It's actually not as hard as you think for experimental design to show strong causitive effects - I think randomized controlled trials (in animal and then human models) and meta-analysis are much more effective than epidemiological studies (which don't have effective controls, often don't appropriately correct for other factors, or are specific enough to target specific lines of inquiry and allow spurious conclusions to be drawn).
I've just started doing my own research digging through the literature in PubMed and there's a huge amount of research over the past decade especially that's really advanced our understanding of nutrition and metabolism (that sadly, has yet to get full traction in conventional wisdom/popular science).
Just a few examples from my collection on fasting (at this point I'm tracking hundreds of publications covering a whole range of different aspects of nutritional science, and this is only over the course of a couple weeks of research):
Nørrelund, Helene, K. Sreekumaran Nair, Jens Otto Lunde Jørgensen, Jens Sandahl Christiansen, and Niels Møller. “The Protein-Retaining Effects of Growth Hormone During Fasting Involve Inhibition of Muscle-Protein Breakdown.” Diabetes 50, no. 1 (January 1, 2001): 96–104. https://doi.org/10.2337/diabetes.50.1.96.
Bhutani, Surabhi, Monica C. Klempel, Reed A. Berger, and Krista A. Varady. “Improvements in Coronary Heart Disease Risk Indicators by Alternate-Day Fasting Involve Adipose Tissue Modulations.” Obesity (Silver Spring, Md.) 18, no. 11 (November 2010): 2152–59. https://doi.org/10.1038/oby.2010.54.
Harvie, Michelle N., Mary Pegington, Mark P. Mattson, Jan Frystyk, Bernice Dillon, Gareth Evans, Jack Cuzick, et al. “The Effects of Intermittent or Continuous Energy Restriction on Weight Loss and Metabolic Disease Risk Markers: A Randomised Trial in Young Overweight Women.” International Journal of Obesity (2005) 35, no. 5 (May 2011): 714–27. https://doi.org/10.1038/ijo.2010.171.
Chaix, Amandine, Amir Zarrinpar, Phuong Miu, and Satchidananda Panda. “Time-Restricted Feeding Is a Preventative and Therapeutic Intervention against Diverse Nutritional Challenges.” Cell Metabolism 20, no. 6 (December 2, 2014): 991–1005. https://doi.org/10.1016/j.cmet.2014.11.001.
Rothschild, Jeff, Kristin K. Hoddy, Pera Jambazian, and Krista A. Varady. “Time-Restricted Feeding and Risk of Metabolic Disease: A Review of Human and Animal Studies.” Nutrition Reviews 72, no. 5 (May 1, 2014): 308–18. https://doi.org/10.1111/nure.12104.
Fontana, Luigi, and Linda Partridge. “Promoting Health and Longevity through Diet: From Model Organisms to Humans.” Cell 161, no. 1 (March 26, 2015): 106–18. https://doi.org/10.1016/j.cell.2015.02.020.
I should add that the evidence was so strong for certain behaviors that I have changed my diet (VLCHF + 16+:8IF) and n=1 shows immediate improvements in weight, energy, mental clarity, and even skin health. Somewhat unexpected because I wasn't eating that badly according to common wisdom (and have maintained a moderate overweight, but stable weight for the past 3+yrs).