High-protein breakfast for improving glycemic control Original paper

In this randomized crossover trial, the consumption of a high-protein breakfast, compared to a lower-protein breakfast, resulted in lower postprandial (postmeal) glucose levels after breakfast and at subsequent meals.

The effect of a high protein breakfast with or without lunch on postprandial (postmeal) blood glucose levels throughout the day.

12 healthy, young participants (average age of 23 and BMI of 20; 9 women and 3 men).

In a randomized crossover design, the participants took part in four trials conducted over 2 weeks:

• Normal breakfast + skipped lunch (NS): The participants ate a breakfast meal that contained 26–32 grams of protein, 23–29 grams of fat, and 65–81 grams of carbohydrate. Lunch was not consumed.
• High-protein breakfast + skipped lunch (PS): The participants ate a breakfast meal that contained 82–98 grams of protein, 9–12 grams of fat, and 35–37 grams of carbohydrate. Lunch was not consumed.
• Normal breakfast + lunch (NL): The participants ate the same normal breakfast described above. Their lunch meal contained 7–9 grams of protein, 7–8 grams of fat, and 19–23 grams of carbohydrate.
• High-protein breakfast + lunch (PL): The participants ate the same high-protein breakfast described above, as well as the normal lunch.

During each condition, breakfast was at 8 a.m., lunch was at 1 p.m., and dinner was at 6 p.m. All of the participants consumed the same standardized dinner on the day before each trial and on the day of each trial.

To assess blood glucose levels, the participants wore a continuous glucose monitor throughout the 2-week period. They also wore an accelerometer throughout the 2-week period to monitor physical activity.

Over the 16-hour postprandial period (from 8 a.m. to 12 a.m.), glucose levels were higher in NL than PL.

In the 3-hour period after breakfast, glucose levels were higher in NS than PS and higher in NL than PL.

In the 3-hour period after lunch, there were no differences between groups. When period after lunch was shortened to 1.5 hours after lunch to account for the small size of the meal, glucose levels were higher in NL than PL.

In the 3-hour period after dinner, glucose levels were higher in NL than PL, while there was no difference between NS and PS.

Blood glucose levels 3 hours (bottom 3 charts) and 16 hours (top histogram; iAUC) after meals

Asterisk (*) denotes significant difference between NL and PL.

The main finding from the summarized study is that the consumption of a higher-protein breakfast resulted in lower postprandial glucose levels throughout the day, compared to the consumption of a lower-protein breakfast. However, this was only the case in the groups that consumed lunch. When participants skipped lunch and thus only consumed breakfast and dinner over the assessed 16-hour period, daily postprandial glucose levels were similar between the higher-protein (PS) and lower-protein (NS) breakfast conditions.

Although there was no difference in daily postprandial blood glucose levels between NS and PS, blood glucose levels were significantly lower in the 3-hour period after breakfast in PS, which was also the case for PL compared to NL. The attenuated rise in blood glucose levels following the higher-protein breakfast is attributable to both the higher protein content and the lower carbohydrate content of the meal.

The quantity and type of carbohydrate consumed in a meal is the primary determinant of postprandial glucose levels,^[1] whereas protein has little to no effect on postprandial glucose levels.^[2][3]

In addition, evidence indicates that amino acids stimulate the secretion of insulin^[4] — a hormone that signals cells to absorb glucose, thus reducing blood glucose levels. Furthermore, the consumption of a high-protein meal^[5][6][7] or amino acids^[8][9] increases levels of the incretin hormones glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), which increase insulin secretion.^[10][11]

In accordance with these physiological responses to protein ingestion, studies involving participants with no health conditions or participants with type 2 diabetes (T2D) have found that ingesting carbohydrates alongside protein results in a lower postprandial glucose response compared to the consumption of the same amount of carbohydrates alone.^{[12][13][7][14][15]}

While the above explains why reducing the carbohydrate content and increasing the protein content of a meal results in lower postprandial glucose levels following that meal, there remains the question of why postprandial glucose levels were also lower after lunch and dinner in PL compared to NL.

The answer to this question has to do with the second meal phenomenon — specifically, the way that a prior meal attenuates the rise in blood glucose levels after a subsequent meal. The second meal phenomenon has been observed in both people with T2D and people with no health conditions.

In both populations, it’s been shown that postprandial glucose levels are lower after lunch when breakfast is eaten compared to when breakfast is skipped.^{[16][17][18][19]}

Several potential mechanisms have been proposed to explain these observations. Two particularly compelling mechanisms are that breakfast consumption suppresses blood free fatty acid levels and enhances muscle glycogen storage.^[17][16] Increased levels of free fatty acids have been shown to induce insulin resistance,^[20][21][22] and thus the suppression of free fatty acid levels via breakfast consumption improves insulin action and facilitates greater storage of glycogen from carbohydrate consumption.

Another potential mechanism is that breakfast consumption enhances the insulin-secretion response in subsequent meals.^[18][23] One study in people with T2D found that this effect persisted over both lunch and dinner. In participants who skipped breakfast (compared to those who consumed breakfast), the 3-hour postprandial glucose area under the curve was 37% higher after lunch and 27% higher after dinner, and the 3-hour postprandial insulin area under the curve was 17% and 8% lower.^[24]

These mechanisms provide a basis to potentially explain the lack of difference between NS and PS on daily postprandial glucose levels. A plausible hypothesis provided by the researchers is that the 10-hour period between breakfast and dinner was so long that the lowered levels of free fatty acids and other changes in metabolic factors following breakfast consumption had rebounded to fasted levels, so the potential to influence the subsequent meal was lost.

In summary, the consumption of protein enhances insulin secretion and consuming protein alongside carbohydrates improves the postprandial glycemic response compared to consuming the same amount of carbohydrate alone. In addition, the postprandial glycemic response to a meal is reduced if a meal is consumed within a few hours beforehand.

The remaining question to consider is whether the macronutrient composition of the meal influences the second meal phenomenon. The summarized study found that the consumption of a higher-protein meal enhances the benefits of the second meal phenomenon. Have other studies reported similar findings?

One study in healthy older adults found that the consumption of a higher-protein breakfast (about 50 grams of protein), compared to isocaloric, lower-protein, higher-carbohydrate and higher-fat breakfasts (each containing about 24 grams of protein), attenuated the rise in postprandial glucose levels following the consumption of 50 grams of carbohydrate from white bread.^[25]

A study in young adults with no health conditions found that the consumption of a higher-protein lunch (71–99 grams of protein) reduced postprandial glucose levels after a standardized dinner compared to an isocaloric, lower-protein (19–30 grams of protein), fat-rich lunch, but there was no difference between the consumption of a higher-protein lunch and a lower-protein, carbohydrate-rich lunch.^[26]

A separate study in healthy, young adults reported that the consumption of a higher-protein (28 grams of protein) breakfast attenuated the rise in blood glucose levels after lunch compared to a lower-protein (12 grams of protein) breakfast with equal carbohydrate content in men, but not women.^[27]

A study in participants with T2D found no difference in glucose levels after lunch between the consumption of a higher-protein, lower-carbohydrate (35% protein, 45% carbohydrate) breakfast and a lower-protein, higher-carbohydrate breakfast (15% protein, 65% carbohydrate).^[28]

Taken together, it’s currently unclear whether a higher-protein meal enhances the benefits of the second meal phenomenon compared to a lower-protein, higher-carbohydrate meal. Further research is needed to get to the bottom of this question as well as to determine whether the answer is different for people with T2D compared to people with no health conditions.