The optimal diet for treating metabolic diseases such as obesity and type 2 diabetes remains controversial.
Evidence suggests various nutritional strategies that can be effective as long as people adhere to them.
However, some interventions may be more effective than others. One strategy that is gaining momentum involves eating more protein.
Although the term ‘high protein diet’ varies in definition from study to study, the evidence supports the idea that is consuming more protein than the recommended daily allowance of 0.8 grams per kilogram of body weight results in less hunger and less appetite.
It also results in increased energy expenditure and preservation of lean body mass.
A recent meta-analysis of 74 randomized controlled trials showed that consuming a higher protein diet (27% vs. 18% calories on average) significantly reduced several cardiometabolic risk factors.
These factors included:
- Waist circumference.
- Blood pressure.
- Fasting triglycerides and insulin.
Likewise, it also significantly increases HDL cholesterol and satiety. A substantial body of evidence supports the claim that a high-protein diet can facilitate dietary adherence, improved health, and long-term fat loss.
But all proteins may not be the same when it comes to health.
An observational research meta-analysis totaling more than half a million people suggests that animal protein is associated with an increased risk of developing type 2 diabetes, while plant protein is not.
However, plants are bundles of fibers and bioactive compounds that could explain the difference in risks, and people who eat more vegetables generally have a healthier lifestyle.
These confounding factors make it impossible to relate differences in type 2 diabetes risk to differences in plant and animal protein, per se.
Animal proteins are generally higher in branched-chain amino acids (BCAAs, leucine, isoleucine, and valine) and sulfurous amino acids (methionine and cysteine) than plant proteins.
The role these amino acids play in the pathology of type 2 diabetes is controversial. Dietary restriction of methionine has been shown in animals to increase insulin sensitivity.
Similarly, BCAAs, especially leucine, are potent stimulators of the mTOR pathway, and chronic mTOR stimulation has been implicated in insulin resistance.
Some evidence suggests that BCAA metabolism is altered in people with type 2 diabetes, resulting in a worsening of insulin resistance, and dietary intake of BCAAs has been associated with the risk of developing type 2 diabetes.
On the other hand, animal studies suggest that mTOR activation in the brain reduces food intake and body weight.
These effects and brain mTOR signaling are triggered through increased dietary leucine intake. In addition, the consumption of BCAAs, especially isoleucine, has been shown to have beneficial effects on glucose metabolism.
To investigate how the amino acid composition of a diet influences health outcomes in people with type 2 diabetes, the reviewed study examined the effects of a high-protein diet containing predominantly animal or plant protein on insulin sensitivity in participants with type 2 diabetes.
High protein diets are well established to promote fat loss and improved health in people with obesity and type 2 diabetes.
How differences in the amino acid composition of plant and animal proteins can affect health is not well researched.
The study under review compared the effects of a high protein diet containing mainly animal protein or plant protein in people with type 2 diabetes.
Who and what was studied?
This was a randomized controlled trial in which adults with diagnosed type 2 diabetes and an HbA1c between 6-11% were recruited to consume a high protein diet based on animal protein or a high protein diet based on plant protein for six weeks.
Forty-four people started the intervention and were randomized by matching age, sex, BMI, HbA1c, and diabetic drug use. Still, only 37 people completed the intervention, with a similar dropout rate between the groups.
The participants had an average age of around 64 years, an average HbA1c of about 7%, and an average BMI of approximately 30. Only eight were not taking medication, while the rest used metformin alone or in combination with another drug (the more common) to be a DPP-4 inhibitor.
Using indirect calorimetry, all participants estimated their total daily energy expenditure by measuring their basal metabolic rate.
The researchers also accounted for self-reported levels of physical activity. The calculated energy expenditure was compared to the average daily energy intake obtained through a five-day food log that each participant completed before starting the intervention.
Together, these values were used to create individualized diet plans for each participant who attempted to maintain their body weight.
All participants received a diet plan that provided 30% of calories in protein (approximately two grams per kilogram of body weight), 30% in the form of fat, and 40% in carbohydrates.
However, the animal protein group got their protein primarily from dairy and meat. In contrast, the plant protein group got their protein primarily from pea protein incorporated into specific foods (for example, mashed potatoes, bread, and noodles. ).
Finally, the animal protein group consumed 80% of their protein from animal products, and the plant protein group consumed 72% of their protein from plants.
Participants received approximately half of their food from the meal plans every two weeks to facilitate compliance and were provided with a detailed replacement list to allow for more flexibility and variation.
Additionally, each participant was weighed when they collected their food, and meal plans were adjusted to maintain body weight. Participants were asked to consider and record all the foods they ate, including deviations from meal plans.
At baseline and again after the six-week intervention, participants underwent euglycemic clamp to determine insulin sensitivity, which was the primary outcome.
Several secondary outcomes were also assessed, including blood pressure, blood lipids, serum glycemic control markers, C-reactive protein, and blood and urinary markers of kidney function.
