Will a Low-Carb Diet Reduce Racial & Ethnic Health Disparities?

Coauthored with Nicholas Norwitz, Diana Thomas, and Cara Ebbeling. Image modified from ChatGTP by Mr Andrew Degryse

Main Messages:

People in some racial-ethnic groups tend to produce more of the hormone insulin after a high-carbohydrate meal than others, increasing risk for obesity and metabolic diseases related to insulin resistance.

A new clinical trial suggests that Black women who produce a lot of insulin respond exceptionally well to a low-carb diet.

Targeting carbs, not calories, may reduce or eliminate the excess risk for obesity and metabolic syndrome among high-risk demographic groups.

The prevalence of obesity is greater among Black than White women in the US (57 to 40%), contributing to differential burdens of diabetes, atherosclerosis, cancer and other weight-related diseases [1]. In this context, identifying reasons for the relatively poor outcomes of behavioral obesity treatments in Black women [2–5] assumes major public health significance.

One study from 1991 [2] examined race-specific results in two multi-center trials on hypertension and reported that Black women lost about 6 lbs less over 18 to 36 months than White women. Among more recent trials, investigators observed low dietary quality and physical activity level among Black women [3–5], potentially related to stress or other sociocultural factors.

Reports such as these raise the discouraging possibility that unmodifiable biological factors (e.g., genetics) or seemingly intractable environmental and behavioral influences impede healthful lifestyle change. However, another more hopeful explanation for this health disparity is the presence of a modifiable biological factor — specifically, how much insulin the body produces in response to carbohydrate.

An important study from 1996 [6] found that Black adults had greater insulin response to glucose and greater insulin resistance than non-Hispanic White adults, after taking into account body weight and body fat distribution. Another study [7] reported a similar finding in Black vs. White children. Could a reduced-carbohydrate diet, which stimulates less insulin production, improve the efficacy of weight loss treatment in Black women with a disorder in insulin dynamics?

The New Study

A study now online at the journal Obesity provides data to address this question. Martins and colleagues [8] randomly assigned 69 Black women with obesity to low-fat (55% carb, 20% fat) or low-carbohydrate (20% carb, 55% fat) weight-loss diets. They provided 60% of estimated calorie requirements for 10 weeks and 100% of calorie requirement during a subsequent 4-week weight maintenance period.

The investigators report that participants lost ∼ 12 lb overall — an impressive outcome. However, insulin sensitivity (the opposite of insulin resistance) influenced response. Among participants in the lower half of insulin sensitivity, fat mass decrease was substantially greater on the low-carbohydrate vs. low-fat diet (11 versus 4.5 lbs). Also among the low insulin sensitivity group, total energy expenditure was much lower following weight loss on the low-fat diet than the low-carbohydrate diet (-230 versus +67 calories [kcal] per day), consistent with results from a large feeding study [9].

Implications

These results have potentially huge implications: The type of foods you eat can affect the number of calories you burn. If true, it means, conceptually, that 1 diet with 2000 kcal could cause weight gain, and another with the exact same number of calories could cause weight loss … undermining foundations of the conventional “energy balance” view of obesity.

This provocative study does have limitations, most notably a small number of participants and short duration. We need additional research in this and other racial-ethnic groups.

Limitations notwithstanding, the trial by Martins and colleagues adds to already compelling evidence relating individual metabolic characteristics to weight loss on diets differing in macronutrients [10, 11], consistent with the new National Institutes of Health Precision Nutrition initiative.

The Carb-Insulin Model

What mechanisms could explain these effects? According to the carbohydrate-insulin model of obesity (CIM) [12], the rapid rise of blood glucose after a high-glycemic load meal (i.e., high in total carbohydrate and/or glycemic index) raises the insulin-to-glucagon ratio. This hormonal response shifts “fuel partitioning” (where the calories you eat wind up) toward fat deposition, leaving fewer calories for the brain, muscle, and other metabolically active organs. Consequently, we get hungrier or metabolism slows to compensate for those calories being sequestered into fat. Thus, in contrast to the usual way of thinking about obesity, “overeating” (a positive energy balance) is a consequence, not cause, of weight gain.

For individuals with a tendency to secrete large amounts of insulin, whether inherited or acquired (e.g., due to prior adverse life experiences, such as in early life), the CIM predicts that a high-carbohydrate diet would exacerbate this high risk predisposition, leading to a vicious cycle of higher insulin production, fat deposition, and hunger [11]. Conventional behavioral weight loss targets (to “eat less” and “move more”) don’t target the underlying metabolic drivers, making long-term weight loss maintenance unsustainable for most people.

From this perspective, racial disparities in weight and health that might appear to be behavioral in origin could instead arise from hormonal differences. Specifically, the difficulty adhering to a calorie-restricted diet (due to hunger) and physical activity goals (due to slowing metabolism and fatigue) may be caused by the effects of a conventional high-carbohydrate diet, more so than unmodifiable genes or intractable environmental factors.

Summary

The study by Martins and colleagues suggests that a low-carbohydrate diet may have special benefits among Black women and other demographic groups with prevalent abnormalities of insulin dynamics — for not only weight control, but also other diseases associated with insulin resistance. In addition to enhancing treatment outcomes, this biologically oriented approach may lessen implicit stigma, by reconceptualizing the reasons why people have difficulty adhering to calorie restricted diets. Considering the public health impacts, well-powered, high quality, long-term trials of a low-carbohydrate diet in Black adults and children should be given priority.

References

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8. Martins C, Bryan DR, Sweatt SK, Garvey WT, Fontaine KR, Dutton GR, Gower BA. The impact of a low-carbohydrate (vs. low-fat) diet on fat mass loss in African American women is modulated by insulin sensitivity. Obesity 2025 Jan 5. doi: 10.1002/oby.24201. Online ahead of print.

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