Scientists study why Over-Processed Foods could be linked to Obesity

A team of scientists from the USA National Institutes of Health (NIH) have found that eating over-processed food could lead to weight gain within 2 weeks, which could lead to obesity. The scientists published the results of their controlled clinical trial which compared the effects of ultra-processed vs unprocessed food in Cell Metabolism. The study was supported by the National Institute of Diabetes and Digestive and Kidney Diseases as well as the NIH.

Scientists study why Over-Processed Foods could be linked to Obesity

Twenty participants were recruited (10 of each gender), who had stable weight and a BMI around 27. They were asked to come as in-patients for the purposes of the trial for 28 days. They were randomly assigned to follow either the ultra-processed diet or the unprocessed diet for 14 days, and were then swapped over. They were offered three meals a day and given 60 minutes to consume as much or little food as they wished. The menus were on a 7 day schedule and had been designed to match for total calories, macronutrients, fibre, sugar, sodium and energy density across both diets. They did however vary in the amount of calories derived from ultra-processed and unprocessed food. These differed in the proportion of total sugar added (approx 54% vs 1%), saturated to total fat (approx 34% vs 19%), insoluble to total fibre (approx 16% vs 77%) and omega-6 to omega-3 fatty acids (approx -11.1 vs 5:1).

The participants spent 1 day a week in a respiratory chamber to measure energy output. They were asked to perform daily cycle ergometry exercise in three 20 minute bouts at a constant intensity linked to their heart rate reserve, so that they did not lead completely sedentary lives while in the unit, and underwent constant glucose monitoring among other tests.

The scientists estimated the weekly cost of preparing 2,000 calories per day of ultra-processed meals to be around $106, while the unprocessed meals cost $151, using the cost of ingredients from a local supermarket. The participants were also offered snacks appropriate to the particular diet. The snacks and meals offered each day were around twice that needed for each participant to maintain their weight according to the formula. The results were compared from the second week of each diet.

Fat Model

Glob of Fat Model.

When the participants were on the unprocessed diet, their energy intake did not significantly change, however there was a significant increase in energy intake when they were on the ultra-processed diet. They consumed more carbohydrate and fat, but not protein. They also had increased gut hormone peptide YY, which is a hunger suppressant and decreased ghrelin, a hormone that signals hunger. Fasting glucose and insulin levels also tended to decrease. Measurements after the ultra-processed diet stayed mostly the same and the scientists hypothesised that the subjects were used to eating over-processed food.

The rate at which they ate their meal increased when consuming the ultra-processed meal and the scientists suggested that this could be that they felt less full. They suggested that future studies should more closely match the diets for protein and non-beverage energy density and include ultra-processed food that can be eaten more slowly.

The participants gained an average of 2lb in weight on the ultra-processed diet, having consumed an average of 508 extra calories each day, and lost a similar amount on the unprocessed diet. The familiarity and pleasantness of both diets seemed to match, but participants consumed an extra 17 calories of food a minute than those on the unprocessed diet. The authors of the study did not plan to take the cost of the food into consideration, but their findings that the unprocessed food cost more agreed with other studies which show that socio-economic circumstances link to obesity. People in lower socio-economic brackets may not have the time, skill, resources or equipment to buy and safely store unprocessed food ingredients and may find it hard to plan meals. This could explain some of the studies linking obesity to socio-economic factors.

Food classed as ultra-processed include packaged snacks, frozen meals, soft drinks, meat nuggets and food which is high in additives and low in unprocessed ingredients. Unprocessed foods were the raw ingredients which make up meals. Recent studies have shown connections between the consumption of processed food and intestinal inflammation as well as obesity, autoimmune illness, cancer and early death.

Hall, K.D., et al., Inpatient Randomized Controlled Trial of Ad Libitum Food Intake, Cell Metabolism, May 2019

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