Previously published data regarding body composition, liver fat, blood lipid and amino acid composition, inflammatory markers, and gene expression.
This was a randomized controlled trial in which 37 people with type 2 diabetes consumed a weight maintenance diet that provided 30% of calories from protein, 30% from fat, and 40% from carbohydrates for six weeks.
One group consumed most of their protein from dairy and meat, while another consumed most of their protein from pea protein. The primary outcome was the difference between the groups in the change in insulin sensitivity.
What were the findings?
There were no significant differences between the groups for insulin sensitivity and other cardiometabolic risk factors or markers of renal function after the intervention.
There were no significant differences between groups regarding changes in body composition, liver fat, blood lipids, amino acid composition, inflammatory markers, or gene expression based on previously published data.
Food adherence to meal plans was vital and dietary records suggested that the actual proportion of macronutrients was within 1% of the planned balance.
However, despite attempts to maintain body weight, both groups showed a similar and significant, albeit slight (0.5-0.8 BMI) reduction during the intervention.
Changes in body composition were not correlated with insulin sensitivity or any other outcome.
There were no significant differences between groups in insulin sensitivity, blood pressure, blood lipids, serum glycemic control markers, C-reactive protein, and blood and urinary markers of kidney function.
What does the study tell us?
The study under review tells us that the amino acid composition of the diet does not affect insulin sensitivity or markers of cardiometabolic health and kidney function.
Importantly, by incorporating pea protein into foods eaten rather than increasing plant consumption, this study avoided the confusion of fiber and bioactive compounds that can influence health outcomes.
Instead, it directly tested the effect of the amino acid profile of the diet.
Significantly, the study methodology limits its external validity, as most people obtain their plant proteins through eating plants rather than fortifying their food with pea protein. But this is not necessarily a bad thing.
The authors did not necessarily care about external validity and tried to answer a more fundamental and specific question:
Does amino acid composition matter for insulin sensitivity?
In future research, if we notice differences in insulin sensitivity between plant-based and animal-based diets, we can confidently say that amino acid composition probably played a minor role.
In a previous study, people with metabolic syndrome were randomized to follow a modified DASH diet rich in plant protein or animal protein for a five-week weight maintenance phase, a six-week weight loss, and a life-free life. 12-week phase.
There were no differences in any outcome between the animal and plant protein groups during the entire six-month intervention.
Unlike the current study, however, the plant protein group ate natural plants for their protein, and the total protein intake was lower, about 18% of the calorie intake.
Both interventions in the study contained around 10% more protein kcal and 10% less fat kcal than the participants’ usual diets (carbohydrate intake was similar).
Both groups reported significant weight loss, which could be due to the established beneficial effects of protein on satiety.
This weight loss may have influenced at least a couple of results. There was a significant interaction between it and systolic blood pressure, total cholesterol, and LDL cholesterol, all of which showed improvement over time within each group.
Other markers that improved over time in both groups were insulin sensitivity, HbA1c, fasting glucose, HOMA-IR, and C-reactive protein.
However, no control group maintained their dietary habits for comparison; the groups showed variable significance within the group.
Notably, previously published data from this study reported that no diet significantly altered fasting plasma amino acid levels. Still, consumption of a test meal based on the dietary parameters of each group did.
Specifically, animal protein meal led to significantly more significant increases in BCAAs and sulfur amino acids plasma concentrations than vegetable protein meal.
Still, both meals resulted in similar increases in mTOR activation. This may be because both meals contained more than 30 grams of protein and three grams of leucine, which is above the suggested threshold intake level for maximizing mTOR stimulation.
The choice of pea protein in the study in question was odd, considering it contains a similar amount of BCAAs as animal protein sources.
After all, one of the proposed mechanisms for substituting plant proteins for animal proteins that would benefit insulin sensitivity for people with type 2 diabetes is lower BCAA content.
However, pea protein is about 17% BCAA, close to the meat and egg content of 18-22% BCAA. Consequently, the test diets were estimated to differ in BCAA content by only about six grams per day.
It is not yet known whether this difference is significant enough to cause differences in insulin sensitivity.
Neither diet differed in its effects on markers of kidney function, which were largely benign.
This is an important finding because diabetes increases the risk of developing kidney disease and protein restriction is the recommended nutrition therapy for managing kidney disease.
Although this study was not designed to assess the effect of protein intake on kidney health directly, its findings support the body of evidence that eating a high-protein diet is not detrimental to kidney function in people with healthy kidneys.
Other limitations of this study include the relatively short duration and the small sample of older (mid-sixties) participants with diabetes.
The intervention also limits external validity, as most people do not get two-thirds of their protein from peas.
Still, one of the study’s strengths is that its primary outcome, the effect of different protein sources on participants’ insulin sensitivity, was measured using the gold standard hyperinsulinemic-euglycemic clamp.
This dramatically increases confidence in the conclusion that plant and animal proteins do not differentially affect insulin sensitivity.
However, there may still be other plant-specific factors that provide a health benefit when plant foods replace animal protein sources.
