR = 2.20 [1.28-3.78], p = 0.004) cancers. However, no association was observed between cancer risk and number of eating episodes, night-time fasting duration, time of first eating episode or macronutrient composition of the last eating episode. This large cohort study suggests that circadian perturbations resulting from late time of last food intake may be involved in carcinogenesis at different locations. Beyond nutritional quality of food intake, nutritional circadian regulation should be further investigated in the context of cancer prevention.

While cancer survivors are at increased risk for negative conditions as second cancers, other comorbidities, and functional decline ; lifestyle factors, such as a healthy diet, regular exercise and weight control, may contribute to prevent these conditions and improve survivors' quality of life. In this context, aims of this thesis were, among cancer survivors from the French prospective NutriNet-Santé study, to 1) investigate nutritional variations between before and after cancer diagnosis as well as dietary supplements use, and 2) evaluate opinions relating to some diet factors and to weight-loss restrictive diets and fasting practices, and to link them on the one hand to their real practices, and, on the other hand, to their sources of nutritional information.In the first part, our results highlight some healthy behaviors such as a decrease in alcohol and sweetened drinks consumption, but also less favorable trends, such as a decrease in vegetable consumption and in many vitamin and mineral intakes. We also observed a decline in overall and vigorous physical activity after diagnosis, especially in prostate and skin cancers, in men and professionally inactive patients. Concomitantly, we observed an increase in sedentary behaviors, especially in women, older subjects and professionally inactive patients. We also observed that while weight loss was reported in many colorectal cancer patients, a substantial proportion of breast cancer patients gained weight. Sociodemographic and economic factors appeared as important determinants of weight gain, illustrating social inequalities in health (higher risk among patients with lower income and lower education). Our results suggest that dietary supplements use was widespread among cancer survivors, a large amount of which being used without any medical supervision, including a non-negligible proportion of patients having dietary supplement practices which can be considered as “at risk”. In a second part, we observed that opinions from cancer survivors regarding some nutritional factors seemed to impact their dietary practices and were themselves impacted by sources of nutritional information. In particular, opinions regarding alcohol consumption were concerning, with an important proportion of cancer survivors who thought that alcohol consumption (and even more regarding red wine) had a positive impact on their disease. Weight-loss restrictive diets were practiced by a large number of cancer survivors since their diagnosis, while fasting was less practiced, but was far from being an isolated phenomenon.; Alors que les survivants du cancer sont à risque plus élevé de second cancer, d’autres pathologies et de déclin fonctionnel ; les facteurs de mode de vie, comme une alimentation équilibrée, une pratique d’activité physique et le contrôle d’un poids sain, peuvent contribuer à prévenir ces risques et améliorer la qualité de vie des survivants du cancer. Dans ce contexte, les objectifs de cette thèse étaient, parmi les survivants du cancer de la cohorte française prospective NutriNet-Santé, 1) d’investiguer les variations du comportement nutritionnel entre avant et après diagnostic de cancer ainsi que la consommation de compléments alimentaires, et 2) d’évaluer les opinions vis-à-vis de certains facteurs nutritionnels, et de les mettre en regard de leurs pratiques alimentaires réelles d’une part, et de leurs sources d’information nutritionnelles d’autre part. Dans un premier temps, nous avons observé des tendances favorables à la santé comme une diminution de la consommation d’alcool et des boissons sucrées non alcoolisées, et d’autres moins favorables comme une diminution de la consommation de légumes et de plusieurs apports en vitamines et minéraux. Un déclin de l’activité physique globale et intense était observé. De manière cohérente, nous avons observé une augmentation des comportements sédentaires, particulièrement chez les femmes, les sujets plus âgés et les patients qui n’étaient pas actif professionnellement. Nous avons observé une perte de poids chez certains cas de cancers colorectaux, alors qu’une prise de poids était observée chez les cas de cancers du sein et de la peau. Les facteurs sociodémographiques et économiques semblaient influencer le risque de prise de poids, suggérant des inégalités sociales de santé (risque plus élevé chez les patients ayant des revenus et un niveau d’étude plus faible). Nos résultats suggéraient que la consommation de compléments alimentaires était largement répandue chez les survivants du cancer, en grande partie sans suivi médical, incluant une proportion non négligeable de patients ayant des pratiques de consommation de compléments alimentaires pouvant être considérées comme « à risque ».Dans un second temps, nous avons observé que les opinions des survivants du cancer concernant certains facteurs nutritionnels semblaient impacter leurs pratiques alimentaires et étaient eux-mêmes impactés par les sources d’informations auxquelles les survivants du cancer avaient eu accès depuis leur diagnostic de cancer. En particulier, les opinions concernant la consommation d’alcool étaient préoccupantes, avec une proportion importante de survivants du cancer qui pensaient que la consommation d’alcool (et plus encore concernant le vin rouge) avait un impact positif sur leur maladie. Les régimes restrictifs pour perdre du poids étaient pratiqués par une grande partie de notre échantillon ; le jeûne était moins pratiqué, mais était loin d’être un phénomène isolé.

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