- Milk’s casein and whey proteins have beneficial effects on blood glucose levels and can also influence daily food intake by increasing satiety.
- A new study found that consuming both regular and high-protein cow milk with a carbohydrate-laden breakfast cereal slowed the digestion of carbohydrates and kept blood sugars low after breakfast and lunch.
- Adding milk to the breakfast menu may offer health benefits for type 2 diabetics as well as individuals looking to keep blood sugar levels in check and reduce overall food intake.
Odds are, your mother told you “breakfast is the most important meal of the day.” And, as is usually the case, your mother was right. Scientific studies continually support this folk wisdom—people that eat breakfast weigh less [1, 2], are less likely to gain lost weight back [2, 3], and have more stable blood sugar levels throughout the day [4, 5] compared with those who skip the first meal of the day. Not all breakfast foods offer these health benefits, however. Sorry bagels and donuts, but studies consistently show that protein-packed breakfast foods may make the biggest impact when it comes to jump-starting your metabolism and limiting spikes in blood sugar and overall daily food intake . But a new study  shows that you may be able to have your breakfast carbohydrates and eat them too—as long as you eat them with milk.
Milk and other dairy products provide two unique classes of proteins, whey and casein. Whey proteins, which contain all nine essential amino acids, are known to have insulin-stimulating effects . Insulin is a protein produced by the pancreas tasked with moving glucose from the bloodstream into cells throughout the body that use that glucose for energy. If cells already have enough glucose, insulin helps the body convert glucose into its storable form, glycogen. Thus, although milk contains carbohydrates (mostly in the form of the sugar lactose), it has a low glycemic index because the insulin stimulating effects of whey protein’s amino acids help keep blood sugar levels relatively stable after milk is consumed [7, 8]. Moreover, the boost in pancreatic insulin production from whey protein can also lower the glucose response to other foods consumed at the same time as milk .
But don’t forget about casein proteins. Although they do not directly stimulate insulin production, they play an important role in carbohydrate digestion and metabolism . Whey proteins are digested quickly and are believed to send signals of satiety (that is “you are full, you can stop eating”) soon after consumption. Casein proteins, in contrast, are digested slowly and send out signals of satiety later than whey proteins. Studies have demonstrated that casein proteins stimulate the production of gastric hormones that also slow down the digestion of other foods . And when it comes to avoiding spikes in blood sugar, slower carbohydrate digestion is better.
In a new study, Kung et al.  investigated these synergistic effects of whey and casein protein on glucose metabolism and food intake. Specifically, they were interested in how milk consumption alongside carbohydrates in the form of cereal at breakfast influenced blood glucose levels and satiety after breakfast, before lunch, and after lunch.
To tease out the effects of whey, casein, and total milk protein, they had their 32 healthy, young adult participants rotate (in random order) among 5 different study treatments (one treatment day per week): water with whey permeate (as the control); cow milk with normal protein content (3.1 %) and normal casein to whey ratio (80:20); cow milk with normal protein content (3.1 %) and modified casein to whey ratio (40:60); cow milk with high protein content (9.3 %) and normal casein to whey ratio (80:20); and cow milk with high protein content and modified casein to whey ratio (40:60).
Participants consumed 250 ml of each “milk” group with two servings of Honey Nut Cheerios (a high glycemic index oat-based cereal) after a 12-hour overnight fast. Blood glucose (via finger prick) and appetite (self-reported from a questionnaire) were measured immediately before and after cereal consumption and assessed again every 15 minutes for the next 200 minutes. In what may be the coolest study protocol ever, participants were provided cheese pizza and given 20 minutes to eat however many pre-weighed slices they desired 2 hours after cereal consumption. At the end, study researchers had 12 blood glucose measurements and 12 appetite responses for each participant at each treatment (and study participants had sore fingers and full stomachs).
