The first issue is that it's extremely difficult to run a long term controlled study on nutrition. Adherence to diet interventions is difficult to measure and it drops off over time. As a result most nutrition studies are epidemiological cohort studies which are not particularly good at determining causality.
The second issue is a question of funding. Diet and exercise interventions are not something that are typically monetized be the pharmaceutical industry. As such there is little private sector funding going into nutrition studies. Couple that with the fact that they're extremely expensive and you have a serious deficit in studies.
The first issue is that it's extremely difficult to run a long term controlled study on nutrition. Adherence to diet interventions is difficult to measure and it drops off over time. As a result most nutrition studies are epidemiological cohort studies which are not particularly good at determining causality.
The second issue is a question of funding. Diet and exercise interventions are not something that are typically monetized be the pharmaceutical industry. As such there is little private sector funding going into nutrition studies. Couple that with the fact that they're extremely expensive and you have a serious deficit in studies.