Last semester I couldn’t help but complain to friends that nutrition science simply “sucks”: never-ending conflicting information, inability to prove even “common sense” knowledge (e.g. refined sugar is bad!- nutrition science can not definitely prove that it is…is it because people overeat sugar? is it because they overeat in general? is it because they lack key other nutrients?]. Uh, I have read scientific papers showing that adding refined sugar to diet does NOT lead to weight gain, others that showed it definitely does, etc etc etc.
Now that I sit in my nutritional epidemiology class… i finally “feel” for nutrition and understand why it tends to be so messy.
So why is nutrition SCIENCE tricky? (science, not nutrition. i still believe eating well is not a complicated thing at all…in theory. Not saying it’s easy to change habits). Mostly- because it is hard to perform nutrition research. And the major problem with such research is the hardship of measuring the actual DIET of people.
Well hold on..before we even measure the diet, we have the problem of how to establish causation with nutrition (let us say eating fried meat causes cancer).. since the studies now focus on chronic disease (which take long to develop and have multiple factors), no single cause (eating fried meat) is sufficient for the disease to occur. There might be many predisposing or protective factors either related to diet (micronutrients, overall energy consumed) or not (exercise, stress levels, exposure to carcinogens, etc etc etc).
It is simply hard to develop a study that would prove that X food causes Y disease. And even if you design a super study to see if fried meat really causes cancer (and keep people in a lab overtime so you could control their diet perfectly…for years 0_o), there are numerous problems due to the nature of nutrition, such as nutrient interactions: bioavailability (while protein or fat is absorbed at 90%, minerals are absorbed at anywhere from 1 to 70%! to even assess available iron in the diet, you also need to know vitamin C intake, tea/coffee, heme and non-heme iron.. ), collinearity (e.g. correlation between high meat intake could be strong for high fat intake…. so is it the fat or the animal protein that’s problematic? High fat intake is correlated with low vitamin C intake.. so it is the lack of vitamin C that’s the issue? WHICH NUTRIENT IS DRIVING THE EFFECT!?). I’d give you more cool examples but i’m not actually a nutritionist by training 😉
Yet the biggest challenge to nutritional science… is trying to measure people’s DIET. [If you want to feel first-hand the horribly annoying task of analyzing 1 day of eating for yourself, try it here: http://asa24demo.westat.com/]. You can interview people to ask what they ate in the last 24 hours or what their usual diet is like, you can have them keep a food diary- then you run into problems of folks forgetting or lying or simply not being educated enough about what’s in their food. The only way to check on whether people are lying or forgetting is to either spy on them or use biomarkers (e.g. blood test to measure the amount of cholesterol for example, or plasma carotenoids for fruit&vegetable intake). Surprisingly, there are few valid biomarkers for few nutrients (like a urine sample will allow measurement of protein and energy in the diet), so you can’t double-check someone’s reported diet completely (+ it’s expensive).
And even if you carefully collect dietary information (which can be a very long and expensive process), you now have to analyze this info in terms of nutrients. You can use databases [like this one http://ndb.nal.usda.gov/ndb/search/list%5D for this that will give you the nutritional profile of many foods.. But even there you have issues- many of the foods are not chemically analyzed (as i assumed) but had their nutrient info calculated via formulas because chemical analysis is very expensive.
So I shall stop throwing around phrases like “nutrition science sucks”. It is truly a challenging field- results need to be taken with caution and validated by repeated studies!!*
*which, by the way, scientists are good at doing….yet the media tends to sensationalize diet studies insanely causing overreaction, misrepresentation of results, and confusion.