More than 40 million low-income Americans benefit from the Department of Agriculture’s Supplemental Nutrition Assistance Program (SNAP), which replaced “food stamps” in 2008. SNAP’s eligibility criteria vary by state, but recipients generally must be at or below 130 percent of the federal poverty line, which, in most states, ranges from $15,650 for a one-person household to $54,150 or higher for a family of eight or more. Monthly benefit amounts, fully funded by federal taxpayers, assume that the average household spends 30 percent of its gross income on food. The maximum monthly SNAP benefit (“allotment”) starts at $292 and increases with household size.

After Donald Trump’s election and Robert F. Kennedy Jr.’s confirmation as secretary of the Department of Health and Human Services, proposals are being advanced in Congress to prohibit SNAP benefits from being used to purchase ultra-processed “junk food,” ostensibly to help respond to America’s obesity epidemic and the diseases (Type 2 diabetes, cardiovascular) related to it. (Recipients already cannot buy tobacco products, alcoholic beverages, nutritional supplements or “hot” prepared foods with SNAP.)

Will it? I am skeptical for several reasons. Like most free-market economists, I oppose lecturing adults about what they can and cannot eat or drink. Because of tight budgets, low-income households rationally spend their money on calorie-dense foods.

The devil, of course, is in the details. How will junk food be defined? As with the construction of the USDA’s “food pyramid,” we can expect lobbying by the processed food industry, nutritionists and other special-interest groups to influence the standards, such as sugar and fat content, of the food and drink products classified as “junk.”

As Adam Smith taught, human beings are not mindless pawns that can be moved around the chessboard at public officials’ will. Once the junk food category is delineated, food manufacturers will likely modify their ingredients to avoid the junk food label. Coke Zero and other artificially sweetened soft drinks, which are not necessarily healthful, have recently been introduced partly to evade the selective local taxation of sugar-sweetened beverages. Similarly, many salt-free, sugar-free and caffeine-free beverages have also been introduced. Growing awareness of the adverse health effects of sugary soft drinks, at least among certain elites, has further induced ingredient substitution.

Although the results of academic studies are unsettled (science never is), evidence indicates that banning the sale of flavored vaping products led young consumers to switch to a more dangerous product: cigarettes. Rather than reducing smoking, heavy taxation or outright prohibitions of flavored tobacco products are countered by cross-border shopping or black-market smuggling. Denmark’s “saturated fat tax” was repealed quickly because of opposition from “influential adversaries” (local retailers who lost sales to foreign sellers and nutritionists who questioned its public health benefits).

An analogy to proposals prohibiting SNAP-financed junk food purchases can be found in a rule proposed by the Federal Trade Commission in the late 1970s, which contemplated banning television advertising of ready-to-eat cereals during Saturday morning cartoon time. Even the Washington Post called the FTC a “National Nanny” for its temerity in telling mothers what their children should be allowed to eat for breakfast.

American obesity is a public health concern only because of the socialization of health insurance (primarily through Medicaid for the SNAP-eligible population), which shifts the costs of poor diets and sedentary lifestyles onto taxpayers’ shoulders and reduces recipients’ incentives to watch their waists (behavioral changes that insurers call “moral hazard”). Obesity-related diseases would be private costs to overweight Americans if they had to pay their medical bills themselves.

I suspect programmatic fraud is a more serious problem than the groceries SNAP recipients can buy. It certainly was with the food stamp program that SNAP replaced. A dollar’s worth of food stamps typically sold for 50 cents in cash, which recipients could then use to buy ineligible items.

The plans for reforming SNAP represent nanny-ism on steroids. The proposals will fatten the pockets of lobbyists and federal office-seekers and their consultants and advisers, but they’re likely to have only minor effects on obesity rates and public health.