Yes, this new study has limitations. The authors do note that and aren't pretending otherwise. This is coming in the context of other studies with similar conclusions which the original article talks about. This new study is a singular imperfect data point, but is combined with other data points that point in the same direction.

What it is primarily helpful for is in that it has a large N value of 79,468 participants and the population they are looking at doesn't partake in as many carcinogens that make it harder to tell cancer rates apart (which is both a strength and also a study limitation too)

From the study

This study has several other strengths. 1) This is probably the single cancer cohort with the largest number of vegetarians, and especially vegans, who have rarely been studied effectively for cancer incidence. This allows consistent definitions and methods to be applied across all variables; 2) in many studies of vegetarians, vegetarian diets may be relatively transient for some subjects, but less so in AHS-2; 3) the level of validation available for the main variables on which the assignment to vegetarian diets is based; 4) the relatively large Black subgroup in which vegetarian diets have rarely been studied. Race is always a co-variate in our statistical models; and 5) the absence (practically) of cigarette smoking, a common confounder for many cancers, and very little alcohol

There are also study limitations, the most prominent of which is still the relatively small numbers of less common cancers, particularly among the less common dietary patterns (vegans and pesco-vegetarians) that diminish statistical power; second, there is the relatively health-conscious low-meat-consuming reference group, the Adventist nonvegetarians, that also limits power; third, that we were able to measure dietary and other data only at study baseline and not during follow-up. Finally, there are the limitations of all observational studies, particularly the possibility of unmeasured confounding, which can be limited but never avoided

[–] rowinxavier@lemmy.world -5 points 2 weeks ago (1 children)

Yeah, so this study basically tells us nothing but can be used for propagandistic purposes. If I were a journal editor I would not publish a study that tells us nothing while being ripe for political and ideological use. It is unethical to act as if this is a purely scientific study when it obviously is not, and the editors of the journal are supposed to be experts in the field, they should be very aware of this issue and be taking appropriate steps.

[–] usernamesAreTricky@lemmy.ml 8 points 2 weeks ago (1 children)

No, it doesn't tell us nothing. These kinds of limitations are not uncommon for nutrition studies. It just is weaker evidence that doesn't tell everything we ever might want. Studies will always have some methodological limitations. There is always some factor you might be forgetting or could do better. Science doesn't work by looking at induvidial studies alone. We take things in aggregate

That being said, of course things like RCTs will always be preferred and considered much stronger evidence. On that front, there have been some RCTs in other related health risk incidents with similar findings. For instance, I have read about some RCT studies for cardiovascular health. One meat industry funded review of RCT studies on cardiovascular risk for red meat found plant substitution improved predictors of cardiovascular health

Substituting red meat with high-quality plant protein sources, but not with fish or low-quality carbohydrates, leads to more favorable changes in blood lipids and lipoproteins.

https://www.ahajournals.org/doi/10.1161/CIRCULATIONAHA.118.035225#d3646671e1

Or from another review looking at larger changes

Nevertheless, several RCTs have examined the effect of vegetarian diets on intermediate risk factors of cardiovascular diseases (Table 1). In a meta-analysis of RCTs, Wang et al. (22) found vegetarian diets to significantly lower blood concentrations of total, LDL, HDL, and non-HDL cholesterol relative to a range of omnivorous control diets. Other meta-analyses have found vegetarian diets to lower blood pressure, enhance weight loss, and improve glycemic control to greater extent than omnivorous comparison diets (23-25). Taken together, the beneficial effects of such diets on established proximal determinants of cardiovascular diseases found in RCTs, and their inverse associations with hard cardiovascular endpoints found in prospective cohort studies provide strong support for the adoption of healthful plant-based diets for cardiovascular disease prevention

https://www.sciencedirect.com/science/article/am/pii/S1050173818300240

[–] rowinxavier@lemmy.world 8 points 2 weeks ago

I think I have reasonable grounds to disagree, but I don't want to cause offense or upset, so to be clear I am not attacking you or your thought process, just the conclusions.

There has been a long history of these religious approaches to diet influencing scientific research. If we discount all science done by ideologically biased institutions such as those in this study the actual field looks very different. If we further discount known bad methodologies, for example food frequency questionnaires (FFQs) which are shown to be absolutely inaccurate in most cases, then we again find less support for the conclusion that plant based diets are better for humans than meat based diets. In fact, we will find that there are almost no interventional studies of any length that even potentially could tell us that a meat based diet is worse than a plant based diet.

In most cases the researchers fail the very first step of defining plant based, meat based, high carb, low carb, ketogenic, Mediterranean, and so on. The studies are all way too short, most being done over less than 12 weeks, and very few have any sort of cross over or similar control. Most of the studies are purely observational and have no intervention, so no change can seen as causally linked to an outcome. Most studies are funded in such a way as to bias the outcomes. Most studies are not preregistered. Many suffer from p hacking. Many have no clear outcome measure but instead target a proxy, for example blood cholesterol, but they do not actually look at the true target outcome, heart disease and death.

The whole field of nutritional science is unfortunately very unreliable at this time due to ideological and financial conflicts of interest. This study is a great example. Given that it is well known that FFQs are unreliable why was this study approved at the outset? Why was a further clarification of the actual diets of participants not taken at some point in the study, even from a subset? Why is this type of study funded, executed, and then passed through peer review? If this arrived on my desk I would not approve it for publication simple for methodological reasons. Why does the journal allow a title which is so provocative and clearly useful for pushing an agenda when their supposed scientific credibility are riding on their reputation as gatekeepers of truth?

If we had real science I would be keen to see it. This does not meet that level of quality and the continued publication of this type of unfit paper is dragging down the whole scientific endeavour. If we continue to allow people to claim to know what they cannot show we will end up believing anything and making grave mistakes in our choices about how to live.