Mainly fruit diet study: biochemical effects in 1971

What the 1971 study was trying to measure

Meyer and colleagues investigated whether a mainly fruit diet supplemented with nuts affected glucose handling, insulin secretion, plasma lipids and proteins. Published in the South African Medical Journal, the biochemical paper accompanied a report on physiological and nutritional observations. Historical racial labels distinguished groups with different follow-up schedules but are outdated today.

How the testing was organized

Participants underwent oral glucose testing before and during the intervention. Blood samples were collected fasting and at 30, 60, 90 and 120 minutes after a 50 g glucose load. One group was reassessed after 12 and 24 weeks, the other after 6 and 12 weeks. A woman habitually consuming a similar diet provided an additional comparison rather than a matched control.

Biochemical outcomes

Researchers measured glucose, insulin, cholesterol, triglycerides, phospholipids, β-lipoproteins and plasma proteins. Dietary fat supplied 36.7% and 37.6% of energy in the intervention groups and 45% in the habitual fruit-diet participant. Nuts and avocado therefore made this substantially different from a fruit-only or very-low-fat diet.

Glucose tolerance changed less than some insulin measures

Neither intervention group showed a significant change in glucose tolerance. Fasting glucose increased significantly after 12 or 24 weeks in the group historically described as Bantu but remained unchanged in the white group.

Insulin responses declined significantly in most participants in the first group, whereas the second group showed mixed results. Limited statistical detail restricts interpretation of these differences.

Lipids and plasma proteins showed time-dependent shifts

Total plasma lipids decreased significantly in both groups after 12 weeks and remained lower at 24 weeks in the longer-followed group.

Cholesterol declined significantly at 24 weeks in one group and at 6 weeks in the other, although the latter difference disappeared by 12 weeks. Triglycerides declined initially in the longer-followed group but later tended to increase in four participants as avocado availability changed. Albumin and globulin also shifted, particularly among participants with unusual baseline values.

The findings describe short-term biochemical responses, not evidence of nutritional completeness or clinical superiority..

Why the evidence remains limited

Small samples, differing follow-up schedules and simultaneous dietary changes limit interpretation. The investigators also suggested that hospitalization and inactivity could explain elevated baseline lipids in one group. Biochemical changes alone cannot demonstrate long-term health benefits or safety.

Frequently asked questions

Did the 1971 fruit diet improve glucose tolerance?

No significant improvement in glucose tolerance was reported in either group. Fasting glucose increased in one group, while insulin responses varied. The small samples and limited statistical reporting prevent conclusions about improved glycaemic control. The findings describe biochemical responses to one dietary intervention rather than evidence supporting its general use.

Was the diet made only of fruit?

No. The intervention combined fruit with nuts and substantial avocado consumption. Changes in avocado availability were considered when interpreting triglyceride results. Dietary fat contributed more than one-third of energy in both intervention groups, making the regimen substantially different from an exclusively fruit-based or very-low-fat diet.

Did cholesterol fall during the study?

Cholesterol decreased significantly after 24 weeks in one group and after 6 weeks in the other. However, the second group's reduction was no longer significant at 12 weeks. These inconsistent findings do not establish that the diet reliably lowers cholesterol or produces lasting cardiovascular benefits in larger populations.

Does this study show that fruitarian diets are safe long term?

No. Follow-up lasted weeks rather than years, and the small groups did not provide evidence of long-term nutritional adequacy. Restrictive plant-based diets may require careful attention to vitamin B12, vitamin D, iodine, calcium, iron and zinc. The study cannot establish the long-term safety of predominantly fruit-based diets.

How should the study be viewed today?

The experiment offers historical observations about glucose, insulin, lipids and plasma proteins during an unusual dietary intervention. Modern research generally supports whole fruit within varied dietary patterns, without establishing fruit-dominant diets as superior. Its findings remain relevant to nutrition history but should not be generalized into dietary recommendations.