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Processed foods cause us to overeat and gain weight

Apr 1, 2020

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Most people already know processed foods aren’t health foods, but that doesn’t stop most people from eating them. They’re tasty, they’re convenient, and they’ve got billions of marketing dollars behind them that play with the brain and reel people in. Speaking of playing with the brain… These foods are typically high in sugar AND high in fat – the killer combo that keeps the brain wanting more. More often than not, processed foods are composed of three main ingredients: refined grains, sugar, and vegetable oils, three ingredients that have only been around for a sliver of human history. In many ways processed foods can work against human nature and are simply not what the body is designed to eat or expect. And while we don’t want to sound like conspiracists, aligning well with the introduction of these foods is the increasing rate of chronic disease – a finding difficult to ignore.

Could this have to do with their obesogenic nature?

Researchers at the NIH’s National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) compared an ultra-processed diet to an unprocessed diet to see whether food processing influences how much we eat and how this affects body weight.

The study was designed as a randomized controlled trial, meaning participants were randomized to one of two diets: ultra-processed diet or unprocessed diet. Foods considered ultra-processed contained manufactured ingredients, such as industrial seed oils, high-fructose corn syrup, artificial flavourings, and emulsifiers, based on the NOVA classification system.

The study involved 20 healthy adults – 10 male, 10 female – and each individual was to follow their given diet for 2-weeks before immediately transitioning to the alternative diet for the following 2-weeks, for a total of 4-weeks. For the duration of the study, participants resided in the Metabolic Clinical Research Unit at the NIH Clinical Centre, in an inpatient setting, allowing for tight control of the investigation, and every week, each subject spent 24-hours in a respiratory chamber to measure energy expenditure.

Subjects were given three meals a day, and were told to eat as much or as little as they desired within a time period of 1-hour. Attempts were made to match the diets for total calories, macronutrients, fibre, sugar, and sodium, with the only difference being the level of processing. However, the ultra-processed diet ended up containing more total sugar, less insoluble fiber, more saturated fat, and a higher omega-6 to omega-3 ratio, which was hard to avoid due to the reality and nature of ultra-processed foods. All in all, the nutritional variables for both diets were relatively well-matched.

The major finding was that ultra-processed foods caused subjects to eat on average 500 calories per day more than they did eating unprocessed foods. And, in direct correlation with calorie intake, weight was gained with the ultra-processed diet, and weight was lost with the unprocessed diet. Total energy expenditure was higher on the ultra-processed diet, however, the 24-hour respiratory quotient was significantly higher, indicating less fat oxidation (burning fat for energy) compared to the unprocessed diet. Furthermore, average daily glucose and 24-hour insulin levels were slightly higher with the ultra-processed diet, but no significant differences were observed in glucose tolerance or insulin sensitivity between the diets.

So, why did people overeat the ultra-processed foods but not the unprocessed foods?

A few observations may help explain why ultra-processed foods caused increased caloric intake. In this study, carbohydrates and fats, but not of protein, accounted for the extra calories from ultra-processed foods. When we consider how different macronutrients affect satiety, this makes sense. Protein is the most satiating macronutrient and typically difficult to over consume. In contrast, carbohydrates and fat are much easier to over consume, especially when combined, which is the case for processed foods. Additionally, the protein leverage hypothesis that describes eating until we consume enough protein, may also help explain these observations. In essence, the more diluted your protein, the greater the chance of overeating calories from fats and/or carbohydrates. The ultra-processed foods contained slightly less protein and may have caused an increase in total food intake in attempt to reach adequate protein consumption. With that said, the protein leverage hypothesis was calculated to account for at most 50% of the additional calories consumed. Another potential contributor was the rate of food consumption. The ultra-processed foods were eaten at a rate faster than unprocessed foods, and it’s been reported that the faster you consume your food, the more likely you are to overeat. In fact, a 20% increase in eating rate can increase intake by up to 13%. Lastly, the ultra-processed diet included caloric beverages that were not included in the unprocessed diet. Caloric beverages increase overall calorie consumption with very little, if any, contribution to satiety, and can also be consumed rapidly, thus likely accounting for a portion of the increased calories.

It’s worth mentioning that there were no significant differences in pleasantness, familiarity, or subjective appetite between the two diets. This suggests that the unprocessed diet was enjoyed just as much as the highly processed diet, which counters the common assumption that processed foods are more hyper-palatable than unprocessed foods. Further suggesting that palatability was not the cause of the increased calorie intake.

This study highlights how food processing can influence food intake and body weight. The results suggest that a diet of highly processed foods can lead to increased calorie intake and weight gain, compared to a diet of unprocessed foods which was shown to reduce calorie intake and lead to weight loss.

The takeaway is that food choices matter. Eliminating processed foods from your diet and replacing them with unprocessed (i.e. whole foods), without changing anything else, can lead to reduced intake and weight loss. In other words, turning your attention to restricting food choices (restrict processed foods) rather than food quantity (restrict food altogether) may offset the need to focus on the latter, which can often require even more discipline if revolving around highly processed foods.

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