One size doesn’t fit all (revisited) – how far have we come?

One size doesn’t fit all (revisited)

One Size Doesn’t Fit All

Dr. Sean Wheatley, PhD – Science and Research Lead/18 July 2019 

One Size Doesn’t Fit All. In this week’s blog I am revisiting something I wrote in November 2016, when I discussed why one size doesn’t fit all. The “one size doesn’t fit all” sentiment will be no surprise to anyone familiar with what we (X-PERT Health) do, as it is at the heart of so much of our content. But the specifics of why this is the case, and whether it is possible to have an educated guess at what is the right approach for an individual, are questions that still need answering.

In my previous blog on the subject I discussed a number of factors that might have an impact on what is, biologically speaking, the “right” diet for someone. The main things I discussed were genetics, the microbiome, and the degree of insulin resistance. So do we know more about these things now?

Clearly this isn’t the place for an in depth review. But recently there have been a number of studies that relate to these topics that I’ll quickly touch upon. What these studies have in common is that they show just how far we actually have to go before we unravel the mystery of true personalised nutrition.

So is insulin resistance important?

I’m going to cheat a bit here (and in the next section too actually) by using changes in body weight rather than using evidence directly looking at the immediate responses to different food.

The study I want to cover here is called DIETFITS, and a publication from it received quite a lot of coverage last year as there were essentially no differences in outcomes between those who were randomised to follow a low fat diet compared to those who were randomised to a low carb one.

The key to the similar outcomes is perhaps that the diet for both groups was based on high quality foods, but that is beside the point. The point is that when the researchers looked to see if insulin sensitivity had an effect on how their diet affected them they found essentially nothing.

Now a single study can never prove anything conclusively. But this does raise some questions over how important insulin sensitivity/resistance is. Or at least that it might not always provide the answer. The big caveat here is that the participants in this particular study were overweight but otherwise healthy. So it is possible they simply weren’t insulin resistant enough for it to have a big effect. It is still possible that people with a greater degree of metabolic dysfunction might be better served by a dietary approach that accounts for this.

What about the microbiome?

The importance of the microbiome is often highlighted using tales of body weight changing when someone has someone else’s microbiota transplanted into them. I won’t go into how they do this, but the answer is essentially poo tablets. The story goes that a lean person receiving the gut microbiota from an obese person will become obese, or vice versa.

The proposed explanation is that the microbiome has a profound effect on what nutrients and energy we absorb from the food we eat. But a recent study did not support this. Finding that although there was a clear and measurable change in the composition of the microbiota after the transplant this didn’t translate to any changes in body weight. Again, no one study can prove anything. But, again, this study raises some doubts over whether the gut microbiome is always key in explaining individual differences when it comes to diet and health.

How about genetics?

In the DIETFITS study publication I mentioned before there was no effect of someone’s genotype on how they responded to their diet. This is essentially a map of the genes that are supposedly relevant to the outcomes they were assessing.

There is also evidence related to genetics from a recent study, which so far has only been presented as, well, a presentation – rather than being published in a peer reviewed journal. The study in question is the PREDICT 1 study. In the results shown here genetics could not explain most of the differences in responses to food. These results were based on findings in identical twins. And there is no better way to assess the effect of genetics than by looking at people with identical genetics! So if two people have big differences in their response to a food despite having the same genes this shows that genetics don’t explain the whole picture either. In fact, in this study genetics only explained 30% of the differences in blood glucose level changes and 5% of the difference in blood fat changes.

These findings which question how good genetic testing is in isolation for predicting outcomes related to diet should be a big knock to companies who are selling people personalised nutrition based on genetic tests. But unfortunately it is rare for money making enterprises to let evidence get in the way of promoting and selling their product.

So what does work?

I’d be lying if I said I knew. It is still likely that the factors above all play a role. Indeed in the PREDICT 1 study alluded to above machine learning, taking into account a number of different factors was pretty effective. Explaining 73% of the variation in blood glucose responses for example. So our knowledge in this areas IS improving. Unsurprisingly the true explanation seems to be more complex than narrowing it down to one or two things.

It is important to take stock of what we don’t know though before we put all of our eggs in one basket, no matter how convincing or science-y that basket might seem.

For now less complex factors, like your personal likes and dislikes, will still play a key role when picking something that will work for you. The best diet is always going to be one you can actually adhere to, and one that you can afford! So basing your diet on foods that you like (within reason) and tracking the health markers that are important to you is a good place to start. You can then use this to guide you, and to help you make any necessary changes. At least until truly personalised nutrition is here! If you are really struggling then seeking advice from an appropriately qualified professional can help to streamline the process.

So what’s the bottom line?

We know much more than we did when I wrote the original blog. However, part of knowing more tends to be that it highlights just how much we still don’t know. The age of personalised nutrition is almost certainly on its way. However for now, anyone trying to sell you a personalised solution is probably more interested in making a profit.

As with all our blogs and other work we’d love to hear your thoughts and feedback, so feel free to leave a comment on our Facebook page, drop me an e-mail at, or tweet us/me at @XPERTHealth or @SWheatley88. If you have attended an X-PERT Programme or are an X-PERT Educator you can also register for our online forum.

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