Using the gold standard to determine insulin sensitivity, the study under review suggests that animal and plant proteins have similar effects on insulin sensitivity and most other cardiometabolic risk factors.
On the other hand, the real-world relevance is questionable due to the use of pea protein-enriched functional foods.
Most people obtain plant protein from plant foods, and other factors inherent in plant foods may provide a health benefit when plant foods replace dietary sources of animal protein.
Increasing insulin sensitivity through dietary means is a thoroughly investigated topic.
Any diet that promotes fat loss will result in long-term benefits, and consuming more protein is a viable way to increase dietary adherence and promote favorable changes in glucose metabolism.
The study under review further suggests that the type of protein is largely irrelevant.
However, what cannot be ignored is how food protein sources are prepared.
The high cost of high-heat cooking was discussed in a randomized controlled trial. People with metabolic syndrome (but not type 2 diabetes) were randomly assigned to continue their usual diet.
Or maybe use gentler cooking methods in food preparation (boiling, poaching, stewing, or cooking instead of frying, baking, or broiling) for a year.
The premise was that more complex cooking methods increase the formation of advanced glycation end products (AGEs) that exacerbate insulin resistance.
The study showed that consuming an AGE-restricted diet for one year led to significant reductions in insulin resistance and numerous inflammation and oxidative stress markers.
The order in which we eat our meals can also influence our insulin sensitivity, even though most people do not consider this in their daily dietary routines—carbohydrates-proteins or carbohydrates of proteins (the order of the food matters).
This topic addresses the results of a small study involving people with type 2 diabetes who were randomized to eat the same meal with carbohydrates first followed by protein and vegetables or vice versa.
Again, although the actual reduction in diabetes risk and long-term implications have yet to be determined, the study showed that blood sugar and insulin levels were lower after meals that started with protein and vegetables earlier than carbohydrates.
Finally, vinegar shots before meals have a surprising amount of literature investigating their effect on blood sugar levels.
A recent meta-analysis of controlled trials investigating the impact of vinegar consumption on insulin and glucose levels after meals.
The study suggests that consuming one or two tablespoons (15-30 ml) of vinegar with or shortly before a carbohydrate-containing meal lowers the total glucose response by an average of 60% and reduces the overall insulin response by an average of 130 % compared to the same food without vinegar.
In particular, subgroup analysis suggested that healthy and insulin-resistant people saw a significant benefit, although the effect was more pronounced.
If we combine all this knowledge, it is not difficult to develop a basic insulin-sensitive meal plan (which, honestly, can reduce food palatability).
Evidence suggests that eating a high-protein diet in which meat-based protein sources are boiled, poached, stewed, or steamed rather than fried, baked, or broiled may increase insulin sensitivity.
Also, consuming high-protein and fiber-rich vegetables with vinegar before meals with starchy carbohydrates is likely to affect insulin sensitivity positively.
Although the type of dietary protein may not have a significant effect on insulin sensitivity, simple nutritional changes can, such as cooking meats using mild rather than brutal methods, eating starchy carbohydrates one last time at a meal (i.e., after protein and fibrous vegetables) and take a shot of vinegar with or before dinner.
Do plant and animal proteins differ in other ways?
Plants have lower protein content and require a higher consumption to obtain the same amount of amino acids obtained in a portion of the protein of animal origin.
There are also issues with the amino acid profile and digestibility of many plant proteins that can impede their ability to support the body’s growth and repair.
It’s also important to note that not all protein sources are created equal, even within the broad categories of plants and animals.
But in general, plant proteins have lower digestibility unless they are highly processed (e.g., plant-based protein powders) and have a reduced ability to stimulate protein synthesis and promote muscle growth due to their lower content of leucine and essential amino acids.
They also do not always contain the full spectrum of essential amino acids and, therefore, may contain limiting amino acids that prevent protein synthesis.
Research suggests that ingesting higher amounts of plant protein may make up for their deficits despite the notable differences in protein quality. However, there are still caloric intake, feasibility, and cost issues to consider.
What should I know?
Although the insulin-sensitizing effects of high-protein diets in people with type 2 diabetes are well established, less is known about how the protein composition of the diet influences insulin sensitivity.
In the study under review, people with type 2 diabetes ate a weight maintenance diet that provided 30% of calories from protein, 30% from fat, and 40% from carbohydrates for six weeks.
One group consumed most of their protein from dairy and meat products, while another group consumed most of their protein from foods enriched with pea protein.
Using the gold standard euglycemic hyperinsulinemic clamp technique, the study found no significant differences between groups for insulin sensitivity.
There were also no differences between groups for blood pressure, blood lipids, serum markers of glycemic control, C-reactive protein, and blood and urinary markers of kidney function.
This suggests that the amino acid composition of high-protein diets does not influence health markers studied in people with type 2 diabetes.
The use of pea protein-enriched functional foods in this study limits the external validity of the findings as most people obtain plant protein from natural plant foods that also contain fiber and bioactive compounds that can provide health benefits.