This was published 7 years ago
The real reason why you don't lose weight on the same diet as your friend
By Sarah Berry
We are one step closer to understanding why different diets work for some people but not others and vice versa.
It all comes down to one common factor – nitrogen.
Blame your nitrogen balance: We can eat the same foods and have very different responses.Credit: iStock
A new study by researchers from the University of Sydney has found that the amount of nitrogen, a component of protein foods, in our guts plays a key role in the health of our microbiome (our community of gut bacteria) which in turn affects everything from our weight to our mood to our physical health.
"There's enormous confusion about what makes one diet good and one diet bad," explains lead author Associate Professor Andrew Holmes, from the Charles Perkins Centre and School of Life and Environmental Sciences. "What might be good in one person might be bad in another person."
For example, you could give 20 people exactly the same amount of a certain type of bread and they will respond differently (that is, have a different blood sugar response).
"Some people will have a very high blood sugar peak after a meal of bread, other people will have absolutely none at all," Holmes explains.
Similarly, you could give the same group a pre or probiotic and it will benefit some but not others.
The question has always been why?
Researchers understood that there are three factors affecting our response: the food itself, our unique microbiome (everyone's is different) and our unique biology. Because we have differences in both our microbiome and our biology there are many ways in which we can respond differently to any food.
They also understood that, generally speaking, a high-carb, low-protein diet was linked to longevity, weight was linked to excess calorie intake (regardless of the macronutrient breakdown), while a high-protein diet is "a very strong driver of poor metabolic health".
What they didn't understand exactly was the mechanism tying all these factors together.
So, for the study published in Cell Metabolism, the team put 858 mice on 25 different diets comprising of varying amounts of protein, carbohydrates and fat.
What they found was that there was a "tipping point" across all diets that related to how nutrients from the diet became available to nitrogen in the gut.
In particular, the bacterial need for nitrogen helps to regulate the balance of gut bacteria and avoid health problems developing.
"The largest nutrient requirements for our gut bacteria are carbon and nitrogen in the foods we eat. As carbohydrates contain no nitrogen but protein does, the bacterial community response to the host animal's diet is strongly affected by this diet's protein-carbohydrate ratio," said Holmes.
"The fact that this same pattern was seen across almost all groups of gut bacteria indicates that the make-up of the microbial ecosystem is fundamentally shaped by a need to access nitrogen in the intestinal environment."
The trick is that the tipping point varies based on the microbiome's existing composition making it impossible to pin down a perfect, one size fits all, ratio of carbs to protein.
Our bodies also respond differently depending on the form of the protein or carbs you are eating. For instance, if you have a high-protein diet (hello Paleo people), eating complex carbohydrates (from plants) can "mop up" some of the excess nitrogen.
Similarly, nitrogen levels are less affected by eating slow release plant proteins, like legumes and pulses, than readily absorbed animal proteins.
This means, Holmes explains: "We can adjust the protein to carb ratio by EITHER eating more microbe available carbs OR by eating less total protein. This means we can make two simple messages:
1. Don't eat more protein than your recommended daily allowance (that increases the risk of an excess of nitrogen).
2. Try to eat more complex carbohydrates from plants (e.g. fibre). This complex carbohydrate is more likely to make it down to the microbes so it gives you more "wiggle room" with regard to how much protein you are eating.
Interestingly, in terms of microbial response, fat seemed to play a very little part.
Where does all of this leave us?
Another step closer to understand and predicting, with accuracy, personalised nutrition.
In practice, this would mean that a practitioner could "easily personalise the most effective diet for the person", Holmes says by understanding the interplay of our individual physiology and metabolism, diet history and microbiome.
In the meantime, it means not subscribing to any idea that one type of diet or macronutrient ratio is perfect for everyone, but the more we incorporate fibre and whole, unprocessed foods into our diets, the happier most microbiomes will be.
And forget that you'll put on weight with one diet while your friend loses it: blame it on your nitrogen balance.