While Engel’s Law explains how the share of household spending on food rises as incomes fall, Bennett’s Law focuses on how diets change as incomes increase.

Formulated in the 1940s by American statistician and economist John Bennett, the law states that diet composition shifts as incomes rise. The share of staple foods such as cereals, roots and tubers declines, while consumption of higher-value foods increases, including dairy, meat, fruits and vegetables, as well as processed products.

Bennett’s Law complements Engel’s Law, which states that as incomes rise, the share of household budgets spent on food falls. The United Nations Food and Agriculture Organization (FAO) notes that this shift in consumption patterns, first observed in the United States and Europe, has been under way for decades across Asia and Africa.

“On average and across countries, the higher the income, proxied by GDP per capita, the lower the share of staple foods in total calories available. For example, in Mali – a low-income country – staple foods account for 73 percent of total calories, compared with 51 percent in relatively more affluent South Africa, an upper-middle-income country,” the FAO said in The State of Agricultural Commodity Markets 2024.

Opportunities for the agricultural economy

This nutritional transition, driven by urbanization and rising incomes, creates significant opportunities for African agriculture. Urban households consume a wider range of foods in larger quantities, including dairy products, fish, meat, legumes, fresh fruits and vegetables, as well as processed foods. This shift away from traditional cereals toward more diversified diets is boosting demand and driving growth in the food sector.

In Africa, where urbanization is advancing rapidly, urban food demand is attracting investment from small and medium-sized enterprises to add value along the agricultural supply chain — from cold storage for fruits and vegetables to the processing of legumes into ready-to-use flours, as well as the sorting and packaging of meat and fish.

Urbanization is also reducing the time available for meal preparation, supporting the growth of fast food, takeout, app-based delivery and purchases at supermarkets and convenience stores. These changes are accompanied by a diversification of retail outlets — from informal markets to street stalls and restaurants — as supply chains become more commercialized. The result is higher food consumption and concrete opportunities for African farmers.

Challenges and nuances

In developing countries, the shift toward more diversified diets presents both benefits and risks. On one hand, higher consumption of fruits, vegetables, fish and dairy products can improve nutrition. On the other, it can drive the spread of ultra-processed foods high in fat, sugar and salt, increasing rates of overweight and obesity.

Bennett’s Law does not operate automatically. It depends largely on the physical and economic accessibility of diverse foods. If fruits, vegetables, dairy or fish remain expensive or scarce, additional income may instead be spent on cheap processed products, at the expense of healthier diets.

Moreover, rising household income does not always translate into dietary change. In some cases, additional income is used to increase consumption of staple foods such as cereals and tubers, or redirected toward other essential needs, including transport, education and healthcare.

Espoir Olodo