Twin truths: New research says your genes are not to blame for weight gain

Twin truths: New research says your genes are not to blame for weight gain

By Sharon Ní Chonchúir

A new study says genes have less to do with weight loss than we previously thought — it’s more about how our gut reacts to different foods, writes Sharon Ní Chonchúir.

We're getting bigger. According to new figures released by the Central Statistics Office this year, 62% of Irish people were overweight or obese in 2017, up from 60% in 2015.

Many of us blame our genes for our expanding waistlines. If our parents were overweight, surely it was inevitable that we too would struggle with weight gain?

But a groundbreaking new study — which included 700 identical twins — disputes this, saying our lifestyle choices are the main factors that determine whether or not we pile on the pounds.

The study found that DNA and genes play a lesser role in weight loss than previously believed. It also discredits generalised diet plans, such as the ketogenic diet, the paleo diet or whatever diet trend happens to be hitting the headlines this week.

Pictured is Dietitian Aveen Bannon
Pictured is Dietitian Aveen Bannon

Instead, the results of the PREDICT trial show that a range of lifestyle choices — from when we eat and how we exercise to how much we sleep — influence how our bodies metabolise food. The trial concludes that if we want to maintain a healthy weight, we need a personalised programme that takes all these factors into consideration.

“We expected to find large variations in the way people responded to food, which would be determined by many factors and not just genetics,” says Dr Sarah Berry, nutritional scientist and senior lecturer at the Department of Nutritional Sciences in King’s College London and one of the study’s lead investigators.

The researchers anticipated individual responses to identical meals might vary by up to 100% — but in some cases, the response was 800% higher.

We were surprised by the extent of the variation in whether people’s blood levels would spike, even among identical twins

This wasn’t their only surprising finding. They also found that in the case of glucose (sugar) response, only 30% was related to genes and in the case of triglycerides (fat), that figure was less than 5%.

These findings are significant as the rise and fall of glucose, triglycerides, and insulin in the blood have an important bearing on health. Spikes that are too high, too prolonged, and too frequent are associated with inflammation, weight gain, heart disease, and diabetes.

If genes are not the determining factor in these spikes and lifestyle has more of a role to play, this has major ramifications for us all. While there is nothing we can do about our genes, we can modify our lifestyle.


The PREDICT trial was the largest-ever attempt to look at why different diets suited different people. Researchers at King’s College London and Harvard and Stanford medical schools in the US asked 1,100 participants, including 700 identical twins, to consume set meals and log every mouthful of food and drink over the course of two weeks. They also asked them to record their sleep patterns, hunger and levels of physical activity.

Samples of gut bacteria were taken throughout the trial. So were blood tests to measure glucose, triglyceride, and insulin levels — the three main indicators of the body’s metabolic system.

What lifestyle factors did they identify? Levels of gut bacteria were a key factor.

We all have up to 2kg of bacteria, yeast, and other microbes in our gut and the way they interact affects everything from appetite to how we metabolise food.

The variety of our gut bacteria is determined by the variety of what we eat. This means that a varied diet is another factor.

Sleep and exercise patterns were also found to be important. So too was our body clock.

The participants in the PREDICT trial were given a meal at breakfast one day and the same meal for lunch the next day. They had twice the response to the carbohydrates and fats when they ate it on the second day. We might all have a different circadian rhythm, which could explain why some people can easily skip breakfast while others struggle to function if they do.


The findings of this trial chime with dietitian Aoife Deane’s personal and professional experience of nutrition.

“I used to be three stone overweight and would tell myself it was because I was from the fat side of the family,” she says. “But now that I’ve changed how I eat, I maintain a healthy weight. It doesn’t matter what your family weigh or what they eat. My whole philosophy is that no one size fits all. You have to personalise how you eat.”

Deane believes that standardised diet plans cause over- and undereating. “For example, if your diet advises a high-calorie breakfast and you eat it when you’re not hungry, that’s overeating,” she says.

If you skip breakfast when you’re hungry, that’s undereating. Much better to learn to listen to your own body and to recognise its hunger pangs. It will tell you what it needs.

