That was just an example, but we can go into more detail - at every point, nutrition studies are highly questionable. Sample sizes are usually minuscule (you can find important studies with 5-10 subjects, and even the largest studies rarely have more than a few hundred), they are often biased samples (only overweight people, only people with heart disease or diabetes etc.), they often don't account for likely confounding factors (using weight without accounting for muscle mass, no accounting for stress, time in the sun etc.) and on and on. And all of this in a subject where we basically don't have any clear idea about how much metabolism differs from one person to the next, based on what factors (e.g. gut microbiome has only been recently identified as a major component of digestion that can differ significantly between people; psychological effects of diet are even less well understood, even though your food choices are obviously not coming from some pure realm of reason).

Basically the digestive system is far too complex for us to understand from first principles at this time. Your diet has a very complex and very slow effect on your body, with some exceptions. Numerous diseases and environmental factors impact how this plays out exactly. So, to do real research in nutrition, the only chance right now would be to conduct massive studies over long periods of time with rigorous controls on subjects' nutrition and activities - which is basically impossible, or at least prohibitively expensive.

Instead, we get conclusions drawn from studies of a few dozen people over a few months or at best a year (in "long-term" studies). Or, we get conclusions drawn from comparing diet across huge populations ("the Mediterranean diet", "the Japanese diet", "the American diet" etc) with no possibility to control for obvious differences in nutrients, environmental factors, lifestyle differences, access to healthcare etc. Both of these are worthless conclusions, they don't tell you anything at all.

The only successes of nutrition science have been identifying the most basic nutrients we need to survive at a basic level (protein, fat, carbohydrates, and the various vitamins and minerals). Basically nothing beyond that should be trusted.

As a fun historical note, after the discovery of the macro-nutrients there was a budding field of nutrition scientists confidently recommending optimal diets using scientific methods. Unfortunately, they had no idea about the existence of micro-nutrients, so actually following some of their diets you could actually end up getting scurvy or other serious malnutrition diseases. The current slew is not that bad, but I wouldn't be surprised if in the future we will look back similarly at some common diet advice of today.