By James J. Kenney, PhD, RD, FACN.
This article and CPE post test are good through December 2016.
The effect of snacking, or meal frequency, on body weight, blood lipids, and the risk of heart disease has long been a matter of debate. Some researchers have found an association between snacking between meals and obesity. By contrast, another group of researchers found that increasing meal frequency was associated with lower body weight in men, but not in women. On the basis of both animal and human research, other researchers have also suggested that larger, less frequent meals increase the risk of obesity, heart disease, and diabetes.
Smaller, More Frequent Meals Improve Blood Lipids
An epidemiological study found significantly lower blood concentrations of total cholesterol (TC) and low-density lipoprotein cholesterol (LDL-C) but no difference in high-density lipoprotein cholesterol (HDL-C) in people who ate smaller, more frequent meals. Smaller, more frequent meals appear to lower LDL-C primarily by reducing cholesterol synthesis in the liver.
A controlled clinical trial compared the effects of eating the exact same diet either as 3 meals daily or by nibbling (17 snacks daily). Blood lipid and lipoprotein concentrations were measured after 2 weeks of meal eating or nibbling. TC, LDL-C, and apolipoprotein B decreased by 8.5%, 13.5%, and 15.1%, respectively. More modest changes in blood lipids were observed when subjects were fed either 3 or 9 meals daily for 3 weeks.
It seems likely that increasing meal frequency, independent of changes in body weight, will improve blood lipids and reduce the risk of cardiovascular disease in most patients. However, if snacking leads to weight gain, then it seems likely that its metabolic effects would increase the risk of cardiovascular disease.
Snacking May Mimic Some of the Metabolic Effects Of Low-Glycemic Index Meals
The metabolic impact of a low-glycemic index meal and a high-glycemic index meal is quite different. In some regards, a low-glycemic index meal mimics the impact of eating smaller, more frequent meals. In both cases the rise in blood sugar and insulin output will be less when the absorption of carbohydrate is delayed. Indeed, compared to a high-glycemic index breakfast, a low-glycemic index breakfast has been shown to lower concentrations of insulin, blood sugar, free fatty acids (FFA), and triglycerides (TG) after the meal and after a subsequent standard lunch meal.
It seems likely that the improvement in glucose tolerance to a second meal following a low-glycemic index meal is due to metabolic changes that result from the delayed digestion and absorption of dietary carbohydrate. The slower absorption of carbohydrate would be expected to suppress the release of FFA from adipose tissue. An elevated plasma concentration of FFA increases glucose intolerance by impairing insulin-mediated glucose disposal and enhancing liver glucose output. A higher plasma concentration of FFA also increases the production and release of very low-density lipoprotein triglycerides (VLDL-TG) from the liver.
In the 1930s, it was demonstrated that spreading out the glucose load (the amount of glucose delivered to the bloodstream over a set period) reduced insulin requirements in diabetics. This reduced need for insulin may be due to a diminished release of counter-regulatory hormones such as glucagon and catecholamines, which is related to the prolonged suppression of FFA that occurs when glucose enters the bloodstream more slowly.
It appears that a diet consisting of a large amount of highly processed and refined high-carbohydrate foods such as breads, dry cereals, crackers, cookies, and cakes would lead to a rapid rise in blood sugar and an increased need for insulin. However, because the carbohydrate in such foods is absorbed too quickly, it can be expected that blood sugar would drop precipitously an hour or so after such a meal, which would trigger the release of glucagon and catecholamines. These hormones would increase the release of FFA from adipose tissue. Elevated FFA would then increase glucose intolerance. Consuming high-glycemic index foods in smaller, more frequent meals could diminish their metabolic insult. Alternatively, if foods with a lower glycemic index were consumed, which are digested and absorbed more slowly, there would appear to be less metabolic advantage to spreading those foods out over smaller, more frequent meals.
When Is Snacking Most Likely To Be Metabolically Harmful?
A recent study examined the impact of a snack consumed after a standard lunch but before the subjects became hungry. The researchers fed subjects a snack (400 kcal) at various times after a 1300-kcal lunch when they were not hungry. The snack neither reduced the amount of food consumed at the dinner meal nor increased the time before the subjects requested their dinner meal. The results of this study suggest that snacking when not hungry may promote increased calorie intake and weight gain over time. When rich, highly palatable foods are readily available, food consumption may be triggered by mealtime or pleasure and not necessarily because of hunger or the need for extra calories. It appears that human beings have not evolved physiological mechanisms to prevent overeating when rich, highly palatable foods are readily available.