Kung et al.  hypothesized that, compared with the control and normal milk treatments, milk treatments with higher whey content (60% as opposed to 20% whey) and milk treatments with higher protein content (9.3% compared with 3.1%) would be associated with lower blood glucose measurements before and after the pizza lunch, higher scores of fullness on the appetite questionnaire, and reduced food intake (fewer slices of pizza) during lunch.
Consistent with their prediction, treatments with higher protein content were associated with the lowest blood glucose levels both before and after lunch . However, the high-protein treatments had no effect on how full participants were immediately before lunch or on their energy intake (i.e., total slices of pizza) during lunch. Also contrary to predictions, both normal and high-protein treatments with modified casein-to-whey ratios (40:60) had only a modest effect on pre-lunch blood glucose measurements  and there was no significant difference in the blood glucose response curves (a measure of glycemic index) between the normal (80:20) and modified (40:60) protein ratio treatments .
The treatment with the highest whey protein (high protein, 40:60 casein-to-whey ratio) was associated with the smallest changes in blood glucose levels relative to baseline (or, to put another way, had the lowest glycemic index), supporting the role of whey protein in stimulating pancreatic production of insulin. However, the significant reduction in blood glucose associated with a normal milk protein ratio suggests that the delayed digestion of casein proteins (and the associated delay in gastric emptying of carbohydrates) may beneficially influence blood glucose levels independent of insulin production .
The fact that all treatments made participants equally full is attributed to the high-energy content of the cereal plus the treatment milk (estimated to be around 500 kcal) and the short amount of time that elapsed between breakfast and lunch (120 minutes). Thus, the study authors concluded something college students have always known: when you are full from Honey Nut Cheerios, you just don’t eat as much pizza.
All joking aside, the study results are welcome news for the more than 100 million Americans living with type 2 diabetes. Weight loss through diet and exercise may be the long-term strategy to improve insulin sensitivity, but dietary recommendations that are easy to implement, like adding milk to the breakfast menu, can have immediate health benefits for individuals looking to control their blood glucose levels.
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2. Kahleova H, Lloren JI, Mashchak A, Hill M, Fraser GE. Meal frequency and timing are associated with changes in body mass index in Adventist health study 2. The Journal of Nutrition. 2017. Jul 12;147(9):1722-8.
3. Wyatt HR, Grunwald GK, Mosca CL, Klem ML, Wing RR, Hill JO. Long‐term weight loss and breakfast in subjects in the National Weight Control Registry. Obesity Research. 2002. Feb;10(2):78-82.
4. Farshchi HR, Taylor MA, Macdonald IA. Deleterious effects of omitting breakfast on insulin sensitivity and fasting lipid profiles in healthy lean women. The American Journal of Clinical Nutrition. 2005. 81:388-396.
5. Kung B, Anderson GH, Paré S, Tucker AJ, Vien S, Wright AJ, Goff HD. Effect of milk protein intake and casein-to-whey ratio in breakfast meals on postprandial glucose, satiety ratings, and subsequent meal intake. Journal of Dairy Science. 2018 Oct 1;101(10):8688-701.
6. Park YM, Heden TD, Liu Y, Nyhoff LM, Thyfault JP, Leidy HJ, Kanaley JA. A high-protein breakfast induces greater insulin and glucose-dependent insulinotropic peptide responses to a subsequent lunch meal in individuals with type 2 diabetes. Journal of Nutrition. 2015. Mar;145(3):452-8.
7. Frid AH, Nilsson M, Holst JJ, Björck IM. Effect of whey on blood glucose and insulin responses to composite breakfast and lunch meals in type 2 diabetic subjects–. The American Journal of Clinical Nutrition. 2005. Jul 1;82(1):69-75.
8. Ballard KD, Bruno RS. Protective role of dairy and its constituents on vascular function independent of blood pressure-lowering activities. Nutrition Reviews. 2015. Jan;73(1):36-50.
Dr. Lauren Milligan Newmark
Researcher, Science Writer