She is not surprised that the PREDICT trial has found there is an interplay of factors involved in metabolising food. “I always take a holistic view with my clients,” she says. “I have clients who think it makes sense to go to sleep at 1am and wake at 6am so they can exercise before work. I try to explain to them that sleep is a factor in their weight-loss programme. Without enough sleep, their body will be stressed, and it will hoard fat because it will panic that there may be a shortage of food ahead.”


Dietitian Aveen Bannon is not as convinced that the impact of genes should be discounted.

“This trial is interesting, but I think there are lots of factors involved when it comes to weight loss, including environment, lifestyle choices, and genetics,” she says.

“Nutrigenetics and epigenetics are hot topics of the 21st century and we are seeing positive correlations on how genetics influence how we react to foods.” She refers to a 2012 US study ( This found that people who carried a certain variant of the FTO gene, which is linked to obesity, lost weight more successfully if they followed a high-protein diet.

“Similar results have been shown with high- and low-carb and low-fat diets,” she says. “This certainly suggests that our genes have an influence. I think the key takeaway from this research is that everyone is different, and it reinforces that lifestyle factors such as meal patterns, sleep, and exercise can influence our overall health and wellbeing.”


With dietitians in agreement as to the importance of lifestyle factors on metabolism, do the results of the PREDICT trial mean that we should all be encouraged to follow a personalised diet? Berry does not believe this should happen.

“Current eating guidelines recommending a diversity of foods, reduction in sugar and saturated fat alongside a good intake of wholegrains and healthy unsaturated fats are relevant to most of the population,” she says.

“We’re not suggesting this should be changed, only that it be used as a guide within which individuals should adjust according to what is best for them and their individual health response.”

She advises anyone thinking about creating their own personalised programme to start small. “Take steps that work with your current lifestyle, schedule and preferences. Remember that food is there to be enjoyed and to bring pleasure, so avoid guilt and denial. The results of this study are not about telling people what they can’t eat, but informing them of the best choices they can make to optimise their health.”

For successful weight loss, Berry recommends experimenting with your eating habits. Skip breakfast or change the timing of your meals to see how it makes you feel. Vary your foods too— it’s good for your gut bacteria, and could also limit the influence of foods that don’t agree with you.


PREDICT lead researcher Professor Tim Spector wishes he had known this years ago. He used to eat the same tuna and sweetcorn sandwich every day but having tested his blood, he sees the impact bread has on his metabolism. He says he could have saved himself from gaining a kilo a year by eating a potato with tuna instead.

“We are omnivores and we need a diverse diet,” he says. “But if you can swap some foods around so that you have exactly the same calories and enjoyment but a lower peak in either glucose or triglycerides, then you’re going to put on less weight and be healthier long term.”

Spector and Berry want everyone to have access to this information about their bodies. Which is why they are developing a home test and app that will inform users about their personal nutritional responses and help them to choose foods that optimise metabolism and aid weight loss.

“The basic premise is simple,” says Berry. “It will allow you to understand how you respond to any food so you can make confident decisions about what to eat.”

Dean disagrees, however. “Apps are not the answer,” she says. “What we need to do is to learn to listen to our bodies. These tests found that the body responded differently to food at different times. It would have felt different too. Our body gives us physical feedback regarding what we should eat, when and what is right for us. We should learn to trust that feedback.”

So, the next time you’re tempted to jump on the latest diet bandwagon, remember — you might lose more weight if you started experimenting with your eating habits to find out what works best for you and your body.

  • Find your ideal eating pattern

  • Here are Professor Tim Spector’s key recommendations.
  • Move away from generalised diet plans and start thinking about what works for your body. Forget the standard advice and pay attention to how you feel after you eat different foods.
  • Avoid processed foods. They have a detrimental impact on your gut microbiome.
  • Start experimenting with a wider variety of ingredients. This will have a positive effect on your gut bacteria and if nothing else, it will help to minimise the impact of foods that affect you adversely.
  • If you eat breakfast because you’ve been told it will kickstart your metabolism and yet you still struggle to lose weight, try skipping it for a week to see what happens.

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