The diet foods and drinks that could make you fatter as prominent health expert professor Tim Spector warns against ultra-processed foods.

The global weight-loss market is valued at over $250 billion, with the majority of its profitability dependent on calorie counts and rising sales of diet foods and beverages. Something is not working when you compare the soaring success of this industry to the ever-rising global obesity rates.

I’ve been an epidemiologist for thirty years, examining how and why diseases occur in various populations, and for the past ten years I’ve led teams at the forefront of nutrition science. My colleagues at King’s College London have pioneered research into the far-reaching impact of the bacteria that reside in our guts, establishing new benchmarks for the comprehension of how our systems metabolize food.

And what is evident to me and many with whom I work is that calorie tracking and diet drinks are more likely to have a detrimental influence than a favorable one — not only on your weight but also on your long-term health.

Today, in the second excerpt from my book Food For Life, I hope to refute many of the outmoded illusions that have benefited the food business, namely: all calories are the same, low-calorie meals are healthy, artificial sweeteners are safe, and high processing levels are innocuous. I will also explain why I am persuaded that diet beverages are not the solution.

It would be reasonable to think that switching from sugary drinks to diet drinks would result in weight loss, and early scientific research confirmed this hypothesis. However, the evidence no longer supports the claim. According to studies, artificial sweeteners are neither safe nor inert, and they can induce sugar surges that hurt your metabolic health and weight.

Although food businesses market them as healthy options, they are anything but. The only benefit we observe is a reduction in tooth decay, but this does not make these beverages a healthy alternative.

A 2017 review by independent epidemiologists from the United Kingdom, the United States, and Brazil revealed that many of the studies were biassed; the results for and against artificial sweeteners could go in either direction, positively if the study was sponsored by the diet drink industry and negatively if it was sponsored by the sugar industry. Their conclusion, supported by a 2019 BMJ study, was that these beverages should not be recommended as part of a healthy diet in any country.

It is difficult to comprehend how a beverage with zero calories might cause weight gain.

A suggested reason is that the mismatch between the impression of sweetness and the shortage of energy caused by the consumption of the sweetened substance disrupts the brain’s usual responses. Your brain likely sends messages to your body to recoup the energy it anticipated, causing you to consume more food.

Another explanation is that artificial sweeteners alter the delicate balance of gut microorganisms, eliminating some of the beneficial ones, reducing their species variety, and inducing others to generate harmful chemicals that disrupt our normal metabolism and predispose us to poor sugar metabolism.

It is possible that both methods, together with the additional ten or so artificial substances required to manufacture these beverages, lead to a disruption of our metabolism. These’so-called diet drinks are engineered to meet a high sweetness threshold, so you keep a sweet tooth even if you switch to artificial sweeteners. This is a significant issue for children, who will seek alternative sources of sweetness.

Xylitol, erythritol, mannitol, and isomalt are frequently blended with other purportedly ‘natural’ sweeteners, which is cause for concern. This increasing complexity of chemical mixes, which we have never before seen in nature, threatens further confounding our body and microbiota, perhaps disrupting our regular metabolic processes and behaviors. Diet foods are no better.

We once believed that the only negative aspect of ultra-processed foods like sweetened morning cereals, ice cream, packaged soups and sauces, and tortilla chips was that they included excessive amounts of fat, sugar, and salt. We anticipated that a reformulated version with reduced levels of these components and fewer calories would be safe to consume.

However, we have ignored for too long the fact that these foods lack any natural structure, also known as the food matrix, and are composed of numerous chemicals, which make us feel hungrier and are associated with an increased risk of disease and earlier death, as demonstrated by numerous published studies.

Ultra-processed foods (UPFs) often have very low levels of high-quality protein, fiber, and beneficial plant compounds (polyphenols) — since any original plant material has been stripped of nutrients and exterior coats before being ground into a pulp. Ultra-processed foods (UPFs) are so appealing that we often consume far more than we require. Which is fantastic news for food producers but poor news for our health, and it worries me that UPFs now account for more than half of the calories consumed by the average British adult. And this is considerably higher among children and adolescents, up from 30% in the early 2000s (according to the European Food Safety Association food consumption database).

According to the UK Public Health Policy Evaluation Unit, the United Kingdom has the dubious distinction of being one of the nations that eat the most UPFs worldwide. The specific mechanism by which eating ultra-processed meals increases overeating is still unknown based on the available studies. It could be that the additives and chemicals used in processing have a direct effect on our brains or gut microorganisms, or it could be that the softer textures make them easier and faster to eat, disrupting the brain’s natural signals of satiety and raising our eating rate.

Because the sugars in UPFs have been “stripped” from their original food matrix, they are readily available and produce higher sugar peaks and troughs, which result in hunger sensations and consequent overeating.

There are no foods in nature that provide this high, addictive combination, so we lack any evolutionary defense mechanism to prevent us from devouring them. No matter how many ‘light’ or reduced fat’ or ‘low-calorie’ ultra-processed meals we consume, we continue to gain weight and become less fed. And becoming a vegetarian is not necessarily a healthy option if you consume highly processed meat substitutes. A new observational study published in The American Journal of Clinical Nutrition demonstrated that vegetarian populations consuming UPFs devoid of meat have the same risk of premature death as red meat eaters.

If you do wish to maintain a healthy weight, I do not encourage calorie counting. The calculations disregard the complexity of human metabolism and our unique, varying responses to each meal.

Even if calorie counts were accurate (on apps, websites, and packaging), 350 calories of focaccia, mushroom omelet, deep-pan pizza, gingerbread latte, panini, Caesar salad, or ratatouille will not have the same effect on your metabolism and appetite.

No two people will react the same way, and our reactions to any food will vary based on a variety of factors, such as the composition of your gut microbiome, your metabolic responses to fat and sugar, the time of day, how hungry you are, how well or poorly you’ve slept, whether you’ve exercised or are stressed.

Aging alters our response to food, which is influenced by stress, sleep quality, hormones, and sickness — and in women, the beginning of menopause can significantly modify this response. Eating habits that worked for us in our thirties will likely need to be reevaluated later in life.

In the future, I hope that everyone will understand how their body reacts to food and that digital menus in restaurants would link with your smartphone to provide personalized suggestions.

In the meanwhile, my best suggestion is to consume more whole plants to nourish your gut bacteria, pay attention to your body’s changing demands, and avoid ultra-processed foods and artificial sweeteners that your microbiome cannot live on.

We now know that the individual microbes in our gut function as chemical factories or pharmacies, and it pays to keep them happy by eating a wide variety of high-fiber, plant-based foods (a ‘rainbow’ of different fruits and vegetables, legumes, spices, nuts, seeds, mushrooms, and whole grains) and limiting our consumption of highly processed, sugary, fatty foods loaded with chemical additives that inhibit the good work our microbiome is trying to do.

Polyphenols are an essential class of plant compounds that can only be utilized by our microorganisms. These are compounds produced by plants to defend against environmental threats, such as severe weather or insects.

The amount of polyphenols in foods varies greatly, with a tenfold range between similarly colored vegetables of the same type, which can further alter if processed or superheated.

It is time to reconsider the low-calorie strategy; instead of promoting ‘five a day,’ promote ‘four colorful vegetables and a mouthful of protein’