When you're choosing protein for dinner, more grams alone can't settle how that choice relates to diabetes or heart disease. In studies following US adults, protein from plants and protein from animals had different links with those outcomes. The source and, in modeled comparisons, the food it replaced added information the total couldn't give.
These were observational links. They help explain differences between eating patterns without proving that protein itself caused the differences in disease risk.
Protein source and type 2 diabetes
An analysis of US men and women compared the share of daily calories coming from different protein sources.
People getting the highest share from animal protein had a higher risk of developing type 2 diabetes than those getting the lowest share. A higher share from plant protein was associated with modestly lower risk.
Those comparisons describe eating patterns followed over time. To examine a change in source more directly, the researchers also modeled replacing some animal protein with plant protein.
What the animal-to-plant protein exchange meant
Imagine a whole day's calories as 100 counters. Five counters represent calories supplied by animal protein. In the modeled exchange, those five counters come from plant protein instead. The total stays at 100.
That represents replacing five percentage points of the day's total calories from animal protein with plant protein. The five points refer to all daily calories.
In the cohort analysis, this modeled exchange was associated with a 23% lower relative risk of type 2 diabetes. The reported 95% confidence interval ranged from 16% to 30% lower.
“Relative” describes a proportional difference in risk between the diets being compared. It doesn't mean 23 fewer cases for every 100 people. The abstract does not report the absolute difference in cases.
Researchers estimated this association from reported eating patterns. They did not assign people to make the exchange and then measure its effects. The counters explain the comparison, rather than a guaranteed result from changing a meal.
For heart health, the replaced food mattered
A separate analysis of three US adult cohorts examined how much protein came from plants relative to animals.
A higher plant-to-animal protein ratio was associated with lower risk of cardiovascular disease—disease affecting the heart and blood vessels overall—and coronary artery disease, which affects the arteries supplying the heart. The same analysis found no association with stroke risk.
The food replacements made the dinner question more specific. Among the exchanges researchers modeled, replacing red and processed meat with several plant protein sources was linked with the greatest reduction in cardiovascular disease risk.
That result stays tied to those foods. The analysis compared whole foods in observed diets and did not isolate protein as the cause of the difference. It therefore adds context to a choice involving red and processed meat; it doesn't establish a ranking of every protein food for every health outcome.
Body-fat findings answer a separate question
The human comparisons above concerned later diabetes and cardiovascular disease. A result about body fat came from a separate experiment in mice.
That experiment tested a specially formulated low-protein diet with added methionine, a protein building block. The diet reduced body fat and frailty, meaning physical vulnerability, in those mice.
The recipe was described as modeling traditional Mediterranean and Okinawan diets. Those names explain the experimental diet's construction; the experiment did not test people in those populations. It also does not isolate a benefit from methionine by itself or establish that the same diet would have those effects in humans.
High-protein diets' effectiveness and safety for weight reduction remain unsettled by the findings discussed here. The human disease associations and the mouse experiment cannot supply a universal daily protein target.
Keep the food alongside the protein number
A dinner choice involving plant protein in place of red and processed meat names a concrete food exchange. Simply saying “more protein” leaves that exchange unspecified.
For diabetes and heart disease, the human studies give a reason to keep protein amount, source and replacement together in the question. They show associations with later disease while leaving the right daily amount for any one person unanswered.