In short: low-carbohydrate diets may be associated with less health benefits than the high-carbohydrate ones. A carefully designed low-carbohydrate diet high in whole fruits, grains and vegetables and low in added sugars and saturated and trans fats should not cause side effects within 1-2 years. Long-term effects of low-carb diets are not known due to lack of studies (APJCN, 2003).
A low-carbohydrate non-ketogenic diet is the one in which carbohydrates represent 20-40% of energy intake (Current Diabetes Reports ; APJCN). The current Acceptable Macronutrient Distribution Ranges (AMDR) are: carbohydrate: 45-65%, fat: 20-35%, protein: 10-35% (National Academics).
Low vs high carbohydrate diets
The diet quality as measured by Healthy Eating Index (HEI) was highest for the high
carbohydrate groups and lowest for the low carbohydrate groups. The
BMIs were significantly lower for men and women on the high
carbohydrate diet; the highest BMIs were noted for those on a low
carbohydrate diet (Jandonline, 2001).
For macronutrients, more favorable Chinese Healthy Eating Index (CHEI) scores were associated with
higher intakes of protein, carbohydrate, dietary fiber and lower fat
intake...(Nutrients, 2018 ; Table 6)
Low-carbohydrate diets were associated with a significantly higher
risk of all-cause mortality and they were not significantly associated
with a risk of CVD mortality and incidence (Plos One, 2013).
The available scientific literature shows that controlled diet studies
(several weeks to < 2 year) with low-carb high-fat in persons with obesity and
diabetes do induce favorable effects on weight loss, blood glucose and
insulin as well as some less desirable effects (increase LDL
cholesterol, decrease vascular reactivity) (European Journal of
Nutrition, 2018).
Carbohydrate quality
However, carbohydrate quality seems to have a more important role in
population health than carbohydrate amount. A strong case can be made
for consumption of high glycemic load grains, potato products, and added sugars
(especially in drinks) being causally related to obesity, diabetes,
cardiovascular disease, and some cancers; whereas non-starchy
vegetables, whole fruits, legumes, and whole kernel grains appear
protective (BMJ, 2018).
In the Nurses' Health Study (NHS) and the Health Professionals Follow-Up Study (HPFS), including 203,457 adults, the intake of vegetables, whole fruit and whole grains was associated with lower and intake of red and processed meat with higher risk of cardiovascular disease; the intake of green leafy vegetables (but not potatoes) and whole grains was associated with a lower and intake of sugar-sweetened beverages (soda, fruit juices) with higher risk of diabetes (Journal of Nutrition, 2012 ; Table 1).
In summary, there seems to be no evidence that a carefully designed low-carbohydrate non-ketogenic diet (33% energy from carbohydrates and enough polyunsaturated fats as suggested in the question) would cause any short-term (1-2 years) side effects on the gastrointestinal, urinary, circulatory, neurological or other system, but long-term effects are not known. When lowering carbohydrates, it may be a good idea to increase protein rather than fat intake.