Snacking May Improve Satiety
A recent study examined the impact of feeding an isoenergetic preload consisting of 1/3 of the subject's average daily calorie intake either as a single meal or as several small meals. This study found that obese men consumed 27% fewer calories at their next meal (which was given 5 1/2 hours after the single meal) when the same food was divided into several small meals compared to when it was consumed as a single meal. The results of this study suggest that eating smaller, more frequent meals may help people feel satisfied while consuming fewer calories.
Eating breakfast may impact snacking behavior in overweight people. Many obese people skip breakfast. However, a study that randomly assigned overweight people to various weight-loss programs showed that those who consumed breakfast lost significantly more weight over a 12-week program. This may be because those who skip breakfast tend to select more calorie-dense foods later in the day than those who regularly eat breakfast.
Another potential advantage in eating smaller, more frequent meals is that stomach capacity is likely to shrink. A study that examined the stomach capacity of people on a very-low-calorie diet for 4 weeks showed that it was reduced by 27 to 36% compared to control subjects who maintained their usual eating habits. This study suggests that people who regularly eat smaller, more frequent meals will begin to feel more satisfied with less food over time. In theory, this might be why those eating smaller, but more frequent meals tend to be thinner.
However, any metabolic advantage from snacking or eating more frequent meals disappears if the total calorie intake goes up. A study of eight normal weight young men examined the impact of consuming either no snack, a high protein or a high carbohydrate snack either when subjects were hungry or when they were in a satiety (or nonhungry) state. The subjects were blinded to time and were given the snacks between a standard lunch and diner. The high protein snack did delay (by 38 minutes) when the evening meal was requested and eaten but it did not reduce the energy intake compared to when no snack was offered. Indeed, energy intake was slightly (but not significantly) higher when they consumed the high-protein snack compared to the basal condition. The mean energy intake was higher on the day they consumed the high-carbohydrate snack than when they had no snack at all or when they consumed the high-protein snack. The authors concluded that "a snack consumed in a satiety state has poor satiating efficiency irrespective of its composition, which is evidence that snacking plays a role in obesity."
The Bottom Line
More research is needed to further clarify the metabolic impact of snacking or increasing meal frequency on weight control and overall health. Whether or not the metabolic benefits of eating smaller, more frequent meals are blunted when the diet consists largely of low-glycemic index foods is unclear. However, based on the available evidence, eating more natural, high-fiber foods with a low energy density should be encouraged for those who wish to lose weight without hunger. Low energy dense meals have been shown to reduce the ad libitum consumption of energy at meals, which over time might lead to an increase in meal frequency. It seems likely that eating such foods will naturally lead to a meal pattern that improves glucose tolerance and lowers plasma concentrations of insulin, fasting and postprandial TG, and LDL-C. These metabolic changes should dramatically reduce the risk of atherosclerosis, heart attack, and stroke over time.
People who are overweight should also be discouraged from a cycle of starving and stuffing. Skipping meals and going hungry appears to increase the desire for foods with a high energy density. This is likely detrimental for weight control because foods with a high energy density generally provide less satiety per calorie, which may lead to consuming more calories over time. Eating large, infrequent meals also may promote obesity and contribute to the development of Type 2 diabetes mellitus in genetically susceptible people. All of these metabolic changes have been associated with an increased risk of atherosclerotic disease. Increased physical activity may blunt many of the adverse metabolic effects of large meals composed largely of high glycemic index foods. Regular aerobic exercise should be encouraged particularly in people who are more prone to eat large but infrequent meals composed largely of high glycemic index foods.
Eating until one is uncomfortably full or "stuffed" will likely increase stomach capacity over time. This could lead to the consumption of even larger meals and more metabolic stress. Consuming large, infrequent meals appears to raise blood cholesterol (mainly LDL-C) and promote insulin resistance. Larger meals increase postprandial plasma TG concentration. Higher postprandial TG levels have been associated with an increased risk of coronary heart disease.
It seems reasonable to discourage overweight people from snacking or eating meals when they are not hungry. The one possible exception to this rule would be breakfast. A healthy breakfast such as a hot whole grain cereal with fresh fruit and NF milk is the one meal each day that people should be encouraged to eat even when they not hungry. Focusing on what people eat (e.g. choosing more whole grains, beans, fruits, vegetables, versus processed foods that are high in calorie density) and getting them to eat only when hungry the rest of the day is likely to prove more beneficial than focusing on how much they are eating (counting calories) in the long term. This is because snacks and meals that provide more such foods will provide more satiety per calorie and enable people to lose weight without constantly having to battle hunger.
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