To stay slim, avoid ultra-processed foods

Updated May 30, 2019

Our eating habits have changed considerably in recent years. Sometimes for the better, for example by taking advantage of the availability of several foods, ingredients and spices from around the world to diversify our diet and broaden our culinary horizons. But also sometimes for the worse, especially because of a very significant increase in the consumption of ultra-processed industrial foods high in fat, sugar, and salt (see box for the definition of ultra-processed foods). The revolution generated by these “new” foods is particularly remarkable: even though these products did not even exist just a century ago, they currently account for about 50% of all calories consumed by the population.

Unfortunately, most of these ultra-processed products should be considered foods of poor nutritional quality. Not only does their high sugar and fat content give them a very high energy density that promotes the overconsumption of calories, but the use of inexpensive ingredients for their manufacture also ensures that they are deprived of several essential elements found in unprocessed foods (fibres, omega-3s, polyphenols, vitamins, minerals, etc.). For all these reasons, it is recommended that the consumption of these ultra-processed foods be limited as much as possible and that home-cooked meals should be preferred, as proposed in the latest version of Canada’s Food Guide.

Ultra-processed foods and weight gain
One of the main arguments against ultra-processed foods is that it has been suspected for several years that their high caloric content could promote the development of obesity. For example, all countries, without exception, that have increased the proportion of ultra-processed industrial foods in their diets must deal with a greater proportion of obese individuals. This is particularly striking in countries in economic transition, where the high availability and low cost of ultra-processed foods mean that the incidence of obesity has skyrocketed, even among the poor.

A remarkable study has just confirmed the close link between ultra-processed food consumption and weight gain. In this randomized clinical trial by Dr. Kevin Hall’s team (National Institute of Health), researchers compared the effects of a diet consisting exclusively of ultra-processed foods to that of a diet based on minimally processed foods. They recruited 20 healthy young people (but who were slightly overweight with an average BMI of 27), and, for a financial compensation of $6,000, the volunteers agreed to be accommodated for 28 consecutive days in the center’s laboratories, with no possibility of going out and with the obligation to eat meals exclusively made from ultra-processed (group 1) or minimally processed (group 2) foods prepared by the research team. In all cases, meals were developed to be equivalent in terms of calories, energy density, fat, sugar and salt, but necessarily differed greatly in the types of sugars and fats present. For example, ultra-processed foods contained significantly more added sugars (54% of total sugars, compared to 1% for unprocessed foods), saturated fats (34% of total fat, compared with 19%), and four times less omega-3s. The subjects were instructed to eat their fill, without worrying about the quantities ingested.

For the first two weeks, each participant ate 3 meals per day from group 1 (ultra-processed foods such as cereals, muffins, white bread or flavoured yogurts for breakfast, deli sandwiches for lunch, and chicken nuggets for dinner) or group 2 (unprocessed foods such as fresh fruits and vegetables, eggs, fish, poultry, whole grains, nuts) (the difference in the types of meals consumed by participants can be viewed here). Snacks were available to volunteers all day long (potato chips, crackers and granola bars for group 1 or nuts, almonds and fruits for group 2). For the next two weeks, the volunteers switched to the other diet, that is, those who ate the ultra-processed foods were now fed the unprocessed foods and vice versa.

The most dramatic result of the study is that the mere fact of being exposed to ultra-processed foods causes a very large increase in the number of calories consumed throughout the duration of the study (Figure 1). Overall, this increase is about 510 kcal per day, the result of an increase in carbohydrate intake (280 kcal/day) and fat intake (230 kcal/day) (but not protein).

Figure 1. Comparison of calorie intake in people on diets of ultra-processed or unprocessed foods. From Hall et al. (2019).

Such a large increase in the calorie intake is obviously not without consequence: the daily weighing of participants shows a rapid increase in body weight which reached 1 kg at the end of the first week of the study (Figure 2). Conversely, people who had eaten the unprocessed diet had lost 1 kg over the course of the study, resulting in a net difference of 2 kg with those fed ultra-processed foods. This is huge, especially considering that these differences can be observed in just two weeks.

Figure 2. Variation in body weight associated with the consumption of ultra-processed or unprocessed foods. From Hall et al. (2019).

Why eat more?
Ultra-processed foods are designed first and foremost to create sensory pleasure (appearance, texture) and satisfy our natural inclination for fat, sugar and salt. It could thus have been expected that the increased intake of these foods by study participants was due to the fact that they preferred to eat these meals rather than those prepared with unprocessed foods. This is not the case, however, as the level of satisfaction of the participants with respect to both food classes was identical, both in terms of appetite and pleasure derived from their consumption. The main difference observed between the two groups is that people ate almost twice as fast when their meals consisted of ultra-processed rather than unprocessed foods (50 kcal/min versus 30 kcal/min). This is probably due to the fact that ultra-processed foods are generally easier to chew and swallow, allowing more food to be ingested in a shorter amount of time and thus excess calories. In this sense, it should be noted that researchers observed that blood levels of peptide YY (a hormone that reduces appetite) were increased in people who ate low-processed foods, while levels of ghrelin (a hormone that stimulates the appetite) were diminished. It is therefore possible that the consumption of ultra-processed food disrupts mechanisms involved in satiety, which promotes overconsumption of food.

These observations strongly suggest that consumption of ultra-processed foods plays a predominant role in the dramatic increase in the incidence of overweight people worldwide. This is truly a major breakthrough that has the potential to revolutionize our understanding of the mechanisms behind this obesity epidemic. For several years, excess weight has always been considered in terms of excessive fat or sugar intake and there are countless “miracle” diets that promise significant weight loss by cutting one or the other. Yet, as we mentioned in another article, there is really no clinically significant difference in the effectiveness of low-fat or low-carb diets in inducing long-term weight loss. Instead of being overly concerned about the amount of fat and/or sugar ingested, Dr. Hall’s study strongly suggests that it is the source of these nutrients that is the most important factor in controlling body weight. To stay slim, the key is to eat unprocessed foods as often as possible and minimize the consumption of ultra-processed industrial foods. Especially since several recent studies have shown that people who regularly consume such foods are at higher risk of cardiovascular disease and premature mortality.