How would you define “junk food?” I have spent a bit of time pondering this question and my best definition is a highly palatable food high in salt, sugar, and/or fat. Clearly this doesn’t cover everything, but I would argue it encompasses most things people agree shouldn’t be dietary staples.
Given my definition above, I was both surprised and joyful that Bolhuis et al from Deakin University recently published a study investigating what impact fat and salt, alone and in combination, have on appetite, food and energy intake, and food palatability.
In a randomized crossover design, 48 healthy adults (16 men) completed 4 experimental trials in which they consumed a standardized breakfast followed by an all-you-can-eat lunch. The lunch consisted of elbow macaroni and tomato sauce made with 100% tomato passata (low-fat; LF) or 60% tomato passata, 30% canola oil, and 10% heavy cream (high-fat; HF). The sauce had no added salt (low-salt; LS) or had 3.3 grams of table salt (high-salt; HS). Thus, our 4 trials are LF/LS, LF/HS, HF/LS, and HF/HS.
As can be seen in figure 1 above, this means that the meals were identical in their protein and carbohydrate content, but differed substantially in their caloric, fat, and salt content. Notably, the amount of salt was chosen based on previous research examining the salt concentration required for optimal pleasantness.
Lunch consumption and appetite
The high-salt meals resulted a significantly greater food and calorie intake (~11%) compared to low-salt meals. Fat didn’t impact food intake amount, but the high-fat meals did result in significantly greater caloric intake (+60%) than the low-fat meals. Additionally, there were no significant differences between the meals in terms of eating rate (grams per minute). These findings emphasize how dietary fat can easily lead to overconsumption of calories during a meal (figure 2).
Participants completed several appetite and hedonic questionnaires before and after eating lunch. The high-salt meals were rated significantly more pleasant than the low-salt meals, but fat content had no effect. Interestingly, BMI was positively correlated with food intake for the HF/HS meal only, suggesting that this meal promotes greater consumption in individuals with higher BMIs. Sex differences were also noted, with women showing roughly 15% less intake of the high-fat meals (in grams) compared to the low-fat meals, while men showed no significant difference (figure 3).
As we could reasonably expect, hunger decreased, fullness increased, and prospective food consumption decreased significantly after eating lunch for each type of meal. The amount of fat didn’t have an impact on these ratings, but the high-salt meals resulted in a significant reduction in prospective food consumption and marginally significant reduction in hunger compared to low-salt meals.
Fat taste sensitivity
Before beginning the study, each participant underwent two sessions where fat taste sensitivity was assessed. The participants had to consume numerous skim milk beverages with increasing amounts of oleic acid, so as to determine their individual detection threshold. Those who detect the oleic acid sooner (at a lower concentration) are more sensitive to the taste of fatty acids.
The degree of fat sensitivity had no significant influence on food intake when the meals were high in salt. However, during the low-salt meals, there was a significant curve linear relationship between food intake and fat taste sensitivity whereby greater fat taste sensitivity predicted less food intake during the high-fat meal (figure 4). Despite these differences in food intake, changes in appetite ratings after eating were not affected by fat taste sensitivity, meaning that the more sensitive individuals ate less and were just as satiated. Interestingly, no relationship between sex or BMI and fat taste threshold was observed.
Overeating fat is easy, and salt makes it easier
This study has several notable conclusions. First, it emphasizes how easy it is to consume an excessive amount of calories from dietary fat. A limitation of this conclusion is that the study tested a single meal only, and these results may not apply to other fatty foods. However, other research has found similar excessive energy intake findings using potato chips and even a buffet of various foods differing only in whether they were fat-free or regular versions (tuna mayonnaise sandwiches, coleslaw, pizza, biscuits, etc.). If anything, it would appear safe to generalize this conclusion to added fats such as the oils and dressings often added to food dishes, which is vastly different from eating foods naturally high in fat, such as nuts.
It must also be recognized that food intake later in the day was not measured, so a possibility exists the increased caloric intake with these high-fat meals was compensated for later on. But others have shown that this is unlikely to occur. Even in normal-weight women, food intake is more dependent on weight rather than energy density, leading to significantly greater calorie consumption when meals are higher in fat and therefore more energy-dense.
The second and third notable conclusions are that fat-mediated satiation is influenced by fat taste sensitivity, but that salt overrides this feedback mechanism. Accordingly, there may very well be a genetic component to fat-food preferences and their impact on food intake. The problem for the general population is that fatty food consumption typically has another dominant taste such as salty or sweet (potato chips and doughnuts, for example). This study suggests that if the food is salty-fatty, then how fat-sensitive an individual is may play a minor role in adjusting food intake. A similar relationship has been suggested for sugary-fatty foods as well.
The sex differences in food intake, with women but not men consuming less high-fat food than low-fat food, will require further exploration. Caution is also warranted in drawing any conclusions, as the amount of men participating in the study (33%) was markedly lower than the amount of women.
As a final thought, I have to question the motives of the person who invented salted butter.