I know it's confusing.
Vegan zealots say that red meat will kill you. Paleo extremists say that grains will kill you. One day butter is evil and causing heart disease and the next day it's being deified on the front of Time magazine.
Rather than oscillating between different dietary opinions like an old dusty fan at a summer house you need to be able to understand what is hype, what is media sensationalism and what is actually good science.
Here are four things to consider about all the conflicting evidence out there.
1. Epidemiological Studies
Epidemiological studies look at patterns and trends in defined populations over time and try to deduce causes and effects of different lifestyle choices on health outcomes. For example, a study may follow 20,000 US veterans for 20 years and look at certain parameters such as how much red meat they eat or if they have pets.
These studies can be helpful at proposing certain hypotheses. For example it seems that those vets that ate more red meat had higher instances of heart disease, but, by definition, they cannot be conclusive because they cannot demonstrate cause and effect.
2. Causation vs. Correlation
The key here is the difference between causation and correlation. Epidemiological studies demonstrate correlation, not causation. They can only infer causation. You cannot say definitively that eating red meat increases your risk of heart disease. You can say that there was a correlation between red meat consumption and heart disease but that is fairly meaningless when we know that there are countless factors that contribute to heart disease and every individual is different. Often people who eat more red meat also eat more processed foods, exercise less and smoke cigarettes, but not necessarily.
Furthermore, many of these epidemiological studies rely on retrospective questionnaires with flawed definitions. A classic example would be asking people what they ate in the previous three months. If they ate a pepperoni pizza that would be classified as a serving of red meat. This is not very rigorous science. Do you remember what you ate three months ago?
The only way to truly determine causation is to do a randomized controlled trial to test one variable against a control group. These are often prohibitively expensive and difficult to do with diet, which is why epidemiological studies get the most air time in the media. Unfortunately, since they only demonstrate patterns they really don't apply to you as an individual, especially if you're idea of red meat is an organic, pastured steak and not a pepperoni pizza.
3. Absolute vs. Relative Risk
It is important to understand the difference between absolute risk and relative risk as this is a major source of sensationalism in the media.
For example, an (epidemiological) study done into indoor UV tanning found that those who used a solarium had a risk of developing melanoma of 0.007 (7 in 1000), while people who did not use sunbeds had a risk of 0.004 (4 in 1000).
In terms of relative risk those who tanned had a 75% increased risk of developing melanoma. Sounds pretty scary doesn't it? But in terms of absolute or 'overall' risk they still only had less than a one percent chance of developing melanoma and just 0.3 of one percent more chance of developing it than those who didn't tan. Big difference in terminology isn't it?
4. Bad Science
As I've stated many times, there is a lot of bad science out there. Gary Taubes (Good Calories, Bad Calories) did an excellent job exposing just how corrupt, self-minded, political, egotistical and biased a lot of science research can be.
Research is expensive and someone always has to pay for it. More often than not the people who pay have vested interests, whether they be multinational food companies, single-agenda associations like the Heart Foundation or (lobby) organizations like the Dairy council.
I'm not saying that all studies are biased like this. I'm sure most scientists have a high level of integrity and funding should not influence their results. But it never makes sense to bite the hand that feeds you.
Furthermore, researches often have their own agenda or 'shtick', and it can be very easy to cherrypick data to reinforce your beliefs - either intentionally or subconsciously.
What can we conclude?
Please don't be misled by the media or doubt your own convictions each time a new study comes out with conflicting evidence. The media's only intention is to get eyeballs on the page.
If you see a new study saying, "Spirulina Decreases Risk Of Balding By 37%" just take a minute before you dash out to buy three pounds of the stuff and consider the following: was this an epidemiological study? Who funded it? Are they talking about relative or absolute risk?
As I always say, you are in control of your health. Trust your intuition, trust your own experience and trust your proven results. When it comes to nutrition advice you need to foster a healthy skepticism. Ignore today's hype and focus on what you know will improve your lifestyle long term. I'm off to the solarium.